Programming in Go

ptg7913109 ptg7913109 Programming in Go ptg7913109 The Developer’s Library Series from Addison-Wesley provides practicing programmers with unique, high-quality references and tutorials on the latest programming languages and technologies they use in their daily work. All books in the Developer’s Library are written by expert technology practitioners who are exceptionally skilled at organizing and presenting information in a way that’s useful for other programmers. Developer’s Library books cover a wide range of topics, from open- source programming languages and databases, Linux programming, Microsoft, and Java, to Web development, social networking platforms, Mac/iPhone programming, and Android programming. Visit for a complete list of available products Developer’s Library Series ptg7913109 Programming in Go Creating Applications for the 21st Century Mark Summerfield Upper Saddle River, NJ · Boston · Indianapolis · San Francisco p New York · Toronto · Montreal · London · Munich · Paris · Madrid p Capetown · Sydney · Tokyo · Singapore · Mexico City ptg7913109 Many of the designationsused by manufacturersand sellersto distinguish their productsare claimed as trademarks. Where those designationsappear in this book,and the publisher was aware of a trademark claim, the designations have been printed with initial capital letters or in all capitals. The author and publisher have taken care in the preparation of this book, but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of the use of the information or programs contained herein. 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This publication is protected by copyright, and permission must be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. To obtain permission to use material from this work, please submit a written request to Pearson Education, Inc., Permissions Department, One Lake Street, Upper Saddle River, New Jersey 07458, or you may fax your request to (201) 236-3290. ISBN-13: 978-0-321-77463-7 ISBN-10: 0-321-77463-9 Text printed in the United States on recycled paper at RR Donnelley in Crawfordsville, Indiana. First printing, April 2012 ptg7913109 This book is dedicated to Jasmin Blanchette and Trenton Schulz ptg7913109 This page intentionally left blank ptg7913109 Contents at a Glance Tables .............................................................. xv Introduction ...................................................... 1 Chapter 1. An Overview in Five Examples ...................... 7 Chapter 2. Booleans and Numbers ................................ 51 Chapter 3. Strings ................................................ 81 Chapter 4. Collection Types ...................................... 139 Chapter 5. Procedural Programming ............................ 185 Chapter 6. Object-Oriented Programming ...................... 253 Chapter 7. Concurrent Programming ............................ 315 Chapter 8. File Handling .......................................... 361 Chapter 9. Packages .............................................. 407 Appendix A. Epilogue ............................................ 435 Appendix B. The Dangers of Software Patents .................. 437 Appendix C. Selected Bibliography .............................. 441 Index .............................................................. 443 ptg7913109 This page intentionally left blank ptg7913109 Contents Tables .............................................................. xv Introduction ...................................................... 1 Why Go? ........................................................ 1 The Structure of the Book ........................................ 4 Acknowledgments ................................................ 5 Chapter 1. An Overview in Five Examples ........................ 7 1.1. Getting Going ................................................ 7 1.2. Editing, Compiling, and Running ............................ 9 1.3. Hello Who? .................................................. 14 1.4. Big Digits—Two-Dimensional Slices .......................... 16 1.5. Stack—Custom Types with Methods .......................... 21 1.6. Americanise—Files, Maps, and Closures ...................... 29 1.7. Polar to Cartesian—Concurrency ............................ 40 1.8. Exercise .................................................... 48 Chapter 2. Booleans and Numbers ................................ 51 2.1. Preliminaries ................................................ 51 2.1.1. Constants and Variables ................................ 53 Enumerations ...................................... 54 2.2. Boolean Values and Expressions .............................. 56 2.3. Numeric Types .............................................. 57 2.3.1. Integer Types .......................................... 59 Big Integers ........................................ 61 2.3.2. Floating-Point Types .................................... 64 Complex Types .................................... 70 2.4. Example: Statistics .......................................... 72 2.4.1. Implementing Simple Statistics Functions ................ 73 2.4.2. Implementing a Basic HTTP Server ...................... 75 2.5. Exercises .................................................... 78 ix ptg7913109 Chapter 3. Strings ................................................ 81 3.1. Literals, Operators, and Escapes .............................. 83 3.2. Comparing Strings .......................................... 86 3.3. Characters and Strings ...................................... 87 3.4. Indexing and Slicing Strings .................................. 90 3.5. String Formatting with the Fmt Package ...................... 93 3.5.1. Formatting Booleans .................................... 97 3.5.2. Formatting Integers .................................... 98 3.5.3. Formatting Characters .................................. 99 3.5.4. Formatting Floating-Point Numbers .................... 100 3.5.5. Formatting Strings and Slices ............................ 101 3.5.6. Formatting for Debugging .............................. 103 3.6. Other String-Related Packages .............................. 106 3.6.1. The Strings Package .................................... 107 3.6.2. The Strconv Package .................................... 113 3.6.3. The Utf8 Package ...................................... 117 3.6.4. The Unicode Package .................................... 118 3.6.5. The Regexp Package .................................... 120 3.7. Example: M3u2pls .......................................... 130 3.8. Exercises .................................................... 135 Chapter 4. Collection Types ...................................... 139 4.1. Values, Pointers, and Reference Types ........................ 140 4.2. Arrays and Slices ............................................ 148 4.2.1. Indexing and Slicing Slices .............................. 153 4.2.2. Iterating Slices .......................................... 154 4.2.3. Modifying Slices ........................................ 156 4.2.4. Sorting and Searching Slices ............................ 160 4.3. Maps ........................................................ 164 4.3.1. Creating and Populating Maps .......................... 166 4.3.2. Map Lookups .......................................... 168 4.3.3. Modifying Maps ........................................ 169 4.3.4. Key-Ordered Map Iteration .............................. 170 4.3.5. Map Inversion .......................................... 170 4.4. Examples .................................................... 171 4.4.1. Example: Guess Separator .............................. 171 4.4.2. Example: Word Frequencies .............................. 174 4.5. Exercises .................................................... 180 x ptg7913109 Chapter 5. Procedural Programming ............................ 185 5.1. Statement Basics ............................................ 186 5.1.1. Type Conversions ...................................... 190 5.1.2. Type Assertions ........................................ 191 5.2. Branching .................................................. 192 5.2.1. If Statements .......................................... 192 5.2.2. Switch Statements ...................................... 195 Expression Switches ................................ 195 Type Switches ...................................... 197 5.3. Looping with For Statements ................................ 203 5.4. Communication and Concurrency Statements ................ 205 5.4.1. Select Statements ...................................... 209 5.5. Defer, Panic, and Recover .................................... 212 5.5.1. Panic and Recover ...................................... 213 5.6. Custom Functions ............................................ 219 5.6.1. Function Arguments .................................... 220 Function Calls as Function Arguments .............. 220 Variadic Functions ................................ 221 Functions with Multiple Optional Arguments ........ 222 5.6.2. The init() and main() Functions .......................... 224 5.6.3. Closures ................................................ 225 5.6.4. Recursive Functions .................................... 227 5.6.5. Choosing Functions at Runtime .......................... 230 Branching Using Maps and Function References .... 230 Dynamic Function Creation ........................ 231 5.6.6. Generic Functions ...................................... 232 5.6.7. Higher Order Functions ................................ 238 Memoizing Pure Functions ........................ 241 5.7. Example: Indent Sort ........................................ 244 5.8. Exercises .................................................... 250 Chapter 6. Object-Oriented Programming ...................... 253 6.1. Key Concepts ................................................ 254 6.2. Custom Types ................................................ 256 6.2.1. Adding Methods ........................................ 258 Overriding Methods ................................ 261 Method Expressions ................................ 263 6.2.2. Validated Types ........................................ 263 xi ptg7913109 6.3. Interfaces .................................................... 265 6.3.1. Interface Embedding .................................... 270 6.4. Structs ...................................................... 275 6.4.1. Struct Aggregation and Embedding ...................... 275 Embedding Values ................................ 276 Embedding Anonymous Values That Have Methods .. 277 Embedding Interfaces .............................. 279 6.5. Examples .................................................... 282 6.5.1. Example: FuzzyBool—A Single-Valued Custom Type ...... 282 6.5.2. Example: Shapes—A Family of Custom Types ............ 289 Package-Level Convenience Functions .............. 289 A Hierarchy of Embedded Interfaces ................ 294 Freely Composable Independent Interfaces .......... 294 Concrete Types and Methods ........................ 295 6.5.3. Example: Ordered Map—A Generic Collection Type ...... 302 6.6. Exercises .................................................... 311 Chapter 7. Concurrent Programming ............................ 315 7.1. Key Concepts ................................................ 317 7.2. Examples .................................................... 322 7.2.1. Example: Filter ........................................ 322 7.2.2. Example: Concurrent Grep .............................. 326 7.2.3. Example: Thread-Safe Map .............................. 334 7.2.4. Example: Apache Report ................................ 341 Synchronizing with a Shared Thread-Safe Map ...... 341 Synchronizing with a Mutex-Protected Map ........ 345 Synchronizing by Merging Local Maps via Channels 347 7.2.5. Example: Find Duplicates .............................. 349 7.3. Exercises .................................................... 357 Chapter 8. File Handling .......................................... 361 8.1. Custom Data Files .......................................... 362 8.1.1. Handling JSON Files .................................... 365 Writing JSON Files ................................ 366 Reading JSON Files ................................ 368 8.1.2. Handling XML Files .................................... 371 Writing XML Files ................................ 371 Reading XML Files ................................ 375 8.1.3. Handling Plain Text Files ................................ 377 xii ptg7913109 Writing Plain Text Files ............................ 378 Reading Plain Text Files ............................ 380 8.1.4. Handling Go Binary Files ................................ 385 Writing Go Binary Files ............................ 385 Reading Go Binary Files ............................ 386 8.1.5. Handling Custom Binary Files .......................... 387 Writing Custom Binary Files ...................... 388 Reading Custom Binary Files ...................... 392 8.2. Archive Files ................................................ 397 8.2.1. Creating Zip Archives .................................. 397 8.2.2. Creating Optionally Compressed Tarballs ................ 399 8.2.3. Unpacking Zip Archives ................................ 401 8.2.4. Unpacking Optionally Compressed Tarballs .............. 403 8.3. Exercises .................................................... 405 Chapter 9. Packages .............................................. 407 9.1. Custom Packages ............................................ 408 9.1.1. Creating Custom Packages .............................. 408 Platform-Specific Code .............................. 410 Documenting Packages ............................ 411 Unit Testing and Benchmarking Packages .......... 414 9.1.2. Importing Packages .................................... 416 9.2. Third-Party Packages ........................................ 417 9.3. A Brief Survey of Go’s Commands ............................ 418 9.4. A Brief Survey of the Go Standard Library .................... 419 9.4.1. Archive and Compression Packages ...................... 419 9.4.2. Bytes and String-Related Packages ...................... 419 9.4.3. Collection Packages .................................... 421 9.4.4. File, Operating System, and Related Packages ............ 423 File Format-Related Packages ...................... 424 9.4.5. Graphics-Related Packages .............................. 425 9.4.6. Mathematics Packages .................................. 425 9.4.7. Miscellaneous Packages ................................ 425 9.4.8. Networking Packages .................................. 427 9.4.9. The Reflect Package .................................... 427 9.5. Exercises .................................................... 431 xiii ptg7913109 Appendix A. Epilogue ............................................ 435 Appendix B. The Dangers of Software Patents .................. 437 Appendix C. Selected Bibliography .............................. 441 Index .............................................................. 443 xiv ptg7913109 Tables 2.1. Go’s Keywords .............................................. 52 2.2. Go’s Predefined Identifiers .................................. 52 2.3. Boolean and Comparison Operators .......................... 57 2.4. Arithmetic Operators Applicable to All Built-In Numbers ...... 59 2.5. Go’s Integer Types and Ranges .............................. 60 2.6. Arithmetic Operators Applicable Only to Built-In Integer Types 60 2.7. Go’s Floating-Point Types .................................... 64 2.8. The Math Package’s Constants and Functions #1 .............. 65 2.9. The Math Package’s Constants and Functions #2 .............. 66 2.10. The Math Package’s Constants and Functions #3 .............. 67 2.11. The Complex Math Package’s Functions ...................... 71 3.1. Go’s String and Character Escapes .......................... 84 3.2. String Operations .......................................... 85 3.3. The Fmt Package’s Print Functions .......................... 94 3.4. The Fmt Package’s Verbs .................................... 95 3.5. The Fmt Package’s Verb Modifiers ............................ 96 3.6. The Strings Package’s Functions #1 .......................... 108 3.7. The Strings Package’s Functions #2 .......................... 109 3.8. The Strconv Package’s Functions #1 .......................... 114 3.9. The Strconv Package’s Functions #2 .......................... 115 3.10. The Utf8 Package’s Functions ................................ 118 3.11. The Unicode Package’s Functions ............................ 119 3.12. The Regexp Package’s Functions ............................ 121 3.13. The Regexp Package’s Escape Sequences .................... 121 3.14. The Regexp Package’s Character Classes .................... 122 3.15. The Regexp Package’s Zero-Width Assertions ................ 122 3.16. The Regexp Package’s Quantifiers ............................ 123 3.17. The Regexp Package’s Flags and Groups ...................... 123 3.18. The *regexp.Regexp Type’s Methods #1 ...................... 124 3.19. The *regexp.Regexp Type’s Methods #2 ...................... 125 4.1. Slice Operations ............................................ 151 xv ptg7913109 4.2. The Sort Package’s Functions ................................ 161 4.3. Map Operations ............................................ 165 5.1. Built-In Functions .......................................... 187 8.1. Format Speed and Size Comparisons ........................ 363 8.2. The Fmt Package’s Scan Functions .......................... 383 xvi ptg7913109 Introduction The purpose of this book is to teach solid idiomatic Go programming using all the features the language provides, as well as the most commonly used Go packages from Go’s standard library. The book is also designed to serve as a useful reference once the language is learned. To meet both of these goals the book is quite comprehensive and tries to cover every topic in just one place—and with forward and backward cross-references throughout. Go is quite C-like in spirit, being a small and efficient language with convenient low-level facilities such as pointers. Yet Go also offers many features associated with high- or very high-level languages, such as Unicode strings, powerful built- in data structures, duck typing, garbage collection, and high-level concurrency support that uses communication rather than shared data and locks. Go also has a large and wide-ranging standard library. The reader is assumed to have programming experience in a mainstream pro- gramming language such as C, C++,Java, Python,or similar,although all of Go’s unique features and idioms are illustrated with complete runnable examples that are fully explained in the text. To successfully learn any programming language it is necessary to write pro- grams in that language. To this end the book’s approach is wholly practical,and readers are encouraged to experiment with the examples, try the exercises, and write their own programs to get hands-on experience. As with all my previous books, the quoted code snippets are of “live code”; that is, the code was auto- matically extracted from .go source files and directly embedded in the PDF that went to the publisher—so there are no cut and paste errors, and the code works. Wherever possible, small but complete programs and packages are used as ex- amples to provide realistic use cases. The examples, exercises,and solutions are available online at The book’s key aim is to teach the Go language, and although many of the standard Go packages are used, not all of them are. This is not a problem, since reading the book will provide enough Go knowledge for readers to be able to make use of any of the standard packages, or any third-party Go package, and of course, be able to create their own packages. Why Go? The Go programming language began as an internal Google project in 2007.The original design was by Robert Griesemer and Unix luminariesRob Pike and Ken Thompson. On November 10, 2009, Go was publicly unveiled under a liberal 1 ptg7913109 2 Introduction open source license. Go is being developed by a team at Google which includes the original designers plus Russ Cox, Andrew Gerrand, Ian Lance Taylor, and many others. Go has an open development model and many developers from around the world contribute to it, with some so trusted and respected that they have the same commit privileges as the Googlers. In addition, many third-party Go packages are available from the Go Dashboard ( project). Go is the most exciting new mainstream language to appear in at least 15 years and is the first such language that is aimed squarely at 21st century computers—and their programmers. Go is designed to scale efficiently so that it can be used to build very big appli- cations—and to compile even a large program in mere seconds on a single com- puter. The lightning-fast compilation speed is made possible to a small extent because the language is easy to parse, but mostly because of its dependency management. If file app.go depends on file pkg1.go, which in turn depends on pkg2.go, in a conventional compiled language app.go would need both pkg1.go’s and pkg2.go’s object files. But in Go, everything that pkg2.go exports is cached in pkg1.go’s object file, so pkg1.go’s object file alone is sufficient to build app.go. For just three files this hardly matters, but it results in huge speedups for large applications with lots of dependencies. Since Go programs are so fast to build, it is practical to use them in situations where scripting languages are normally used (see the sidebar “Go Shebang Scripts”, ➤ 10). Furthermore, Go can be used to build web applications using Google’s App Engine. Go uses a very clean and easy-to-understand syntax that avoids the complexity and verbosity of older languages like C++ (first released in 1983) or Java (first released in 1995). And Go is a strongly statically typed language, something which many programmers regard as essential for writing large programs. Yet Go’s typing is not burdensome due to Go’s short “declare and initialize” variable declaration syntax (where the compiler deduces the type so it doesn’t have to be written explicitly), and because Go supports a powerful and convenient version of duck typing. Languages like C and C++ require programmers to do a vast amount of book- keeping when it comes to memory management—bookkeeping that could be done by the computer itself, especially for concurrent programs where keeping track can be fiendishly complicated. In recent years C++ has greatly improved in this area with various “smart” pointers, but is only just catching up with Java with regard to its threading library. Java relieves the programmer from the burden of memory management by using a garbage collector. C has only third- party threading libraries, although C++ now has a standard threading library. However, writing concurrent programs in C, C++, or Java requires considerable ptg7913109 Introduction 3 bookkeeping by programmers to make sure they lock and unlock resources at the right times. The Go compiler and runtime system takes care of the tedious bookkeeping. For memory management Go has a garbage collector, so there’s no need for smart pointers or for manually freeing memory. And for concurrency, Go provides a form of CSP (Communicating Sequential Processes) based on the ideas of com- puter scientist C. A. R. Hoare, that means that many concurrent Go programs don’t need to do any locking at all. Furthermore, Go uses goroutines—very lightweight processes which can be created in vast numbers that are automati- cally load-balanced across the available processors and cores—to provide much more fine-grained concurrency than older languages’ thread-based approach- es. In fact, Go’s concurrency support is so simple and natural to use that when porting single-threaded programs to Go it often happens that opportunities for using concurrency arise that lead to improved runtimes and better utilization of machine resources. Go is a pragmatic language that favors efficiency and programmer convenience over purity. For example, Go’s built-in types and user-defined types are not the same, since the former can be highly optimized in ways the latter can’t be. Go also provides two fundamental built-in collection types: slices (for all practical purposes these are references to variable-length arrays) and maps (key–value dictionaries or hashes). These collection types are highly efficient and serve most purposes extremely well. However, Go supports pointers (it is a fully com- piled language—there’s no virtual machine getting in the way of performance), so it is possible to create sophisticated custom types, such as balanced binary trees, with ease. While C supports only procedural programming and Java forces programmers to program everything in an object-oriented way, Go allows programmers to use the paradigm best suited to the problem. Go can be used as a purely procedural language, but also has excellent support for object-oriented programming. As we will see, though, Go’s approach to object orientation is radically different from, say, C++, Java, or Python—and is easier to use and much more flexible than earlier forms. Like C, Go lacks generics (templates in C++-speak); however, in practice the other facilities that Go provides in many cases obviate the need for generics. Go does not use a preprocessor or include files (which is another reason why it compiles so fast), so there is no need to duplicate function signatures as there is in C and C++. And with no preprocessor, a program’s semantics cannot change behind a Go programmer’s back as it can with careless #defines in C and C++. Arguably, C++, Objective-C, and Java have all attempted to be better Cs (the latter indirectly as a better C++).Go can also be seen as an attempt to be a better C, even though Go’s clean, light syntax is reminiscent of Python—and Go’s slices and maps are very similar to Python’s lists and dicts. However, Go is closer in ptg7913109 4 Introduction spirit to C than to any other language,and can be seen as an attempt to avoid C’s drawbacks while providing all that’s best in C, as well as adding many powerful and useful features that are unique to Go. Originally Go was conceived as a systemsprogramming language for developing large-scale programs with fast compilation that could take advantage of dis- tributed systems and multicore networked computers. Go’s reach has already gone far beyond the original conception and it is now being used as a highly productive general-purpose programming language that’s a pleasure to use and maintain. The Structure of the Book Chapter 1 begins by explaining how to build and run Go programs. The chapter then provides a brief overview of Go’s syntax and features, as well as introduc- ing some of its standard library. This is done by presenting and explaining a se- ries of five very short examples, each illustrating a variety of Go features. This chapter is designed to provide just a flavor of the language and to give readers a feel for the scope of what is required to learn Go. (How to obtain and install Go is also explained in this chapter.) Chapters 2 to 7 cover the Go language in depth. Three chapters are devoted to built-in data types: Chapter 2 covers identifiers, Booleans, and numbers; Chapter 3 covers strings; and Chapter 4 covers Go’s collection types. Chapter 5 describes and illustrates Go’s statements and control structures. It also explains how to create and use custom functions, and completes the chapters that show how to create procedural nonconcurrent programs in Go. Chapter 6 shows how to do object-oriented programming in Go. This chapter includes coverage of Go structs used for aggregating and embedding (delegat- ing) values, and Go interfaces for specifying abstract types, as well as how to produce an inheritance-like effect in some situations. The chapter presents several complete fully explained examples to help ensure understanding, since Go’s approach to object orientation may well be different from most readers’ ex- perience. Chapter 7 covers Go’s concurrency features and has even more examples than the chapter on object orientation, again to ensure a thorough understanding of these novel aspects of the Go language. Chapter 8 shows how to read and write custom binary, Go binary, text, JSON, and XML files. (Reading and writing text files is very briefly covered in Chap- ter 1 and several subsequent chapters since this makes it easier to have useful examples and exercises.) The book’s final chapter is Chapter 9. This chapter begins by showing how to import and use standard library packages, custom packages, and third-party ptg7913109 Introduction 5 packages. It also shows how to document, unit test, and benchmark custom packages. The chapter’s last sections provide brief overviews of the tools provided with the gc compiler, and of Go’s standard library. Although Go is quite a small language, it is a very rich and expressive language (as measured in syntactic constructs,concepts,and idioms),so there is a surpris- ing amount to learn. This book shows examples in good idiomatic Go style right from the start.★ This approach, of course, means that some things are shown be- fore being fully explained. We ask the reader to take it on trust that everything will be explained over the course of the book (and,of course,cross-references are provided for everything that is not explained on the spot). Go is a fascinating language, and one that is really nice to use. It isn’t hard to learn Go’ssyntax and idioms, but it does introduce some novel conceptsthat may be unfamiliar to many readers. This book tries to give readers the conceptual breakthroughs—especially in object-oriented Go programming and in concur- rent Go programming—that might take weeks or even months for those whose only guide is the good but rather terse documentation. Acknowledgments Every technical book I have ever written has benefited from the help and advice of others, and this one is no different in this regard. I want to give particular thanks to two friends who are programmers with no prior Go experience: Jasmin Blanchette and Trenton Schulz. Both have contributed to my books for many years, and in this case their feedback has helped to ensure that this book will meet the needs of other programmers new to Go. The book was also greatly enhanced by the feedback I received from core Go developer Nigel Tao. I didn’t always take his advice, but his feedback was always illuminating and resulted in great improvements both to the code and to the text. I had additional help from others, including David Boddie, a programmer new to Go, who gave some valuable feedback. And Go developers Ian Lance Taylor, and especially Russ Cox, between them solved many problems both of code and concepts, and provided clear and precise explanations that contributed greatly to the book’s accuracy. During the writing of the book I asked many questions on the golang-nuts mail- ing list and always received thoughtful and useful replies from many different ★ The one exception is that in the early chapters we always declare channels to be bidirectional, even when they are used only unidirectionally. Channels are declared to have a particular direction wherever this makes sense, starting from Chapter 7. ptg7913109 6 Introduction posters. I also received feedback from readers of the Safari “rough cut” preview edition that led to some important clarifications. The Italian software company, in the person of Giovanni Bajo, was kind enough to provide me with free Mercurial repository hosting to aid my peace of mind over the long process of writing this book. Thanks to Lorenzo Mancini for setting it all up and looking after it for me. I’m also very grateful to Anton Bowers and Ben Thompson who have been hosting my web site,, on their web server since early 2011. Thanks to Russel Winder for his coverage of software patents in his blog, www Appendix B borrows a number of his ideas. And as always, thanks to Jeff Kingston, creator of the lout typesetting sys- tem that I have used for all my books and many other writing projects over many years. Particular thanks to my commissioning editor, Debra Williams Cauley, who so successfully made the case for this book with the publisher, and who provided support and practical help as the work progressed. Thanks also to production manager Anna Popick, who once again managed the production process so well, and to the proofreader, Audrey Doyle, who did such excellent work. As ever, I want to thank my wife, Andrea, for her love and support. ptg7913109 1 An Overview in Five Examples §1.1. Getting Going ➤ 7 §1.2. Editing, Compiling, and Running ➤ 9 §1.3. Hello Who? ➤ 14 §1.4. Big Digits—Two-Dimensional Slices ➤ 16 §1.5. Stack—Custom Types with Methods ➤ 21 §1.6. Americanise—Files, Maps, and Closures ➤ 29 §1.7. Polar to Cartesian—Concurrency ➤ 40 This chapter provides a series of five explained examples. Although the exam- ples are tiny, each of them (apart from “Hello Who?”) does something useful, and between them they provide a rapid overview of Go’s key features and some of its key packages. (What other languages often call “modules” or “libraries” are called packages in Go terminology, and all the packages supplied with Go as standard are collectively known as the Go standard library.) The chapter’s pur- pose is to provide a flavor of Go and to give a feel for the scope of what needs to be learned to program successfully in Go. Don’t worry if some of the syntax or idioms are not immediately understandable;everything shown in this chapter is covered thoroughly in subsequent chapters. Learning to program Go the Go way will take a certain amount of time and practice. For those wanting to port substantial C, C++, Java, Python, and other programs to Go, taking the time to learn Go—and in particular how its object- orientation and concurrency featureswork—will save time and effort in the long run. And for those wanting to create Go applications from scratch it is best to do so making the most of all that Go offers, so again the upfront investment in learning time is important—and will pay back later. 1.1. Getting Going Go programsare compiled rather than interpreted so as to have the best possible performance. Compilation is very fast—dramatically faster than can be the case with some other languages, most notably compared with C and C++. 7 ptg7913109 8 Chapter 1. An Overview in Five Examples The Go Documentation i Go’s official web site is which hosts the most up-to-date Go docu- mentation. The “Packages” link provides access to the documentation on all the Go standard library’s packages—and to their source code, which can be very helpful when the documentation itself is sparse. The “Commands” link leads to the documentation for the programs distributed with Go (e.g., the compilers, build tools, etc.). The “Specification” link leads to an accessible, in- formal, and quite thorough Go language specification. And the “Effective Go” link leads to a document that explains many best practices. The web site also features a sandbox in which small (somewhat limited) Go programs can be written, compiled, and run, all online. This is useful for be- ginners for checking odd bits of syntax and for learning the Go fmt package’s sophisticated text formatting facilitiesor the regexp package’sregular expres- sion engine. The Go web site’s search box searches only the Go documenta- tion; to search for Go resources generally, visit The Go documentation can also be viewed locally, for example, in a web browser. To do this, run Go’s godoc tool with a command-line argument that tells it to operate as a web server. Here’s how to do this in a Unix console (xterm, gnome-terminal, konsole,, or similar): $ godoc -http=:8000 Or in a Windows console (i.e., a Command Prompt or MS-DOS Prompt window): C:\>godoc -http=:8000 The port number used here is arbitrary—simply use a different one if it conflicts with an existing server. This assumes that godoc is in your PATH. To view the served documentation, open a web browser and give it a location of http://localhost:8000. This will present a page that looks very similar to the web site’s front page. The “Packages” link will show the docu- mentation for Go’s standard library, plus any third-party packages that have been installed under GOROOT.If GOPATH is defined (e.g., for local programs and packages), a link will appear beside the “Packages” link through which the relevant documentation can be accessed. (The GOROOT and GOPATH environment variables are discussed later in this chapter and in Chapter 9.) It is also possible to view the documentation for a whole package or a single item in a package in the console using godoc on the command line. For ex- ample, executing godoc image NewRGBA will output the documentation for the image.NewRGBA() function, and executing godoc image/png will output the docu- mentation for the entire image/png package. ptg7913109 1.1. Getting Going 9 The standard Go compiler is called gc and its toolchain includes programs such as 5g, 6g, and 8g for compiling, 5l, 6l, and 8l for linking, and godoc for viewing the Go documentation. (These are 5g.exe, 6l.exe, etc., on Windows.) The strange names follow the Plan 9 operating system’s compiler naming conventions where the digit identifies the processor architecture (e.g., “5” for ARM, “6” for AMD- 64—including Intel 64-bit processors—and “8” for Intel 386.) Fortunately, we don’t need to concern ourselves with these tools, since Go provides the high-level go build tool that handles the compiling and linking for us. All the examples in this book—available from—have been tested using gc on Linux, Mac OS X, and Windows using Go 1. The Go developers intend to make all subsequent Go 1.x versions backward compatible with Go 1, so the book’s text and examples should be valid for the entire 1.x series. (If incompatible changes occur, the book’s examples will be updated to the latest Go release, so as time goes by, they may differ from the code shown in the book.) To download and install Go, visit which provides instructions and download links. At the time of this writing, Go 1 is available in source and binary form for FreeBSD 7+, Linux 2.6+, Mac OS X(Snow Leopard and Lion), and Windows 2000+, in all cases for Intel 32-bit and AMD 64-bit processor architectures. There is also support for Linux on ARM processors. Go prebuilt packages are available for the Ubuntu Linux distribution, and may be available for other Linuxes by the time you read this. For learning to program in Go it is easier to install a binary version than to build Go from scratch. Programs built with gc use a particular calling convention. This means that programs compiled with gc can be linked only to external libraries that use the same calling convention—unless a suitable tool is used to bridge the difference. Go comeswith support for using external C code from Go programsin the form of the cgo tool (, and at least on Linux and BSD systems, both C and C++ code can be used in Go programs using the SWIG tool ( In addition to gc there is also the gccgo compiler. This is a Go-specific front end to gcc (the GNU Compiler Collection) available for gcc from version 4.6. Like gc, gccgo may be available prebuilt for some Linux distributions. Instructions for building and installing gccgo are given at 1.2. Editing, Compiling, and Running Go programs are written as plain text Unicode using the UTF-8 encoding.★ Most modern text editors can handle this automatically, and some of the most popular may even have support for Go color syntax highlighting and automatic ★ Some Windows editors (e.g., Notepad) go against the Unicode standard’s recommendation and insert the bytes 0xEF, 0xBB, 0xBF, at the start of UTF-8 files. This book’s examples assume that UTF-8 files do not have these bytes. ptg7913109 10 Chapter 1. An Overview in Five Examples Go Shebang Scripts i One side effect of Go’s fast compilation is that it makes it realistic to write Go programs that can be treated as shebang #! scripts on Unix-like systems. This requires a one-off step of installing a suitable tool. At the time of this writing, two rival tools provide the necessary functionality:gonow ( kless/gonow), and gorun ( Once gonow or gorun is available, we can make any Go program into a shebang script. This is done with two simple steps. First, add either #!/usr/bin/env gonow or #!/usr/bin/env gorun, as the very first line of the .go file that contains the main() function (in package main). Second, make the file executable (e.g., with chmod +x). Such files can only be compiled by gonow or gorun rather than in the normal way since the #! line is not legal in Go. When gonow or gorun executes a .go file for the first time, it will compile the file (extremely fast, of course), and then run it. On subsequent uses, the program will only be recompiled if the .go source file has been modified since the previous compilation. This makes it possible to use Go to quickly and conveniently create various small utility programs, for example, for system administration tasks. indentation. If your editor doesn’t have Go support, try entering the editor’s name in the Go search engine to see if there are suitable add-ons. For editing convenience, all of Go’s keywords and operators use ASCII characters; however, Go identifiers can start with any Unicode letter followed by any Unicode letters or digits, so Go programmers can freely use their native language. To get a feel for how we edit, compile, and run a Go program we’ll start with the classic “Hello World” program—although we’ll make it a tiny bit more sophisticated than usual. First we will discuss compiling and running, then in the next section we will go through the source code—in file hello/hello.go—in detail, since it incorporates some basic Go ideas and features. All of the book’s examples are available from and unpack to directory goeg. So file hello.go’s full path (assuming the exam- ples were unpacked in the home directory—although anywhere will do) is $HOME/goeg/src/hello/hello.go. When referring to files the book always assumes the first three components of the path, which is why in this case the path is giv- en only as hello/hello.go.(Windows users must, of course, read “/”s as “\”s and use the directory they unpacked the examples into, such as C:\goeg or %HOME- PATH%\goeg.) If you have installed Go from a binary package or built it from source and in- stalled it as root or Administrator, you should have at least one environment variable, GOROOT, which contains the path to the Go installation, and your PATH should now include $GOROOT/bin or %GOROOT%\bin. To check that Go is installed ptg7913109 1.2. Editing, Compiling,and Running 11 correctly, enter the following in a console (xterm, gnome-terminal, konsole, Termi-, or similar): $ go version Or on Windows in an MS-DOS Prompt or Command Prompt window: C:\>go version If you get a “command not found” or “‘go’ is not recognized…” error message then it means that Go isn’t in the PATH. The easiest way to solve this on Unix-like systems (including Mac OS X) is to set the environment variables in .bashrc (or the equivalent file for other shells). For example, the author’s .bashrc file contains these lines: export GOROOT=$HOME/opt/go export PATH=$PATH:$GOROOT/bin Naturally, you must adjust the values to match your own system. (And, of course, this is only necessary if the go version command fails.) On Windows, one solution is to create a batch file that sets up the environment for Go, and to execute this every time you start a console for Go programming. However, it is much more convenient to set the environment variables once and for all through the Control Panel. To do this, click Start (the Windows logo), then Control Panel, then System and Security, then System, then Advanced system settings, and in the System Properties dialog click the Environment Variables button, then the New… button, and add a variable with the name GOROOT and a suitable value, such as C:\Go. In the same dialog, edit the PATH environment variable by adding the text ;C:\Go\bin at the end—the leading semicolon is vital! In both cases replace the C:\Go path component with the actual path where Go is installed if it isn’t C:\Go.(Again, this is only necessary if the go version command failed.) From now on we will assume that Go is installed and the Go bin directory containing all the Go tools is in the PATH.(It may be necessary—once only—to open a new console window for the new settings to take effect.) Two steps are required to build Go programs: compiling and linking.★ Both of these steps are handled by the go tool which can not only build local programs and packages, but can also fetch, build, and install third-party programs and packages. ★ Since the book assumes the use of the gc compiler, readers using gccgo will need to follow the compile and link process described in Similarly, readers using other compilers will need to compile and link as per their compiler’s instructions. ptg7913109 12 Chapter 1. An Overview in Five Examples For the go tool to be able to build local programs and packages, there are three requirements. First, the Go bin directory ($GOROOT/bin or %GOROOT%\bin) must be in the path. Second, there must be a directory tree that has an src directory and under which the source code for the local programs and packages resides. For example, the book’s examples unpack to goeg/src/hello, goeg/src/bigdigits, and so on. Third, the directory above the src directory must be in the GOPATH environment variable. For example, to build the book’s hello example using the go tool, we must do this: $ export GOPATH=$HOME/goeg $ cd $GOPATH/src/hello $ go build We can do almost exactly the same on Windows: C:\>set GOPATH=C:\goeg C:\>cd %gopath%\src\hello C:\goeg\src\hello>go build In both cases we assume that the PATH includes $GOROOT/bin or %GOROOT%\bin.Once the go tool has built the program we can run it. By default the executable is given the same name as the directory it is in (e.g., hello on Unix-like systemsand hello.exe on Windows). Once built, we can run the program in the usual way. $ ./hello Hello World! Or: $ ./hello Go Programmers! Hello Go Programmers! On Windows it is very similar: C:\goeg\src\hello>hello Windows Go Programmers! Hello Windows Go Programmers! We have shown what must be typed in bold and the console’s text in roman.We have also assumed a $ prompt, but it doesn’t matter what it is (e.g., C:\>). Note that we do not need to compile—or even explicitly link—any other pack- ages (even though as we will see, hello.go uses three standard library packages). This is another reason why Go programs build so quickly. ptg7913109 1.2. Editing, Compiling,and Running 13 If we have several Go programs, it would be convenient if all their executables could be in a single directory that we could add to our PATH. Fortunately, the go tool supports this as follows: $ export GOPATH=$HOME/goeg $ cd $GOPATH/src/hello $ go install Again, we can do the same on Windows: C:\>set GOPATH=C:\goeg C:\>cd %gopath%\src\hello C:\goeg\src\hello>go install The go install command does the same as go build only it puts the executable in a standard location ($GOPATH/bin or %GOPATH%\bin). This means that by adding a single path ($GOPATH/bin or %GOPATH%\bin) to our PATH, all the Go programs that we install will conveniently be in the PATH. In addition to the book’s examples, we are likely to want to develop our own Go programs and packages in our own directory. This can easily be accom- modated by setting the GOPATH environment variable to two (or more) colon- separated paths (semicolon-separated on Windows); for example, export GOPATH=$HOME/app/go:$HOME/goeg or SET GOPATH=C:\app\go;C:\goeg.★ In this case we must put all our program and package’s source code in $HOME/app/go/src or C:\app\go\src. So, if we develop a program called myapp, its .go source files would go in $HOME/app/go/src/myapp or C:\app\go\src\myapp. And if we use go install to build a program in a GOPATH directory where the GOPATH has two or more directo- ries, the executable will be put in the corresponding directory’s bin directory. Naturally, it would be tedious to export or set the GOPATH every time we wanted to build a Go program,so it is best to set this environment variable permanently. This can be done by setting GOPATH in the .bashrc file (or similar) on Unix-like systems (see the book’s example’s file). On Windows it can be done either by writing a batch file (see the book’s example’s gopath.bat file), or by adding it to the system’s environment variables: Click Start (the Windows logo), then Control Panel, then System and Security, then System, then Advanced system settings, and in the System Properties dialog click the Environment Variables button, then the New… button, and add a variable with the name GOPATH and a suitable value, such as C:\goeg or C:\app\go;C:\goeg. Although Go uses the go tool as its standard build tool, it is perfectly possible to use make or some of the modern build tools,or to use alternative Go-specific build ★ From now on we will almost always show Unix-style command lines only, and assume that Windows programmers can mentally translate. ptg7913109 14 Chapter 1. An Overview in Five Examples tools, or add-ons for popular IDEs (Integrated Development Environments) such as Eclipse and Visual Studio. 1.3. Hello Who? Now that we have seen how to build the hello program we will look at its source code. Don’t worry about understanding all the details—everything shown in this chapter (and much more!) is covered thoroughly in the subsequent chapters. Here is the complete hello program (in file hello/hello.go): // hello.go package main import ( ➊ "fmt" "os" "strings" ) func main() { who := "World!" ➋ if len(os.Args) > 1 {/* os.Args[0] is "hello" or "hello.exe" */ ➌ who = strings.Join(os.Args[1:], "") ➍ } fmt.Println("Hello", who) ➎ } Go uses C++-style comments: // for single-line comments that finish at the end of the line and /*…*/ for comments that can span multiple lines. It is conven- tional in Go to mostly use single-line comments, with spanning comments often used for commenting out chunks of code during development.★ Every piece of Go code exists inside a package,and every Go program must have a main package with a main() function which serves as the program’s entry point, that is, the function that is executed first. In fact, Go packages may also have init() functions that are executed before main(), as we will see (§1.7, ➤ 40); full details are given later (§5.6.2, ➤ 224). Notice that there is no conflict between the name of the package and the name of the function. Go operates in terms of packages rather than files. This means that we can split a package across as many files as we like, and from Go’s point of view if they all have the same package declaration, they are all part of the same package and no different than if all their contents were in a single file. Naturally,we can also ★ We use some simple syntax highlighting and sometimes highlight lines or annotate them with numbers (➊, ➋,…), for ease of reference in the text. None of this is part of the Go language. ptg7913109 1.3. Hello Who? 15 break our applications’ functionality into as many local packages as we like, to keep everything neatly modularized, something we will see in Chapter 9. The import statement (14 ➤ , ➊) imports three packages from the standard li- brary. The fmt package provides functions for formatting text and for read- ing formatted text (§3.5, ➤ 93), the os package provides platform-independent operating-system variables and functions, and the strings package provides functions for manipulating strings (§3.6.1, ➤ 107). Go’s fundamental types support the usual operators (e.g., + for numeric addition and for string concatenation),and the Go standard library supplementsthese by providing packages of functions for working with the fundamental types, such asthe strings packageimportedhere. It isalso possibleto createour own custom types based on the fundamental types and to provide our own methods—that is, custom type-specific functions—for them. (We will get a taste of this in §1.5, ➤ 21, with full coverage in Chapter 6.) The reader may have noticed that the program has no semicolons, that the im- ports are not comma-separated, and that the if statement’s condition does not require parentheses. In Go, blocks, including function bodies and control struc- ture bodies (e.g., for if statements and for for loops), are delimited using braces. Indentation is used purely to improve human readability. Technically, Go state- ments are separated by semicolons, but these are put in by the compiler, so we don’t have to use them ourselves unless we want to put multiple statements on the same line. No semicolons and fewer commas and parentheses give Go pro- grams a lighter look and require less typing. Go functions and methods are defined using the func keyword. The main pack- age’s main() function always has the same signature—it takes no arguments and returns nothing. When main.main() finishes the program will terminate and return 0 to the operating system. Naturally, we can exit whenever we like and return our own choice of value, as we will see (§1.4, ➤ 16). The first statement in the main() function (14 ➤ , ➋; using the := operator) is called a short variable declaration in Go terminology. Such a statement both declares and initializes a variable at the same time. Furthermore,we don’t need to specify the variable’s type because Go can deduce that from the initializing value. So in this case we have declared a variable called who of type string, and thanks to Go’s strong typing we may only assign strings to who. As with most languages the if statement tests a condition—in this case, how many strings were entered on the command-line—which if satisfied executes the corresponding brace-delimited block. We will see a more sophisticated if statement syntax later in this chapter (§1.6, ➤ 29), and further on (§5.2.1, ➤ 192). The os.Args variable is a slice of strings (14 ➤ , ➌). Arrays, slices, and other col- lection data types are covered in Chapter 4 (§4.2, ➤ 148). For now it is sufficient ptg7913109 16 Chapter 1. An Overview in Five Examples to know that a slice’s length can be determined using the built-in len() function and its elements can be accessed using the [] index operator using a subset of the Python syntax. In particular,slice[n] returns the slice’s nth element (count- ing from zero), and slice[n:] returns another slice which has the elements from the nth element to the last element. In the collections chapter we will see the full generality of Go’s syntax in this area. In the case of os.Args, the slice should always have at least one string (the program’s name), at index position 0.(All Go indexing is 0-based.) If the user has entered one or more command line arguments the if condition is satisfied and we set the who string to contain all the arguments joined up as a single string (14 ➤ , ➍). In this case we use the assignment operator (=), since if we used the short variable declaration operator (:=) we would end up declaring and initializing a new who variable whose scope was limited to the if statement’s block. The strings.Join() function takes a slice of strings and a separator (which could be empty, i.e., ""), and returns a single string consisting of all the slice’s strings with the separator between each one. Here we have joined them using a single space between each. Finally, in the last statement (14 ➤ , ➎), we print Hello, a space, the string held in the who variable, and a newline. The fmt package has many different print variants, some like fmt.Println() which will neatly print whatever they are given, and others like fmt.Printf() that use placeholders to provide very fine control over formatting. The print functions are covered in Chapter 3 (§3.5, ➤ 93). The hello program presented here hasshown far more of the language’sfeatures than such programs conventionally do. The subsequent examples continue in this vein, covering more advanced features while keeping the examples as short as possible. The idea here is to simply acquire some basic familiarity with the language and to get to grips with building, running, and experimenting with simple Go programs,while at the same time getting a flavor of Go’s powerful and novel features. And,of course,everything presented in this chapter is explained in detail in the subsequent chapters. 1.4. Big Digits—Two-Dimensional Slices The bigdigits program (in file bigdigits/bigdigits.go) reads a number entered on the command line (as a string), and outputs the same number onto the console using “big” digits. Back in the twentieth century, at sites where lots of users shared a high-speed line printer, it used to be common practice for each user’s print job to be preceded by a cover page that showed some identifying details such as their username and the name of the file being printed,using this kind of technique. ptg7913109 1.4. Big Digits—Two-Dimensional Slices 17 We will review the code in three parts: first the imports,then the static data,and then the processing. But right now, let’s look at a sample run to get a feel for how it works: $ ./bigdigits 290175493 222 9999 000 1 77777 55555 4 9999 333 2 2 9 9 0 0 11 7 5 44 9 9 3 3 2 9 9 0 0 1 7 5 4 4 9 9 3 2 9999 0 0 1 7 555 4 4 9999 33 2 9 0 0 1 7 5 444444 9 3 2 9 0 0 1 7 5 5 4 9 3 3 22222 9 000 111 7 555 4 9 333 Each digit is represented by a slice of strings, with all the digits together repre- sented by a slice of slices of strings. Before looking at the data, here is how we could declare and initialize single-dimensional slices of strings and numbers: longWeekend := []string{"Friday","Saturday","Sunday","Monday"} var lowPrimes = []int{2, 3, 5, 7, 11, 13, 17, 19} Slices have the form []Type, and if we want to initialize them we can immedi- ately follow with a brace-delimited comma-separated list of elements of the cor- responding type. We could have used the same variable declaration syntax for both, but have used a longer form for the lowPrimes slice to show the syntactic difference and for a reason that will be explained in a moment. Since a slice’s Type can itself be a slice type we can easily create multidimensional collections (slices of slices, etc.). The bigdigits program needs to import only four packages. import ( "fmt" "log" "os" "path/filepath" ) The fmt package provides functions for formatting text and for reading format- ted text (§3.5, ➤ 93). The log package provides logging functions. The os pack- age provides platform-independent operating-system variables and functions including the os.Args variable of type []string (slice of strings) that holds the command-line arguments. And the path package’s filepath package provides functions for manipulating filenames and paths that work across platforms. Note that for packages that are logically inside other packages, we only specify the last component of their name (in this case filepath) when accessing them in our code. ptg7913109 18 Chapter 1. An Overview in Five Examples For the bigdigits program we need two-dimensional data (a slice of slices of strings). Here is how we have created it, with the strings for digit 0 laid out to illustrate how a digit’s strings correspond to rows in the output, and with the strings for digits 3 to 8 elided. var bigDigits = [][]string{ {" 000 ", " 0 0 ", "0 0", "0 0", "0 0", " 0 0 ", " 000 "}, {"1","11 ","1","1","1","1","111"}, {" 222 ","2 2"," 2 "," 2 "," 2 ","2 ","22222"}, // ... 3 to 8 ... {" 9999","9 9","9 9"," 9999"," 9"," 9"," 9"}, } Variables declared outside of any function or method may not use the := oper- ator, but we can get the same effect using the long declaration form (with key- word var) and the assignment operator (=) as we have done here for the bigDigits variable (and did earlier for the lowPrimes variable).We still don’t need to specify bigDigits’ type since Go can deduce that from the assignment. We leave the bean counting to the Go compiler, so there is no need to specify the dimensions of the slice of slices. One of Go’s many conveniences is its excellent support for composite literals using braces, so we don’t have to declare a data variable in one place and populate it with data in another—unless we want to, of course. The main() function that reads the command line and uses the data to produce the output is only 20 lines. func main() { if len(os.Args) == 1 { ➊ fmt.Printf("usage: %s \n", filepath.Base(os.Args[0])) os.Exit(1) } stringOfDigits := os.Args[1] for row := range bigDigits[0]{ ➋ line := "" for column := range stringOfDigits { ➌ digit := stringOfDigits[column] -'0' ➍ if 0 <= digit && digit <= 9 { ➎ ptg7913109 1.4. Big Digits—Two-Dimensional Slices 19 line += bigDigits[digit][row] + " " ➏ } else { log.Fatal("invalid whole number") } } fmt.Println(line) } } The program begins by checking to see if it was invoked with any command-line arguments. If it wasn’t, len(os.Args) will be 1 (recall that os.Args[0] holds the program’s name, so the slice’s length is normally at least 1), and the first if statement (18 ➤ , ➊) will be satisfied. In this case we output a suitable usage message using the fmt.Printf() function that accepts % placeholders similar to those supported by the C/C++ printf() function or by Python’s % operator. (See §3.5, ➤ 93 for full details.) The path/filepath package provides path manipulation functions—for example, the filepath.Base() function returns the basename (i.e., the filename) of the given path. After outputting the message the program terminates using the os.Exit() function and returns 1 to the operating system. On Unix-like systems a return value of 0 is used to indicate success, with nonzero values indicating a usage error or a failure. The use of the filepath.Base() function illustrates a nice feature of Go: When a package is imported,no matter whether it is top-level or logically inside another package (e.g., path/filepath), we always refer to it using only the last component of its name (e.g., filepath). It is also possible to give packages local names to avoid name collisions; Chapter 9 provides the details. If at least one command-line argument was given, we copy the first one into the stringOfDigits variable (of type string). To convert the number that the user entered into big digits we must iterate over each row in the bigDigits slice to produce each line of output, that is, the first (top) string for each digit, then the second, and so on. We assume that all the bigDigits’ slices have the same number of rows and so take the row count from the first one. Go’s for loop has various syntaxes for different purposes;here (18 ➤ , ➋ and 18 ➤ , ➌) we have used for … range loops that return the index positions of each item in the slices they are given. The row and column loops part of the code could have been written like this: for row := 0; row < len(bigDigits[0]); row++ { line := "" for column := 0; column < len(stringOfDigits); column++ { ... ptg7913109 20 Chapter 1. An Overview in Five Examples This is a form familiar to C, C++, and Java programmers and is perfectly valid in Go.★ However, the for … range syntax is shorter and more convenient. (Go’s for loops are covered in §5.3, ➤ 203.) At each row iteration we set that row’s line to be an empty string. Then we iterate over the columns (i.e., the characters) in the stringOfDigits string we received from the user. Go strings hold UTF-8 bytes, so potentially a character might be represented by two or more bytes. This isn’t an issue here because we are only concerned with the digits 0, 1,…,9 each of which is represented by a single byte in UTF-8 and with the same byte value as in 7-bit ASCII. (We will see how to iterate over a string character by character—regardless of whether the characters are single- or multibyte—in Chapter 3.) When we index a particular position in a string we get the byte value at that position. (In Go the byte type is a synonym for the uint8 type.) So we retrieve the byte value of the command-line string at the given column and subtract the byte value of digit 0 from it to get the number it represents (18 ➤ , ➍). In UTF-8 (and 7-bit ASCII) the character '0' is code point (character) 48 decimal, the character '1' is code point 49, and so on. So if, for example,we have the character '3' (code point 51), we can get its integer value by doing the subtraction '3' -'0' (i.e., 51 − 48) which results in an integer (of type byte) of value 3. Go uses single quotes for character literals, and a character literal is an integer that’s compatible with any of Go’s integer types. Go’s strong typing means we cannot add, say, an int32 to an int16 without explicit conversion, but Go’s numeric constants and literals adapt to their context, so in this context '0' is considered to be a byte. If the digit (of type byte) is in range (18 ➤ , ➎) we can add the appropriate string to the line.(In the if statement the constants 0 and 9 are considered to be bytes because that’s digit’s type, but if digit was of a different type, say, int, they would be treated as that type instead.) Although Go strings are immutable (i.e., they cannot be changed), the += append operator is supported to provide a nice easy-to-use syntax. (It works by replacing the original string under the hood.) There is also support for the + concatenate operator which returns a new string that is the concatenation of its left and right string operands. (The string type is covered fully in Chapter 3.) To retrieve the appropriate string (19 ➤ , ➏) we access the bigDigits’s slice that corresponds to the digit, and then within that to the row (string) we need. If the digit is out of range (e.g., due to the stringOfDigits containing a nondigit), we call the log.Fatal() function with an error message. This function logs the ★ Unlike C, C++, and Java, in Go the ++ and -- operators may only be used as statements, not expressions. Furthermore, they may only be used as postfix operators, not prefix operators. This means that certain order of evaluation problems cannot occur in Go—so thankfully,expressions like f(i++) and a[i] = b[++i] cannot be written in Go. ptg7913109 1.4. Big Digits—Two-Dimensional Slices 21 date, time, and error message—to os.Stderr if no other log destination is explic- itly specified—and calls os.Exit(1) to terminate the program. There is also a log.Fatalf() function that does the same thing and which accepts % placehold- ers. We didn’t use log.Fatal() in the first if statement (18 ➤ , ➊) because we want to print the program’s usage message without the date and time that the log.Fatal() function normally outputs. Once all the number’s strings for the given row have been accumulated the complete line is printed. In this example, seven lines are printed because each digit in the bigDigits slice of strings is represented by seven strings. One final point is that the order of declarationsand definitions doesn’t generally matter. So in the bigdigits/bigdigits.go file we could declare the bigDigits variable before or after the main() function. In this case we have put main() first since for the book’s examples we usually prefer to order things top-down. The first two examples have covered a fair amount of ground, but both of them show material that is familiar from other mainstream languages even though the syntax is slightly different. The following three examples take us beyond the comfort zone to illustrate Go-specific features such as custom Go types, Go file handling (including error handling) and functions as values, and concurrent programming using goroutines and communication channels. 1.5. Stack—Custom Types with Methods Although Go supports object-oriented programming it provides neither class- es nor inheritance (is-a relationships). Go does support the creation of custom types, and Go makes aggregation (has-a relationships) extremely easy. Go also allows for the complete separation of a type’s data from its behavior, and sup- ports duck typing.Duck typing is a powerful abstraction mechanism that means that values can be handled (e.g., passed to functions),based on the methods they provide, regardless of their actual types. The terminology is derived from the phrase, “If it walks like a duck, and quacks like a duck, it is a duck”. All of this produces a more flexible and powerful alternative to the classes and inheritance approach—but does require those of us used to the more traditional approach to make some significant conceptual adjustments to really benefit from Go’s object orientation. Go representsdata using the fundamental built-in typessuch as keyword!!struct bool, int, and string, or by aggregations of types using structs.★ Go’s custom types are based on the fundamental types, or on structs, or on other custom types. (We will see some simple examples later in this chapter; §1.7, ➤ 40.) ★ Unlike C++, Go’s structs are not classes in disguise. For example, Go’s structs support aggregation and delegation, but not inheritance. ptg7913109 22 Chapter 1. An Overview in Five Examples Go supports both named and unnamed custom types. Unnamed types with the same structure can be used interchangeably; however, they cannot have any methods. (We will discuss this more fully in §6.4, ➤ 275.) Any named cus- tom type can have methods and these methods together constitute the type’s interface. Named custom types—even with the same structure—are not inter- changeable. (Throughout the book any reference to a “custom type” means a named custom type, unless stated otherwise.) An interface is a type that can be formally defined by specifying a particular set of methods. Interfaces are abstract and cannot be instantiated. A concrete (i.e., noninterface) type that has the methods specified by an interface fulfills the interface, that is, values of such a concrete type can be used as values of the interface’s type as well as of their own actual type. Yet no formal connection need be established between an interface and a concrete type that provides the methods specified by the interface. It is sufficient for a custom type to have the interface’s methods for it to satisfy that interface. And, of course, a type can satisfy more than one interface simply by providing all the methods for all the interfaces we want it to satisfy. The empty interface (i.e., the interface that has no methods) is specified as interface{}.★ Since the empty interface makes no demands at all (because it doesn’t require any methods), it can stand for any value (in effect like a pointer to any value), whether the value is of a built-in type or is of a custom type. (Go’s pointers and references are explained later; §4.1, ➤ 140.) Incidentally, in Go terminology we talk about types and values rather than classes and objects or instances (since Go has no classes). Function and method parameters can be of any built-in or custom type—or of any interface type. In the latter case this means that a function can have a parameter that says, for example, “pass a value that can read data”, regardless of what that value’s type actually is. (We will see this in practice shortly; §1.6, ➤ 29.) Chapter 6 covers all of these matters in detail and presents many examples to ensure that the ideas are understood. For now, let’s just look at a very simple custom type—a stack—starting with how values are created and used, and then looking at the implementation of the custom type itself. We will start with the output produced by a simple test program: $ ./stacker 81.52 [pin clip needle] -15 hay ★ Go’s empty interface can serve the same role as a reference to a Java Object or as C/C++’s void*. ptg7913109 1.5. Stack—Custom Types with Methods 23 Each item was popped from the custom stack and printed on its own line. The simple test program that produced this output is stacker/stacker.go. Here are the imports it uses: import ( "fmt" "stacker/stack" ) The fmt package is part of Go’s standard library, but the stack package is a local package specific to the stacker application. A Go program or package’s imports are first searched for under the GOPATH path or paths, and then under GOROOT. In this particular case the program’s source code is in $HOME/goeg/src/stacker/ stacker.go and the stack package is in $HOME/goeg/src/stacker/stack/stack.go. The go tool will build both of them so long as the GOPATH is (or includes) the path $HOME/goeg/. Import paths are specified using Unix-style “/”s, even on Windows. Every local package should be stored in a directory with the same name as the package. Local packages can have their own packages (e.g., like path/filepath), in exactly the same way as the standard library. (Creating and using custom packages is covered in Chapter 9.) Here’s the simple test program’s main() function that produced the output: func main() { var haystack stack.Stack haystack.Push("hay") haystack.Push(-15) haystack.Push([]string{"pin","clip","needle"}) haystack.Push(81.52) for { item, err := haystack.Pop() if err != nil { break } fmt.Println(item) } } The function begins by declaring the haystack variable of type stack.Stack.Itis conventional in Go to always refer to types, functions, variables, and other items in packages using the syntax pkg.item, where pkg is the last (or only) component of the package’s name. This helps prevent name collisions. We then push some items onto the stack and then pop them off and print each one until there are no more left. ptg7913109 24 Chapter 1. An Overview in Five Examples One amazingly convenient aspect of our custom stack is that despite Go’s strong typing, we are not limited to storing homogeneous items (items all of the same type), but can freely mix heterogeneous items (items of various types). This is because the stack.Stack type simply stores interface{} items (i.e., values of any type) and doesn’t care what their types actually are. Of course, when those items are used,then their type does matter. Here, though, we only use the fmt.Println() function and this uses Go’s introspection facilities (from the re- flect package) to discover the types of the items it is asked to print. (Reflection is covered in a later chapter; §9.4.9, ➤ 427.) Another nice Go feature illustrated by the code is the for loop with no conditions. This is an infinite loop, so in most situations we will need to provide a means of breaking out of the loop—for example, using a break statement as here, or a return statement. We will see an additional for syntax in the next example (§1.6, ➤ 29); the complete range of for syntaxes is covered in Chapter 5. Go functions and methods can return a single value or multiple values. It is conventional in Go to report errors by returning an error value (of type error) as the last (or only) value returned by a function or method. The custom stack. Stack type respects this convention. Now that we have seen the custom stack.Stack type in use we are ready to review its implementation (in file stacker/stack/stack.go). package stack import "errors" type Stack []interface{} The file starts conventionally by specifying its package name. Then it imports other packages that it needs—in this case just one, errors. When we define a named custom type in Go what we are doing is binding an identifier (the type’s name) to a new type that has the same underlying rep- resentation as an existing (built-in or custom) type—and which is treated by Go as different from the underlying representation. Here, the Stack type is a new name for a slice (i.e., a reference to a variable-length array) of interface{} values—and is considered to be different from a plain []interface{}. Because all Go types satisfy the empty interface, values of any type can be stored in a Stack. The built-in collection types (maps and slices), communication channels (which can be buffered), and strings, can all return their length (or buffer size) using the built-in len() function. Similarly, slices and channels can also report their capacity (which may be greater than the length being used) using the built-in cap() function. (All of Go’s built-in functions are listed in Table 5.1, ➤ 187, with cross-references to where they are covered; slices are covered in Chapter 4; §4.2, ptg7913109 1.5. Stack—Custom Types with Methods 25 ➤ 148.) It is conventional for custom collection types—our own, and those in the Go standard library—to support corresponding Len() and Cap() methods when these make sense. Since the Stack type uses a slice for its underlying representation it makes sense to provide Stack.Len() and Stack.Cap() methods for it. func (stack Stack) Len() int { return len(stack) } Both functions and methods are defined using the func keyword.However,in the case of methods the type of value to which the method applies is written after the func keyword and before the method’s name, enclosed in parentheses. After the function or method’s name comes a—possibly empty—parenthesized list of comma-separated parameters(each written in the form variableName type).After the parameterscomes the function or method’s opening brace (if it has no return value), or a single return value (e.g., as a type name such as the int returned by the Stack.Len() method shown here), or a parenthesized list of return values, followed by an opening brace. In most cases a variable name for the value on which the method is called is also given—as here where we have used the name stack (and with no conflict with the package’s name). The value on which the method is called is known in Go terminology as the receiver.★ In this example the type of the receiver is Stack, so the receiver is passed by value. This means that any changes made to the receiver would be made on a copy of the original value and in effect lost. This is no problem for methods that don’t modify the receiver, such as the Stack.Len() method shown here. The Stack.Cap() method is almost identical to the Stack.Len() method (and so is not shown). The only difference is that the Stack.Cap() method returns the cap() rather than the len() of the receiver stack. The source code also includes a Stack.IsEmpty() method, but this is so similar to Stack.Len()—it just returns a bool indicating whether the stack’s len() equals 0—that again it isn’t shown. func (stack *Stack) Push(x interface{}) { *stack = append(*stack, x) } The Stack.Push() method is called on a pointer to a Stack (explained in a mo- ment), and is passed a value (x) of any type. The built-in append() function takes a slice and one or more values and returns a (possibly new) slice which has the ★ In other languages the receiver is typically called this or self; using such names works fine in Go, but is not considered to be good Go style. ptg7913109 26 Chapter 1. An Overview in Five Examples original slice’s contents, plus the given value or values as its last element or ele- ments. (See §4.2.3, ➤ 156.) If the stack has previously had items popped from it (➤ 28), the underlying slice’s capacity is likely to be greater than its length, so the push could be very cheap: simply a matter of putting the x item into the len(stack) position and increasing the stack’s length by one. The Stack.Push() method always works (unless the computer runs out of memo- ry), so we don’t need to return an error value to indicate success or failure. If we want to modify a value we must make its receiver a pointer.★ A pointer is a variable that holds the memory address of another value. One reason that pointers are used is for efficiency—for example,if we have a value of a large type it is much cheaper to pass a pointer to the value as a parameter than to pass the value itself. Another use is to make a value modifiable. For example, when a variable is passed into a function the function gets a copy of the value (e.g., the stack passed into the stack.Len() function; 25 ➤ ). This means that if we make any changes to the variable inside the function, they will have no effect on the original value. If we need to modify the original value—as here where we want to append to the stack—we must pass a pointer to the original value, and then inside the function we can modify the value that the pointer points to. A pointer is declared by preceding the type name with a star (i.e., an asterisk, *). So here, in the Stack.Push() method, the stack variable is of type *Stack, that is, the stack variable holds a pointer to a Stack value and not an actual Stack value. We can access the actual Stack value that the pointer points to by dereferencing the pointer—this simply means that we access the value the pointer points to. Dereferencing is done by preceding the variable name with a star. So here,when we write stack we are referring to a pointer to a Stack (i.e., to a *Stack), and when we write *stack we are dereferencing the pointer, that is, referring to the actual Stack that the pointer points to. So, in Go (and C and C++ for that matter), the star is overloaded to mean multi- plication (when between a pair of numbers or variables, e.g., x * y), pointer dec- laration (when preceding a type name, e.g., z *MyType), and pointer dereference (when preceding a pointer variable’s name, e.g., *z). Don’t worry too much about these matters for now: Go’s pointers are fully explained in Chapter 4. Note that Go’s channels, maps, and slices are all created using the make() func- tion, and make() always returns a reference to the value it created. References behave very much like pointers in that when they are passed to functions any changes made to them inside the function affect the original channel, map, or slice. However, references don’t need to be dereferenced,so in most cases there’s no need to use stars with them. But if we want to modify a slice inside a func- ★ Go pointers are essentially the same as in C and C++except that pointer arithmetic isn’t supported —or necessary; see §4.1, ➤ 140. ptg7913109 1.5. Stack—Custom Types with Methods 27 tion or method using append() (as opposed to simply changing one of its existing items), then we must either pass the slice by pointer, or return the slice (and set the original slice to the function or method’s return value), since append() some- times returns a different slice reference than the one it was passed. The Stack type uses a slice for its representation and therefore Stack values can be used with functions that operate on a slice, such as append() and len(). Nonetheless, Stack values are values in their own right, distinct from their representation, so they must be passed by pointer if we want to modify them. func (stack Stack) Top() (interface{}, error){ if len(stack) == 0 { return nil, errors.New("can't Top() an empty stack") } return stack[len(stack)-1], nil } The Stack.Top() method returns the item at the top of the stack (the item that was added last) and a nil error value; or a nil item and a non-nil error value, if the stack is empty. The stack receiver is passed by value since the stack isn’t modified. The error type is an interface type (§6.3, ➤ 265) which specifies a single method, Error() string. In general, Go’s library functions return an error as their last (or only) return value to indicate success (where error is nil) or failure. Here, we have made our Stack type work like a standard library type by creating a new error value using the errors package’s errors.New() function. Go uses nil for zero pointers (and for zero references); that is, for pointers that point to nothing and for references that refer to nothing.★ Such pointers should be used only in conditions or assignments; methods should not normally be called on them. Constructorsare never called implicitly in Go. Instead Go guarantees that when a value is created it is always initialized to its zero value. For example, numbers are initialized to 0, strings to the empty string, pointers to nil, and the fields inside structs are similarly initialized. So there is no uninitialized data in Go, thus eliminating a major source of errors that afflicts many other program- ming languages. If the zero value isn’t suitable we can write a construction function—and call it explicitly—as we do here to create a new error. It is also possible to prevent values of a type being created without using a constructor function, as we will see in Chapter 6. ★ Go’s nil is in effect the same as NULL or 0 in C and C++, null in Java, and nil in Objective-C. ptg7913109 28 Chapter 1. An Overview in Five Examples If the stack is nonempty we return its topmost value and a nil error value. Since Go uses 0-based indexing the first element in a slice or array is at position 0 and the last element is at position len(sliceOrArray)-1. There is no formality when returning more than one value from a function or method; we simply list the types we are returning after the function or method’s name and ensure that we have at least one return statement that has a corre- sponding list of values. func (stack *Stack) Pop() (interface{}, error){ theStack := *stack if len(theStack) == 0 { return nil, errors.New("can't Pop() an empty stack") } x := theStack[len(theStack)-1] ➊ *stack = theStack[:len(theStack)-1] ➋ return x, nil } The Stack.Pop() method is used to remove and return the top (last added) item from the stack. Like the Stack.Top() method it returns the item and a nil error, or if the stack is empty, a nil item and a non-nil error. The method must have a receiver that is a pointer since it modifies the stack by removing the returned item. For syntactic convenience, rather than referring to *stack (the actual stack that the stack variable points to) throughout the method, we assign the actual stack to a local variable (theStack), and work with that variable instead. This is quite cheap, because *stack is pointing to a Stack, which uses a slice for its representation, so we are really assigning little more than a reference to a slice. If the stack is empty we return a suitable error. Otherwise we retrieve the stack’s top (last) item and store it in a local variable (x). Then we take a slice of the stack (which itself is a slice). The new slice has one less element than the original and is immediately set to be the value that the stack pointer points to. And at the end,we return the retrieved value and a nil error. We can reasonably expect any decent Go compiler to reuse the slice, simply reducing the slice’s length by one, while leaving its capacity unchanged, rather than copying all the data to a new slice. The item to return is retrieved using the [] index operator with a single index (➊); in this case the index of the slice’s last element. The new slice is obtained by using the [] slice operator with an index range (➋). An index range has the form first:end. If first is omitted—as here—0 is assumed, and if end is omitted, the len() of the slice is assumed. The slice thus obtained has elements with indexes from and including the first up to and ptg7913109 1.5. Stack—Custom Types with Methods 29 excluding the end. So in this case, by specifying the last index as one less than the length, we slice up to the last but one element, effectively removing the last element from the slice. (Slice indexing is covered in Chapter 4, §4.2.1, ➤ 153.) In this example we used Stack receivers rather than pointers (i.e., of type *Stack) for those methods that don’t modify the Stack.For custom types with lightweight representations (say, a few intsorstrings), this is perfectly reasonable. But for heavyweight custom types it is usually best to always use pointer receivers since a pointer is much cheaper to pass (typically a simple 32- or 64-bit value), than a large value, even for methods where the value isn’t modified. A subtle point to note regarding pointers and methods is that if we call a method on a value, and the method requires a pointer to the value it is called on, Go is smart enough to pass the value’s address rather than a copy of the value (providing the value is addressable; §6.2.1, ➤ 258). Correspondingly, if we call a method on a pointer to a value, and the method requires a value, Go is smart enough to dereference the pointer and give the method the pointed-to value.★ As this example illustrates, creating custom types in Go is generally straight- forward, and doesn’t involve the cumbersome formalities that many other lan- guages demand. Go’s object-oriented features are covered fully in Chapter 6. 1.6. Americanise—Files, Maps, and Closures To have any practical use a programming language must provide some means of reading and writing external data. In previous sections we had a glimpse of Go’s versatile and powerful print functions from its fmt package; in this section we will look at Go’s basic file handling facilities. We will also look at some more advanced featuressuch as Go’streatment of functionsand methodsas first-class values which makes it possible to pass them as parameters. And in addition we will make use of Go’s map type (also known as a data dictionary or hash). This section provides enough of the basics so that programs that read and write text files can be written—thus making the examples and exercises more inter- esting. Chapter 8 provides much more coverage of Go’s file handling facilities. By about the middle of the twentieth century, American English surpassed British English as the most widely used form of English. In this section’s example we will review a program that reads a text file and writes out a copy of the file into a new file with any words using British spellings replaced with their U.S. counterparts. (This doesn’t help with differences in semantics or idioms, of course.) The program is in the file americanise/americanise.go, and we will review it top-down, starting with its imports, then its main() function, then the functions that main() calls, and so on. ★ This is why Go does not have or need the -> indirection operator used by C and C++. ptg7913109 30 Chapter 1. An Overview in Five Examples import ( "bufio" "fmt" "io" "io/ioutil" "log" "os" "path/filepath" "regexp" "strings" ) All the americanise program’s imports are from Go’s standard library. Packages can be nested inside one another without formality, as the io package’s ioutil package and the path package’s filepath package illustrate. The bufio package provides functions for buffered I/O, including ones for read- ing and writing strings from and to UTF-8 encoded text files. The io package provides low-level I/O functions—and the io.Reader and io.Writer interfaces we need for the americanise() program. The io/ioutil package provides high-level file handling functions. The regexp package provides powerful regular expres- sion support. The other packages(fmt, log, filepath,and strings) have been men- tioned in earlier sections. func main() { inFilename, outFilename, err := filenamesFromCommandLine() ➊ if err != nil { fmt.Println(err) ➋ os.Exit(1) } inFile, outFile := os.Stdin, os.Stdout ➌ if inFilename != "" { if inFile, err = os.Open(inFilename); err != nil { log.Fatal(err) } defer inFile.Close() ➍ } if outFilename != "" { if outFile, err = os.Create(outFilename); err != nil { log.Fatal(err) } defer outFile.Close() ➎ } if err = americanise(inFile, outFile); err != nil { log.Fatal(err) ptg7913109 1.6. Americanise—Files,Maps, and Closures 31 } } The main() function gets the input and output filenames from the command line, creates corresponding file values, and then passes the files to the americanise() function to do the work. The function begins by retrieving the names of the files to read and write and an error value. If there was a problem parsing the command line we print the error (which contains the program’s usage message), and terminate the program. Some of Go’s print functions use reflection (introspection) to print a value using the value’s Error() string method if it has one, or its String() string method if it has one, or as best they can otherwise. If we provide our own custom types with one of these methods, Go’s print functions will automatically be able to print values of our custom types, as we will see in Chapter 6. If err is nil,wehave inFilename and outFilename strings (which may be empty), and we can continue. Files in Go are represented by pointers to values of type os.File, and so we create two such variables initialized to the standard input and output streams (which are both of type *os.File). Since Go functions and methods can return multiple values it follows that Go supports multiple assignments such as the ones we have used here (30 ➤ , ➊, ➌). Each filename is handled in essentially the same way. If the filename is empty the file has already been correctly set to os.Stdin or os.Stdout (both of which are of type *os.File, i.e., a pointer to an os.File value representing the file); but if the filename is nonempty we create a new *os.File to read from or write to the file as appropriate. The os.Open() function takes a filename and returns an *os.File value that can be used for reading the file. Correspondingly, the os.Create() function takes a filename and returns an *os.File value that can be used for reading or writing the file, creating the file if it doesn’t exist and truncating it to zero length if it does exist. (Go also provides the os.OpenFile() function that can be used to exercise complete control over the mode and permissions used to open a file.) In fact, the os.Open(), os.Create(), and os.OpenFile() functions return two values: an *os.File and nil if the file was opened successfully, or nil and an error if an error occurred. If err is nil we know that the file was successfully opened so we immediately execute a defer statement to close the file. Any function that is the subject of a defer statement (§5.5, ➤ 212) must be called—hence the parentheses after the functions’ names (30 ➤ , ➍, ➎)—but the calls only actually occur when the func- tion in which the defer statements are written returns. So the defer statement “captures” the function call and sets it aside for later. This means that the de- fer statement itself takes almost no time at all and control immediately passes to the following statement. Thus, the deferred os.File.Close() method won’t ptg7913109 32 Chapter 1. An Overview in Five Examples actually be called until the enclosing function—in this case, main()—returns (whether normally or due to a panic,discussed in a moment), so the file is open to be worked on and yet guaranteed to be closed when we are finished with it, or if a panic occurs. If we fail to open the file we call log.Fatal() with the error. As we noted in a previous section, this function logs the date, time, and error (to os.Stderr un- less another log destination is specified), and calls os.Exit() to terminate the program. When os.Exit() is called (directly, or by log.Fatal()), the program is terminated immediately—and any pending deferred statements are lost. This is not a problem, though, since Go’s runtime system will close any open files, the garbage collector will release the program’s memory, and any decent database or network that the application might have been talking to will detect the ap- plication’s demise and respond gracefully. Just the same as with the bigdigits example,we don’t use log.Fatal() in the first if statement (30 ➤ , ➋), because the err contains the program’s usage message and we want to print this without the date and time that the log.Fatal() function normally outputs. In Go a panic is a runtime error (rather like an exception in other languages). We can cause panics ourselves using the built-in panic() function, and can stop a panic in its tracks using the recover() function (§5.5, ➤ 212). In theory, Go’s panic/recover functionality can be used to provide a general-purpose exception handling mechanism—but doing so is considered to be poor Go practice. The Go way to handle errors is for functions and methods to return an error value as their sole or last return value—or nil if no error occurred—and for callers to always check the error they receive. The purpose of panic/recover is to deal with genuinely exceptional (i.e., unexpected) problems and not with normal errors.★ With both files successfully opened (the os.Stdin, os.Stdout, and os.Stderr files are automatically opened by the Go runtime sytem), we call the americanise() function to do the processing, passing it the files on which to work. If ameri- canise() returns nil the main() function terminates normally and any deferred statements—in this case, ones that close the inFile and outFile if they are not os.Stdin and os.Stdout—are executed. And if err is not nil, the error is printed, the program is exited, and Go’s runtime system closes any open files. The americanise() function accepts an io.Reader and an io.Writer, not *os.Files, but this doesn’t matter since the os.File type supports the io.ReadWriter inter- face (which simply aggregates the io.Reader and io.Writer interfaces) and can therefore be used wherever an io.Reader or an io.Writer is required. This is an example of duck typing in action—the americanise() function’s parameters are interfaces, so the function will accept any values—no matter what their types—that satisfy the interfaces, that is, any values that have the methods the ★ Go’s approach is very different from C++, Java, and Python, where exception handling is often used for both errors and exceptions. The discussion and rationale for Go’s panic/recover mechanism is at ptg7913109 1.6. Americanise—Files,Maps, and Closures 33 interfaces specify. The americanise() function returns nil,oran error if an error occurred. func filenamesFromCommandLine() (inFilename, outFilename string, err error){ if len(os.Args) > 1 && (os.Args[1]=="-h" || os.Args[1]=="--help"){ err = fmt.Errorf("usage: %s [<]infile.txt [>]outfile.txt", filepath.Base(os.Args[0])) return "","", err } if len(os.Args) > 1 { inFilename = os.Args[1] if len(os.Args) > 2 { outFilename = os.Args[2] } } if inFilename != "" && inFilename == outFilename { log.Fatal("won't overwrite the infile") } return inFilename, outFilename, nil } The filenamesFromCommandLine() function returns two strings and an error value—and unlike the functions we have seen so far, here the return values are given variable names, not just types. Return variables are set to their zero val- ues (empty strings and nil for err in this case) when the function is entered, and keep their zero values unless explicitly assigned to in the body of the function. (We will say a bit more on this topic when we discuss the americanise() func- tion, next.) The function begins by seeing if the user has asked for usage help.★ If they have, we create a new error value using the fmt.Errorf() function with a suitable usage string, and return immediately. As usual with Go code, the caller is expected to check the returned error and behave accordingly (and this is exactly what main() does).The fmt.Errorf() function is like the fmt.Printf() function we saw earlier, except that it returns an error value containing a string using the given format string and arguments rather than writing a string to os.Stdout. (The errors.New() function is used to create an error given a literal string.) If the user did not request usage information we check to see if they entered any command-line arguments, and if they did we set the inFilename return variable to their first command-line argument and the outFilename return variable ★ The Go standard library includes a flag package for handling command-line arguments. Third-party packages for GNU-compatible command-line handling are available from godashboard. third-party packages is covered in Chapter 9.) ptg7913109 34 Chapter 1. An Overview in Five Examples to their second command-line argument. Of course, they may have given no command-line arguments,in which case both inFilename and outFilename remain empty strings; or they may have entered just one, in which case inFilename will have a filename and outFilename will be empty. At the end we do a simple sanity check to make sure that the user doesn’t over- write the input file with the output file, exiting if necessary—but if all is well, we return.★ Functions or methods that return one or more values must have at least one return statement. It can be useful for clarity, and for godoc-generated documentation, to give variable names for return types, as we have done in this function. If a function or method has variable names as well as types listed for its return values,then a bare return is legal (i.e., a return statement that doesnot specify any variables). In such cases, the listed variables’ values are returned. We do not use bare returns in this book because they are considered to be poor Go style. Go takes a consistent approach to reading and writing data that allows us to read and write to files, to buffers (e.g., to slices of bytes or to strings), and to the standard input, output, and error streams—or to our own custom types—so long as they provide the methods necessary to satisfy the reading and writing interfaces. For a value to be readable it must satisfy the io.Reader interface. This interface specifies a single method with signature, Read([]byte) (int, error). The Read() method reads data from the value it is called on and puts the data read into the given byte slice. It returns the number of bytes read and an error value which will be nil if no error occurred, or io.EOF (“end of file”) if no error occurred and the end of the input was reached, or some other non-nil value if an error occurred. Similarly, for a value to be writable it must satisfy the io.Writer interface. This interface specifies a single method with signature, Write([]byte) (int, error). The Write() method writes data from the given byte slice into the value the method was called on, and returns the number of bytes written and an error value (which will be nil if no error occurred). The io package provides readers and writers but these are unbuffered and operate in termsof raw bytes. The bufio package providesbuffered input/output where the input will work on any value that satisfiesthe io.Reader interface (i.e., provides a suitable Read() method), and the output will work on any value that satisfies the io.Writer interface (i.e., provides a suitable Write() method). The bufio package’s readers and writers provide buffering and can work in terms of bytes or strings, and so are ideal for reading and writing UTF-8 encoded text files. ★ In fact, the user could still overwrite the input file by using redirection—for example, $ ./americanise infile > infile—but at least we have prevented an obvious accident. ptg7913109 1.6. Americanise—Files,Maps, and Closures 35 var britishAmerican = "british-american.txt" func americanise(inFile io.Reader, outFile io.Writer) (err error){ reader := bufio.NewReader(inFile) writer := bufio.NewWriter(outFile) defer func() { if err == nil { err = writer.Flush() } }() var replacer func(string) string ➊ if replacer, err = makeReplacerFunction(britishAmerican); err != nil { return err } wordRx := regexp.MustCompile("[A-Za-z]+") eof := false for !eof { var line string ➋ line, err = reader.ReadString('\n') if err == io.EOF { err = nil // io.EOF isn't really an error eof = true // this will end the loop at the next iteration } else if err != nil { return err // finish immediately for real errors } line = wordRx.ReplaceAllStringFunc(line, replacer) if _, err = writer.WriteString(line); err != nil { ➌ return err } } return nil } The americanise() function buffers the inFile reader and the outFile writer. Then it reads lines from the buffered reader and writes each line to the buffered writer, having replaced any British English words with their U.S. equivalents. The function begins by creating a buffered reader and a buffered writer through which their contentscan be accessed as bytes—or more conveniently in thiscase, as strings. The bufio.NewReader() construction function takes as argument any value that satisfies the io.Reader interface (i.e., any value that has a suitable Read() method) and returns a new buffered io.Reader that reads from the given reader. The bufio.NewWriter() function is synonymous. Notice that the ameri- canise() function doesn’t know or care what it is reading from or writing to—the reader and writer could be compressed files, network connections, byte slices ptg7913109 36 Chapter 1. An Overview in Five Examples ([]byte), or anything else that supports the io.Reader and io.Writer interfaces. This way of working with interfaces is very flexible and makes it easy to com- pose functionality in Go. Next we create an anonymous deferred function that will flush the writer’s buffer before the americanise() function returns control to its caller. The anony- mous function will be called when americanise() returns normally—or abnor- mally due to a panic. If no error has occurred and the writer’s buffer contains unwritten bytes, the bytes will be written before americanise() returns. Since it is possible that the flush will fail we set the err return value to the result of the writer.Flush() call. A less defensive approach would be to have a much simpler defer statement of defer writer.Flush() to ensure that the writer is flushed be- fore the function returnsand ignoring any error that might have occurred before the flush—or that occurs during the flush. Go allows the use of named return values, and we have taken advantage of this facility here (err error), just as we did previously in the filenamesFromCommand- Line() function. Be aware, however, that there is a subtle scoping issue we must consider when using named return values. For example, if we have a named return value of value, we can assign to it anywhere in the function using the as- signment operator (=) as we’d expect. However,if we have a statement such as if value := …, because the if statement starts a new block,the value in the if state- ment will be a new variable, so the if statement’s value variable will shadow the return value variable. In the americanise() function,err is a named return value, so we have made sure that we never assign to it using the short variable decla- ration operator (:=) to avoid the risk of accidentally creating a shadow variable. One consequence of this is that we must declare the other variables we want to assign to at the same time, such as the replacer function (35 ➤ , ➊) and the line we read in (35 ➤ , ➋). An alternative approach is to avoid named return values and return the required value or values explicitly, as we have done elsewhere. One other small point to note is that we have used the blank identifier, _(35 ➤ , ➌). The blank identifier serves as a placeholder for where a variable is expected in an assignment, and discards any value it is given. The blank identifier is not considered to be a new variable, so if used with :=, at least one other (new) variable must be assigned to. The Go standard library contains a powerful regular expression package called regexp (§3.6.5, ➤ 120). This package can be used to create pointers to regexp.Regexp values (i.e., of type *regexp.Regexp). These values provide many methods for searching and replacing. Here we have chosen to use the reg- exp.Regexp.ReplaceAllStringFunc() method which given a string and a “replacer” function with signature func(string) string, calls the replacer function for every match,passing in the matched text,and replacing the matched text with the text the replacer function returns. ptg7913109 1.6. Americanise—Files,Maps, and Closures 37 If we had a very small replacer function, say, one that simply uppercased the words it matched, we could have created it as an anonymous function when we called the replacement function. For example: line = wordRx.ReplaceAllStringFunc(line, func(word string) string { return strings.ToUpper(word) }) However, the americanise program’s replacer function, although only a few lines long, requires some preparation, so we have created another function, makeRe- placerFunction(), that given the name of a file that contains lines of original and replacement words, returns a replacer function that will perform the appropri- ate replacements. If the makeReplacerFunction() returns a non-nil error, we return and the caller is expected to check the returned error and respond appropriately (as it does). Regular expressions can be compiled using the regexp.Compile() function which returns a *regexp.Regexp and nil, or nil and error if the regular expression is invalid. This is ideal for when the regular expression is read from an external source such as a file or received from the user. Here, though, we have used the regexp.MustCompile() function—this simply returns a *regexp.Regexp, or panics if the regular expression, or “regexp”, is invalid. The regular expression used in the example matches the longest possible sequence of one or more English alphabetic characters. With the replacer function and the regular expression in place we start an infi- nite loop that begins by reading a line from the reader. The bufio.Reader.Read- String() method reads (or, strictly speaking, decodes) the underlying reader’s raw bytes as UTF-8 encoded text (which also works for 7-bit ASCII) up to and in- cluding the specified byte (or up to the end of the file).The function conveniently returns the text as a string, along with an error (or nil). If the error returned by the call to the bufio.Reader.ReadString() method is not nil, either we have reached the end of the input or we have hit a problem. At the end of the input err will be io.EOF which is perfectly okay, so in this case we set err to nil (since there isn’t really an error), and set eof to true to ensure that the loop finishes at the next iteration, so we won’t attempt to read beyond the end of the file. We don’t return immediately we get io.EOF, since it is possible that the file’s last line doesn’t end with a newline, in which case we will have received a line to be processed, in addition to the io.EOF error. For each line we call the regexp.Regexp.ReplaceAllStringFunc() method, giving it the line and the replacer function. We then try to write the (possibly modified) line to the writer using the bufio.Writer.WriteString() method—this method ac- cepts a string and writes it out as a sequence of UTF-8 encoded bytes, returning the number of bytes written and an error (which will be nil if no error occurred). We don’t care how many bytes are written so we assign the number to the blank ptg7913109 38 Chapter 1. An Overview in Five Examples identifier, _.If err is not nil we return immediately, and the caller will receive the error. Using bufio’s reader and writer as we have done here means that we can work with convenient high level string values, completely insulated from the raw bytes which represent the text on disk. And, of course, thanks to our deferred anonymous function, we know that any buffered bytes are written to the writer when the americanise() function returns, providing that no error has occurred. func makeReplacerFunction(file string) (func(string) string, error){ rawBytes, err := ioutil.ReadFile(file) if err != nil { return nil, err } text := string(rawBytes) usForBritish := make(map[string]string) lines := strings.Split(text, "\n") for _, line := range lines { fields := strings.Fields(line) if len(fields) == 2 { usForBritish[fields[0]] = fields[1] } } return func(word string) string { if usWord, found := usForBritish[word]; found { return usWord } return word }, nil } The makeReplacerFunction() takes the name of a file containing original and replacement strings and returns a function that given an original string returns its replacement, along with an error value. It expects the file to be a UTF-8 encoded text file with one whitespace-separated original and replacement word per line. In addition to the bufio package’s readers and writers, Go’s io/ioutil package provides some high level convenience functions including the ioutil.ReadFile() function used here. This function reads and returns the entire file’s contents as raw bytes (in a []byte) and an error. As usual, if the error is not nil we immedi- ately return it to the caller—along with a nil replacer function. If we read the bytes okay, we convert them to a string using a Go conversion of form type(vari- able).Converting UTF-8 bytes to a string is very cheap since Go’s strings use the UTF-8 encoding internally. (Go’s string conversions are covered in Chapter 3.) ptg7913109 1.6. Americanise—Files,Maps, and Closures 39 The replacer function we want to create must accept a string and return a corresponding string, so what we need is a function that uses some kind of lookup table. Go’s built-in map collection data type is ideal for this purpose (§4.3, ➤ 164). A map holds key–value pairs with very fast lookup by key. So here we will store British words as keys and their U.S. counterparts as values. Go’s map, slice, and channel types are created using the built-in make() function. This creates a value of the specified type and returns a reference to it. The reference can be passed around (e.g., to other functions) and any changes made to the referred-to value are visible to all the code that accesses it. Here we have created an empty map called usForBritish, with string keys and string values. With the map in place we then split the file’s text (which is in the form of a single long string) into lines, using the strings.Split() function. This function takes a string to split and a separator string to split on and does as many splits as pos- sible. (If we want to limit the number of splits we can use the strings.SplitN() function.) The iteration over the lines uses a for loop syntax that we haven’t seen before, this time using a range clause. This form can be conveniently used to iterate over a map’s keys and values, over a communication channel’s elements, or—as here—over a slice’s (or array’s) elements. When used on a slice (or array), the slice index and the element at that index are returned on each iteration, starting at index 0 (if the slice is nonempty). In this example we use the loop to iterate over all the lines, but since we don’t care about the index of each line we assign it to the blank identifier (_) which discards it. We need to split each line into two: the original string and the replacement string. We could use the strings.Split() function but that would require us to specify an exact separator string, say, "", which might fail on a hand-edited file where sometimes users accidentally put in more than one space, or sometimes use tabs. Fortunately, Go provides the strings.Fields() function which splits the string it is given on whitespace and is therefore much more forgiving of human-edited text. If the fields variable (of type []string) has exactly two elements we insert the corresponding key–value pair into the map. Once the map is populated we are ready to create the replacer function that we will return to the caller. We create the replacer function as an anonymous function given as an argument to the return statement—along with a nil error value. (Of course, we could have been less succinct and assigned the anonymous function to a variable and returned the variable.) The function has the exact signature required by the regexp.Regexp.ReplaceAllStringFunc() method that it will be passed to. Inside the anonymous replacer function all we do is look up the given word. If we access a map element with one variable on the left-hand side, that variable is set to the corresponding value—or to the value type’s zero value if the given ptg7913109 40 Chapter 1. An Overview in Five Examples key isn’t in the map. If the map value type’s zero value is a legitimate value, then how can we tell if a given key is in the map? Go provides a syntax for this case—and that is generally useful if we simply want to know whether a particular key is in the map—which is to put two variables on the left-hand side, the first to accept the value and the second to accept a bool indicating if the key was found. In this example we use this second form inside an if statement that has a simple statement (a short variable declaration), and a condition (the found Boolean). So we retrieve the usWord (which will be an empty string if the given word isn’t a key in the map),and a found flag of type bool.If the British word was found we return the U.S. equivalent; otherwise we simply return the original word unchanged. There is a subtlety in the makeReplacerFunction() function that may not be immediately apparent. In the anonymous function created inside it we access the usForBritish map, yet this map was created outside the anonymous function. This works because Go supports closures (§5.6.3, ➤ 225). A closure is a function that “captures” some external state—for example, the state of the function it is created inside, or at least any part of that state that the closure accesses. So here, the anonymous function that is created inside the makeReplacerFunction() is a closure that has captured the usForBritish map. Another subtlety is that the usForBritish map is a local variable and yet we will be accessing it outside the function in which it is declared. It is perfectly fine to return local variables in Go. Even if they are references or pointers, Go won’t delete them while they are in use and will garbage-collect them when they are finished with (i.e., when every variable that holds, refers, or points to them has gone out of scope). This section has shown some basic low-level and high-level file handling func- tionality using os.Open(), os.Create(), and ioutil.ReadFile(). In Chapter 8 there is much more file handling coverage, including the writing and reading of text, binary, JSON, and XML files. Go’s built-in collection types—slices and maps—largely obviate the need for custom collection types while providing ex- tremely good performance and great convenience. Go’s collection types are cov- ered in Chapter 4. Go’s treatment of functions as first-class values in their own right and its suppport for closures makes it possible to use some advanced and very useful programming idioms. And Go’s defer statement makes it straight- forward to avoid resource leakage. 1.7. Polar to Cartesian—Concurrency One key aspect of the Go language is its ability to take advantage of modern computerswith multiple processorsand multiple cores,and to do so without bur- dening programmers with lots of bookkeeping. Many concurrent Go programs can be written without any explicit locking at all (although Go does have locking ptg7913109 1.7. Polar to Cartesian—Concurrency 41 primitives for when they’re needed in lower-level code, as we will see in Chap- ter 7). Two features make concurrent programming in Go a pleasure. First, goroutines (in effect very lightweight threads/coroutines)can easily be created at will with- out the need to subclass some “thread” class (which isn’t possible in Go anyway). Second,channelsprovide type-safe one-way or two-way communication with gor- outines and which can be used to synchronize goroutines. The Go way to do concurrency is to communicate data, not to share data. This makes it much easier to write concurrent programs than using the traditional threads and locks approach, since with no shared data we can’t get race condi- tions (such as deadlocks), and we don’t have to remember to lock or unlock since there is no shared data to protect. In this section we will look at the fifth and last of the chapter’s “overview” examples. This section’s example program uses two communication channels and does its processing in a separate Go routine. For such a small program this is complete overkill, but the point is to illustrate a basic use of these Go features in as clear and short a way as possible. More realistic concurrency examples that show many of the different techniques that can be used with Go’s channels and goroutines are presented in Chapter 7. The program we will review is called polar2cartesian; it is an interactive console program that prompts the user to enter two whitespace-separated numbers—a radius and an angle—which the program then uses to compute the equivalent cartesian coordinates. In addition to illustrating one particular approach to con- currency,it also shows some simple structs and how to determine if the program is running on a Unix-like system or on Windows for when the difference matters. Here is an example of the program running in a Linux console: $ ./polar2cartesian Enter a radius and an angle (in degrees), e.g., 12.5 90, or Ctrl+D to quit. Radius and angle: 5 30.5 Polar radius=5.00 θ=30.50° → Cartesian x=4.31 y=2.54 Radius and angle: 5 -30.25 Polar radius=5.00 θ=-30.25° → Cartesian x=4.32 y=-2.52 Radius and angle: 1.0 90 Polar radius=1.00 θ=90.00° → Cartesian x=-0.00 y=1.00 Radius and angle: ^D $ The program is in file polar2cartesian/polar2cartesian.go, and we will review it top-down, starting with the imports, then the structs it uses, then its init() function, then its main() function, and then the functions called by main(), and so on. ptg7913109 42 Chapter 1. An Overview in Five Examples import ( "bufio" "fmt" "math" "os" "runtime" ) The polar2cartesian program imports several packages, some of which have been mentioned in earlier sections, so we will only mention the new ones here. The math package provides mathematical functions for operating on floating- point numbers (§2.3.2, ➤ 64) and the runtime package provides functions that access the program’s runtime properties, such as which platform the program is running on. type polar struct { radius float64 θ float64 } type cartesian struct { x float64 y float64 } In Go a struct is a type that holds (aggregatesor embeds) one or more data fields. These fields can be built-in types as here (float64), or structs, or interfaces, or any combination of these. (An interface data field is in effect a pointer to an item—of any kind—that satisfies the interface, i.e., that has the methods the interface specifies.) It seems natural to use the Greek lowercase letter theta (θ) to represent the polar coordinate’s angle, and thanks to Go’s use of UTF-8 we are free to do so. This is because Go allows us to use any Unicode letters in our identifiers, not just English letters. Although the two structs happen to have the same data field types they are dis- tinct typesand no automatic conversion between them is possible. Thissupports defensive programming; after all, it wouldn’t make sense to simply substitute a cartesian’s positional coordinates for polar coordinates. In some cases such con- versions do make sense, in which case we can easily create a conversion method (i.e., a method of one type that returned a value of another type) that made use of Go’s composite literal syntax to create a value of the target type populated by the fields from the source type. (Numeric data type conversions are covered in Chapter 2; string conversions are covered in Chapter 3.) ptg7913109 1.7. Polar to Cartesian—Concurrency 43 var prompt = "Enter a radius and an angle (in degrees), e.g., 12.5 90, " + "or %s to quit." func init() { if runtime.GOOS == "windows" { prompt = fmt.Sprintf(prompt, "Ctrl+Z, Enter") } else {// Unix-like prompt = fmt.Sprintf(prompt, "Ctrl+D") } } If a package has one or more init() functions they are automatically executed beforethe main package’s main() function is called. (In fact, init() functionsmust never be called explicitly.) So when our polar2cartesian program is invoked this init() function is the first function that is called. We use init() to set the prompt to account for platform differences in how end of file is signified—for example, on Windows end of file is given by pressing Ctrl+Z then Enter. Go’s run- time package provides the GOOS (Go Operating System) constant which is a string identifying the operating system the program is running on. Typical values are darwin (Mac OS X), freebsd, linux, and windows. Before diving into the main() function and the rest of the program we will briefly discuss channels and show some toy examples before seeing them in proper use. Channels are modeled on Unix pipes and provide two-way (or at our option, one-way) communication of data items. Channels behave like FIFO (first in, first out) queues, hence they preserve the order of the items that are sent into them. Items cannot be dropped from a channel, but we are free to ignore any or all of the items we receive. Let’s look at a very simple example. First we will make a channel: messages := make(chan string, 10) Channels are created with the make() function (Chapter 7) and are declared us- ing the syntax, chan Type. Here we have created the messages channel to send and receive strings. The second argument to make() is the buffer size (which defaults to 0);here we have made it big enough to accept ten strings. If a channel’s buffer is filled it blocks until at least one item is received from it. This means that any number of items can pass through a channel, providing the items are retrieved to make room for subsequent items. A channel with a buffer size of 0 can only send an item if the other end is waiting for an item. (It is also possible to get the effect of nonblocking channels using Go’s select statement, as we will see in Chapter 7.) Now we will send a couple of strings into the channel: ptg7913109 44 Chapter 1. An Overview in Five Examples messages <- "Leader" messages <- "Follower" When the <- communication operator is used as a binary operator its left-hand operand must be a channel and its right-hand operand must be a value to send to the channel of the type the channel was declared with. Here,we first send the string Leader to the messages channel, and then we send the string Follower. message1 := <-messages message2 := <-messages When the <- communication operator is used as a unary operator with just a right-hand operand (which must be a channel), it acts as a receiver, blocking until it has a value to return. Here, we retrieve two messages from the messages channel. The message1 variable is assigned the string Leader and the message2 variable is assigned the string Follower; both variables are of type string. Normally channels are created to provide communication between goroutines. Channel sends and receives don’t need locks, and the channel blocking behavior can be used to achieve synchronization. Now that we have seen some channel basics, let’s see channels—and goroutines —in practical use. func main() { questions := make(chan polar) defer close(questions) answers := createSolver(questions) defer close(answers) interact(questions, answers) } Once any init() functions have returned, Go’s runtime system then calls the main package’s main() function. Here, the main() function begins by creating a channel (of type chan polar) for passing polar structs, and assigns it to the questions variable. Once the channel has been created we use a defer statement to call the built-in close() function (➤ 187) to ensure that it is closed when it is no longer needed. Next we call the createSolver() function, passing it the questions channel and receiving from it an answers channel (of type chan cartesian). We use another defer statement to ensure that the answers channel is closed when it is finished with. And finally, we call the interact() function with the two channels, and in which the user interaction takes place. ptg7913109 1.7. Polar to Cartesian—Concurrency 45 func createSolver(questions chan polar) chan cartesian { answers := make(chan cartesian) go func() { for { polarCoord := <-questions ➊ θ := polarCoord.θ * math.Pi / 180.0 // degrees to radians x := polarCoord.radius * math.Cos(θ) y := polarCoord.radius * math.Sin(θ) answers <- cartesian{x, y} ➋ } }() return answers } The createSolver() function begins by creating an answers channel to which it will send the answers (i.e., cartesian coordinates) to the questions (i.e., polar coordinates) that it receives from the questions channel. After creating the channel,the function then has a go statement. A go statement is given a function call (syntactically just like a defer statement), which is exe- cuted in a separate asynchronous goroutine. This means that the flow of control in the current function (i.e., in the main goroutine) continues immediately from the following statement. In this case the go statement is followed by a return statement that returnsthe answers channel to the caller. As we noted earlier,it is perfectly safe and good practice in Go to return local variables, since Go handles the chore of memory management for us. In this case we have (created and) called an anonymous function in the go state- ment. The function has an infinite loop that waits (blocking its own goroutine, but not any other goroutines, and not the function in which the goroutine was started), until it receives a question—in this case a polar struct on the questions channel. When a polar coordinate arrivesthe anonymousfunction computesthe corresponding cartesiancoordinateusing somesimplemath (and using thestan- dard library’s math package), and then sends the answer as a cartesian struct (created using Go’s composite literal syntax), to the answers channel. In ➊ the <- operator is used as a unary operator, retrieving a polar coordinate from the questions channel. And in ➋ the <- operator is used as a binary opera- tor;its left-hand operand being the answers channel to send to,and its right-hand operand being the cartesian to send. Once the call to createSolver() returns we have reached the point where we have two communication channels set up and where a separate goroutine is waiting for polar coordinatesto be sent on the questions channel—and without any other goroutine, including the one executing main(), being blocked. ptg7913109 46 Chapter 1. An Overview in Five Examples const result = "Polar radius=%.02f θ=%.02f° → Cartesian x=%.02f y=%.02f\n" func interact(questions chan polar, answers chan cartesian) { reader := bufio.NewReader(os.Stdin) fmt.Println(prompt) for { fmt.Printf("Radius and angle: ") line, err := reader.ReadString('\n') if err != nil { break } var radius, θ float64 if _, err := fmt.Sscanf(line, "%f %f",&radius, &θ); err != nil { fmt.Fprintln(os.Stderr, "invalid input") continue } questions <- polar{radius, θ} coord := <-answers fmt.Printf(result, radius, θ, coord.x, coord.y) } fmt.Println() } This function is called with both channels passed as parameters. It begins by creating a buffered reader for os.Stdin since we want to interact with the user in the console. It then prints the prompt that tells the user what to enter and how to quit. We could have made the program terminate if the user simply pressed Enter (i.e., didn’t type in any numbers), rather than asking them to enter end of file. However, by requiring the use of end of file we have made polar2cartesian more flexible, since it is also able to read its input from an arbitrary external file using file redirection (providing only that the file has two whitespace-separated numbers per line). The function then starts an infinite loop which begins by prompting the user to enter a polar coordinate (a radius and an angle).After asking for the user’sinput the function waits for the user to type some text and press Enter, or to press Ctrl+D (or Ctrl+Z, Enter on Windows) to signify that they have finished. We don’t bother checking the error value; if it isn’t nil we break out of the loop and return to the caller (main()),which in turn will return (and call its deferred statementsto close the communication channels). We create two float64s to hold the numbers the user has entered and then use Go’s fmt.Sscanf() function to parse the line. Thisfunction takesa string to parse, a format—in this case two whitespace-separated floating-point numbers—and one or more pointers to variables to populate. (The & address of operator is used to get a pointer to a value; see §4.1, ➤ 140.) The function returns the number of ptg7913109 1.7. Polar to Cartesian—Concurrency 47 items it successfully parsed and an error (or nil).In the case of an error,we print an error message to os.Stderr—this is to make the error message visible on the console even if the program’s os.Stdout is redirected to a file. Go’s powerful and flexible scan functionsare shown in use in Chapter 8 (§,➤ 380),and listed in Table 8.2 (➤ 383). If valid numberswere input and sent to the questions channel (in a polar struct), we block the main goroutine waiting for a response on the answers channel. The additional goroutine created in the createSolver() function is itself blocked wait- ing for a polar on the questions channel, so when we send the polar, the addi- tional goroutine performs the computation, sends the resultant cartesian to the answers channel, and then waits (blocking only itself) for another question to arrive. Once the cartesian answer is received in the interact() function on the answers channel, interact() is no longer blocked. At this point we print the re- sult string using the fmt.Printf() function, and passing the polar and cartesian values as the arguments that the result string’s % placeholders are expecting. The relationship between the goroutines and the channels is illustrated in Fig- ure 1.1. Main goroutine init() main() createSolver() interact() Solver goroutine func() // anonymous Questions Answers Figure 1.1 Two communicating goroutines The interact() function’s for loop is an infinite loop, so as soon as a result is printed the user is once again asked to enter a radius and angle, with the loop being broken out of only if the reader reads end of file—either interactively from the user or because the end of a redirected input file has been reached. The calculations in polar2cartesian are very lightweight, so there was no real need to do them in a separate goroutine. However, a similar program that needed to do multiple independent heavyweight calculations as the result of each input might well benefit from using the approach shown here, for example, with one goroutine per calculation. We will see more realistic use cases for channels and goroutines in Chapter 7. We have now completed our overview of the Go language as illustrated by the five example programs reviewed in this chapter. Naturally, Go has much more to offer than there has been space to show here, as we will see in the subsequent chapters, each of which focuses on a specific aspect of the language and any relevant packages from the standard library. This chapter concludes with a small exercise, which despite its size, requires some thought and care. ptg7913109 48 Chapter 1. An Overview in Five Examples 1.8. Exercise Copy the bigdigits directory to, say, my_bigdigits, and modify my_bigdigits/big- digits.go to produce a version of the bigdigits program (§1.4, 16 ➤ ) that can optionally output the number with an overbar and underbar of “*”s, and with improved command-line argument handling. The original program output its usage message if no number was given; change this so that the usage message is also output if the user gives an argument of -h or --help. For example: $ ./bigdigits --help usage: bigdigits [-b|--bar] -b --bar draw an underbar and an overbar If the --bar (or -b) option is not present the program should have the same behavior as before. Here is an example of the expected output if the option is present: $ ./bigdigits --bar 8467243 ************************************************* 888 4 666 77777 222 4 333 8 8 44 6 7 2 2 44 3 3 8 8 4 4 6 7 2 4 4 3 888 4 4 6666 7 2 4 4 33 8 8 444444 6 6 7 2 444444 3 8 8 4 6 6 7 2 4 3 3 888 4 666 7 22222 4 333 ************************************************* The solution requires more elaborate command-line processing than the version shown in the text, although the code producing the output only needs a small change to output the overbar before the first row and the underbar after the last row. Overall, the solution needs about 20 extra lines of code—the solution’s main() function is twice as long as the original (~40 vs. ~20 lines), mostly due to the code needed to handle the command line. A solution is provided in the file bigdigits_ans/bigdigits.go. Hints:The solution also has a subtle difference in the way it builds up each row’s line to prevent the bars extending too far. Also, the solution imports the strings package and uses the strings.Repeat(string, int) function. This function returnsa string that containsthe string it is given as its first argument repeated by the number of times of the int given as its second argument. Why not look this function up either locally (see the sidebar “The Go Documentation”, 8 ➤ ), or at, and start to become familiar with the Go standard library’s documentation. ptg7913109 1.8. Exercise 49 It would be much easier to handle command-line arguments using a package de- signed for the purpose. Go’s standard library includes a rather basic command line parsing package, flag, that supports X11-style options (e.g., -option). In ad- dition, several option parsers that support GNU-style short and long options (e.g., -o and --option) are available from ptg7913109 This page intentionally left blank ptg7913109 2 Booleans and Numbers §2.1. Preliminaries ➤ 51 §2.1.1. Constants and Variables ➤ 53 §2.2. Boolean Values and Expressions ➤ 56 §2.3. Numeric Types ➤ 57 §2.3.1. Integer Types ➤ 59 §2.3.2. Floating-Point Types ➤ 64 §2.4. Example: Statistics ➤ 72 §2.4.1. Implementing Simple Statistics Functions ➤ 73 §2.4.2. Implementing a Basic HTTP Server ➤ 75 This is the first of four chapters on procedural programming that lay down the foundations for Go programming—whether procedural, object-oriented, concurrent, or any combination of these approaches. This chapter covers Go’s built-in Boolean type and all of Go’s built-in numeric types, and briefly introduces two of the numeric types from Go’s standard library. Apart from the need to explicitly convert between different types of numbers and the convenience of having a built-in complex type, programmers coming from C, C++, and Java should find few surprises in this chapter. This chapter’s first section covers some of the language’s basics, such as how comments are written, Go’s keywords and operators, what constitutes a valid identifier, and so on. Once these preliminaries have been covered, there are sections on Booleans, integers, and floating-point numbers, the latter including coverage of complex numbers. 2.1. Preliminaries Go supports two kinds of comments, both adopted from C++. Line comments begin with // and end at the newline; these are treated simply as a newline. General comments begin with /* and end with */ and may span multiple lines. When a general comment is all on one line (e.g., /* inline comment */), it is treated 51 ptg7913109 52 Chapter 2. Booleans and Numbers as a space, and when a general comment spans one or more lines it it treated as a newline. (Newlines are significant in Go, as we will see in Chapter 5.) A Go identifier is a nonempty sequence of letters and digits where the first character must be a letter, and which is not the name of a keyword. A letter is the underscore, _, or any character that is in the Unicode categories, “Lu” (letter, uppercase), “Ll” (letter, lowercase), “Lt” (letter, titlecase), “Lm” (letter, modifier), or “Lo” (letter, other); this includes all the English alphabetic characters (A–Z and a–z).A digit is any character in the Unicode category “Nd” (number,decimal digit); this includes the Arabic digits (0–9). The compiler will prevent the use of an identifier that has the same name as a keyword; see Table 2.1. Table 2.1 Go’s Keywords break default func interface select case defer go map struct chan else goto package switch const fallthrough if range type continue for import return var Go has many predefined identifiers; it is possible—but rarely wise—to create an identifier with the same name as a predefined identifier; see Table 2.2. Table 2.2 Go’s Predefined Identifiers append copy int8 nil true bool delete int16 panic uint byte error int32 print uint8 cap false int64 println uint16 close float32 iota real uint32 complex float64 len recover uint64 complex64 imag make rune uintptr complex128 int new string Identifiers are case-sensitive, so for example, LINECOUNT, Linecount, LineCount, lineCount, and linecount are five different identifiers. Identifiers that begin with a capital letter, that is, with a character in Unicode category “Lu” (including A–Z), are considered to be public—exported in Go terminology—while all others are considered to be private—unexported in Go terminology. (This rule does not apply to package names which are conventionally all lowercase.) We will see this distinction in action when we discuss object-oriented programming in Chapter 6, and packages in Chapter 9. The blank identifier, _, serves as a placeholder for where a variable is expected in an assignment, and discards any value it is given. The blank identifier is not ptg7913109 2.1. Preliminaries 53 considered to be a new variable, so if it is used with the := operator, at least one other (new) variable must be assigned to. It is legitimate to discard some or all of a function’s return values by assigning them to the blank identifier. However, if no return values are wanted it is more conventional to simply ignore them. Here are some examples: count, err = fmt.Println(x) // get number of bytes printed and error count, _ = fmt.Println(x) // get number of bytes printed; discard error _, err = fmt.Println(x) // discard number of bytes printed; get error fmt.Println(x) // ignore return values It is not uncommon to ignore the return values when printing to the console, but the error value should always be checked when printing to files, network connections, and so on—for example, using fmt.Fprint() and similar functions. (Go’s print functions are fully covered later; §3.5, ➤ 93.) 2.1.1. Constants and Variables Constants are declared using the const keyword;variables can be declared using the var keyword, or using the short variable declaration syntax. Go can infer the type of the declared type, although it is legal to specify it if we wish to or need to—for example, to specify a type that is different from the type Go would normally infer. Here are some example declarations: const limit = 512 // constant; type-compatible with any number const top uint16 = 1421 // constant; type: uint16 start := -19 // variable; inferred type: int end := int64(9876543210)// variable; type: int64 var i int // variable; value 0; type: int var debug = false // variable; inferred type: bool checkResults := true // variable; inferred type: bool stepSize := 1.5 // variable; inferred type: float64 acronym := "FOSS" // variable; inferred type: string For integer literals Go infers type int, for floating-point literals Go infers type float64, and for complex literals Go infers type complex128 (the numbers in their names refer to how many bits they occupy).The normal practice is to leave types unspecified unless we want to use a specific type that Go won’t infer; we will discuss this further in §2.3, ➤ 57. Typed numeric constants (e.g., top) can only be used in expressions with other numbers of the same type (unless converted). Untyped numeric constants can be used in expressions with numbers of any built-in type, (e.g., limit can be used in an expression with integers or in one with floating-point numbers). The variable i was not given any explicit value. This is perfectly safe in Go since Go always assigns variables their type’s zero value if no other value is specified. ptg7913109 54 Chapter 2. Booleans and Numbers This means that every numeric variable is guaranteed to be zero and every string to be empty—unless we specify otherwise. Thisensuresthat Go programs don’t suffer from the problems of uninitialized garbage values that afflict some other languages. Enumerations Rather than repeat the const keyword when we want to set multiple constants, we can group together several constant declarations using the const keyword just once. (We used the same grouping syntax when importing packages in Chapter 1; the syntax can also be used to group variables declared with var.) For those cases where we just want constantsto have distinct values and don’t really care what those values are, we can use Go’s somewhat bare-bones enumeration support. const Cyan = 0 const Magenta = 1 const Yellow = 2 const ( Cyan = 0 Magenta = 1 Yellow = 2 ) const ( Cyan = iota // 0 Magenta // 1 Yellow // 2 ) These three code snippets all achieve exactly the same thing. The way a group of consts works is that the first one is set to its zero value unless explicitly set (either to a value or to iota), and the second and subsequent ones are set to their predecessor’s value—or to iota if their predecessor’s value is iota. And each subsequent iota value is one more than the previous one. More formally, the iota predefined identifier represents successive untyped integer constants. Its value is reset to zero whenever the keyword const occurs (so every time a new const group is defined), and increments by one for each constant declaration. So in the right-hand code snippet all the constants are set to iota (implicitly for the Magenta and Yellow ones). And since Cyan immediately follows a const, iota is reset to 0 which become’s Cyan’s value; Magenta’s value is also iota but at this point iota’s value is 1. Similarly, Yellow’s value is iota whose value is now 2. And if we added Black at the end (but within the const group) it would be implicitly set to iota whose value at that point would be 3. On the other hand, if the right-hand code snippet didn’t have iota, Cyan would be set to 0 and Magenta would be set to Cyan’s value and Yellow would be set to Magenta’s value—so they would all end up being set to 0.Similarly, if Cyan was set to 9, then they would all be set to 9; or if Magenta was set to 5, Cyan would be set to 0 (first in the group and not assigned an explicit value or iota), Magenta would be 5 (explicitly set), and Yellow would be 5 (the previous constant’s value). It is also possible to use iota with floating-point numbers, simple expressions, and custom types. ptg7913109 2.1. Preliminaries 55 type BitFlag int const ( Active BitFlag = 1 << iota // 1 << 0 == 1 Send // Implicitly BitFlag = 1 << iota // 1 << 1 == 2 Receive // Implicitly BitFlag = 1 << iota // 1 << 2 == 4 ) flag := Active | Send In this snippet we have created three bit flags of custom type BitFlag and then set variable flag (of type BitFlag) to the bitwise OR of two of them (so flag has value 3; Go’s bitwise flags are shown in Table 2.6, ➤ 60). We could have omitted the custom type in which case Go would have made the constants untyped integers and inferred flag’s type as int. Variables of type BitFlag can have any int value; nonetheless BitFlag is a distinct type so can only be used in operations with ints if converted to an int (or if the ints are converted to BitFlags). The BitFlag type is useful as it stands, but it isn’t very convenient for debugging. If we were to print flag we would just get 3 with no indication of what that means. Go makes it really easy to control how values of custom types are print- ed, because the fmt package’s print functions will use a type’s String() method if it has one. So to make our BitFlag type print in a more informative way, we can simply add a suitable String() method to it. (Custom types and methods are covered fully in Chapter 6.) func (flag BitFlag) String() string { var flags []string if flag&Active == Active { flags = append(flags, "Active") } if flag&Send == Send { flags = append(flags, "Send") } if flag&Receive == Receive { flags = append(flags, "Receive") } if len(flags) > 0 {// int(flag) is vital to avoid infinite recursion! return fmt.Sprintf("%d(%s)", int(flag), strings.Join(flags, "|")) } return "0()" } This method builds up a (possibly empty) slice of strings for those bit fields that are set and then prints the bit field’s value as a decimal int and with the strings to indicate its value. (We could easily have printed the value as a binary number by replacing the %d format specifier with %b.) As the comment notes, it is essen- ptg7913109 56 Chapter 2. Booleans and Numbers tial that we convert the flag (of type BitFlag) to its underlying int type when passing it to the fmt.Sprintf() function, otherwise the BitFlag.String() method will be called recursively on the flag which will take us into an infinite recursion. (The built-in append() function is covered in §4.2.3, ➤ 156; the fmt.Sprintf() and strings.Join() functions are covered in Chapter 3.) Println(BitFlag(0), Active, Send, flag, Receive, flag|Receive) 0() 1(Active) 2(Send) 3(Active|Send) 4(Receive) 7(Active|Send|Receive) This snippet shows how BitFlags with the String() method in place look when printed—clearly, this is much more useful for debugging than bare integers. It is,of course, possible to create a custom type that represents a restricted range of integers, and to create a more elaborate custom enumeration type; we cover custom typesmore fully in Chapter 6.Go’sminimalist approach to enumerations is typical of the Go philosophy:Go aims to provide everything that programmers need—including many powerful and convenient features—while keeping the language as small, consistent, and fast (to build and run) as possible. 2.2. Boolean Values and Expressions Go provides two built-in Boolean values, true and false, both of type bool. Go supports the standard logical and comparison operators, all of which produce a bool result; they are shown in Table 2.3. Boolean values and expressions are used in if statements (§5.2.1, ➤ 192), in the conditions of for statements (§5.3, ➤ 203), and sometimes in the conditions of switch statements’ case clauses (§5.2.2, ➤ 195), as we will see in Chapter 5. The binary logical operators (|| and &&) use short-circuit logic. This means that if we have b1 || b2 and expression b1 evaluates to true, the result must be true no matter what b2 is, so true is returned and b2 is not evaluated. Similarly, if we have b1 && b2 and expression b1 evaluates to false, the result must be false,so false is returned and b2 is not evaluated. Go is strict about the values that can be compared using the comparison oper- ators (<, <=, ==,!=, >=, >). The two values must be of the same type, or—if they are interfaces—they must implement the same interface type. If one value is a constant then it must be of a type that is compatible with the other’s type. This means that an untyped numeric constant can be compared with another value of any numeric type, but numbers of different types—and that are not constants—cannot be compared unless one of them is explicitly converted to be of the same type as the other. (Numeric conversions are discussed in §2.3, ➤ 57.) ptg7913109 2.2. Boolean Values and Expressions 57 Table 2.3 Boolean and Comparison Operators Syntax Description/result !b Logical NOT operator; false if Boolean expression b is true a || b Short-circuit logical OR operator; true if either Boolean expression a or b is true a && b Short-circuit logical AND operator; true if both Boolean expressions a and b are true x < y true if expression x is less than expression y x <= y true if expression x is less than or equal to expression y x == y true if expression x is equal to expression y x != y true if expression x is not equal to expression y x >= y true if expression x is greater than or equal to expression y x > y true if expression x is greater than expression y The == and != operators can be applied to operands of any comparable types, in- cluding arrays and structs whose items or fields are comparable using == and !=. These operators cannot be used to compare slices, although such a comparison can be done using the Go standard library’s reflect.DeepEqual() function. The == and != operators can be used to compare two pointers or two interfaces—or to compare a pointer or interface or reference (e.g., to a channel, map, or slice) with nil. The other comparison operators (<, <=, >=, >) may be applied only to numbers and strings. (Since Go—like C and Java—doesn’t support operator overloading, for our own custom types we can implement our own comparison methods or functions if needed, such as Less() or Equal(), as we will see in Chapter 6.) 2.3. Numeric Types Go provides a wide range of built-in numeric types, and the standard library adds integers of type big.Int and rationals of type big.Rat which are of un- bounded size (i.e., limited only by the machine’s memory). Every numeric type is distinct: This means that we cannot use binary arithmetic operations or com- parisons (e.g., + or <) on numeric values of different types (e.g., of type int32 and type int). Untyped numeric constants are compatible with any (built-in) typed number they are in an expression with,so we can add or compare an untyped nu- meric constant with another number, no matter what the other number’s (built- in) type. If we need to perform arithmetic or comparisons on typed numbers of different types we must perform conversions—usually to the biggest type to avoid loss of accuracy. Conversions take the form type(value) and where valid (e.g., from one type of number to another) they always succeed—even if this results in data loss. Here are some examples: ptg7913109 58 Chapter 2. Booleans and Numbers const factor = 3 // factor is compatible with any numeric type i:=20000 // i is of type int by inference i *= factor j := int16(20)// j is of type int16; same as: var j int16 = 20 i += int(j) // Types must match so conversion is required k := uint8(0)// Same as: var k uint8 k = uint8(i) // Succeeds, but k's value is truncated to 8 bits ✗ fmt.Println(i, j, k) // Prints: 60020 20 116 If we want to perform safe downsizing conversions we can always create suitable functions. For example: func Uint8FromInt(x int) (uint8, error){ if 0 <= x && x <= math.MaxUint8 { return uint8(x), nil } return 0, fmt.Errorf("%d is out of the uint8 range",x) } This function acceptsan int argument and returns a uint8 and nil if the int is in range, or 0 and an error otherwise. The math.MaxUint8 constant is from the math package which also has similar constants for Go’s other built-in numeric types. (Of course, there are no minimum constants for the unsigned types since they all share a minimum of 0.) The fmt.Errorf() function returns an error based on the format string and value or values it is given. (String formatting is covered in §3.5, ➤ 93.) Numbers of the same type can be compared using the comparison operators (see Table 2.3, 57 ➤ ). Similarly, Go’s arithmetic operators can be applied to numbers; these are shown in Table 2.4 (➤ 59) applicable to all built-in numbers, and in Table 2.6 (➤ 60) applicable only to integers. Constant expressions are evaluated at compile time; they may use any of the arithmetic, Boolean, and comparison operators. For example: const ( efri int64 = 10000000000 // type: int64 hlutföllum = 16.0 / 9.0 // type: float64 mælikvarða = complex(-2, 3.5)* hlutföllum // type: complex128 erGjaldgengur = 0.0 <= hlutföllum && hlutföllum < 2.0 // type: bool ) The example uses Icelandic identifiers as a reminder that Go fully supports native language identifiers. (We will discuss complex() shortly; §, ➤ 70.) ptg7913109 2.3. Numeric Types 59 Table 2.4 Arithmetic Operators Applicable to All Built-In Numbers Syntax Description/result +x x -x The negation of x x++ Increments x by the untyped constant 1 x-- Decrements x by the untyped constant 1 x += y Increments x by y x -= y Decrements x by y x *= y Sets x to x multiplied by y x /= y Sets x to x divided by y; if the numbers are integers any remainder is discarded; division by zero causes a runtime panic★ x + y The sum of x and y x - y The result of subtracting y from x x * y The result of multiplying x by y x / y The result of dividing x by y; if the numbers are integers any remain- der is discarded; division by zero causes a runtime panic★ Although Go has sensible rules of precedence (unlike, say, C and C++), we recommend using parentheses to make intentions clear. Using parentheses is particularly recommended for programmers who use multiple languages so as to avoid subtle mistakes. 2.3.1. Integer Types Go provides 11 separate integer types, five signed and five unsigned, plus an in- teger type for storing pointers—their names and values are shown in Table 2.5 (➤ 60). In addition, Go allows the use of byte as a synonym for the unsigned uint8 type, and encourages the use of rune as a synonym for the int32 type when working with individual characters (i.e., Unicode code points).For most process- ing purposes the only integer type that we need is int. This is suitable for loop counters, array and slice indexes, and all general-purpose integer arithmetic; it is also normally the integer type that offers the fastest processing speeds. At the time of this writing, the int type is represented by a signed 32-bit integer (even on 64-bit platforms), but is expected to change to 64-bit in a future Go version. The other integer types that Go provides are needed when it comes to reading and writing integers outside the program—for example, from and to files or network connections. In such cases it is essential to know exactly how many bits must be read or written so that integers can be handled without corruption. ★ A panic is an exception; see Chapter 1 (32 ➤ ) and §5.5, ➤ 212. ptg7913109 60 Chapter 2. Booleans and Numbers Table 2.5 Go’s Integer Types and Ranges Type Range byte Synonym for uint8 int The int32 or int64 range depending on the implementation int8 [−128, 127] int16 [−32 768, 32767] int32 [−2147483648,2147483647] int64 [−9223372036854 775808, 9223372036854 775807] rune Synonym for int32 uint The uint32 or uint64 range depending on the implementation uint8 [0, 255] uint16 [0, 65535] uint32 [0, 4 294 967 295] uint64 [0, 18446744 073709551615] uintptr An unsigned integer capable of storing a pointer value (advanced) Table 2.6 Arithmetic Operators Applicable Only to Built-In Integer Types Syntax Description/result ^x The bitwise complement of x x %= y Sets x to be the remainder of dividing x by y; division by zero causes a runtime panic x &= y Sets x to the bitwise AND of x and y x |= y Sets x to the bitwise OR of x and y x ^= y Sets x to the bitwise XOR of x and y x &^= y Sets x to the bitwise clear (AND NOT)of x and y x >>= u Sets x to the result of right-shifting itself by unsigned int u shifts x <<= u Sets x to the result of left-shifting itself by unsigned int u shifts x % y The remainder of dividing x by y; division by zero causes a runtime panic x & y The bitwise AND of x and y x | y The bitwise OR of x and y x ^ y The bitwise XOR of x and y x &^ y The bitwise clear (AND NOT)of x and y x << u The result of left-shifting x by unsigned int u shifts x >> u The result of right-shifting x by unsigned int u shifts ptg7913109 2.3. Numeric Types 61 A common practice is to store integers in memory using the int type, and to convert to or from one of the explicitly signed and sized integer types when writing or reading integers. The byte (uint8) type is used for reading and writing raw bytes—for example, when handling UTF-8 encoded text. We saw the basics of reading and writing UTF-8 encoded text in the previous chapter’s americanise example (29 ➤ ), and will see how to read and write built-in and custom data in Chapter 8. Go integers support all the arithmetic operations listed in Table 2.4 (59 ➤ ), and in addition they support all the arithmetic and bitwise operations listed in Table 2.6 (60 ➤ ). All of these operations have the expected standard behaviors, so they are not discussed further,especially since we will see plenty of examples throughout the book. It is always safe to convert an integer of a smaller type to one of a larger type (e.g., from an int16 to an int32); but downsizing an integer that is too big for the target type or converting a negative integer to an unsigned integer will silent- ly result in a truncated or otherwise unexpected value. In such cases it is best to use a custom downsizing function such as the one shown earlier (58 ➤ ). Of course, when attempting to downsize a literal (e.g., int8(200)), the compiler will detect the problem and report an overflow error. Integers can also be converted to floating-point numbers using the standard Go syntax (e.g., float64(integer)). Go’s support for 64-bit integers makes it realistically possible to use scaled integers for precise calculations in some contexts. For example, computing the finances for a business using int64s to represent millionths of a cent allows for calculations in the range of billions of dollars with sufficient accuracy for most purposes—especially if we are careful about divisions. And if we need to do financial calculations with perfect accuracy and avoid rounding errors we can use the big.Rat type. Big Integers In some situations we need to perform perfectly accurate computations with whole numbers whose range exceeds even that of int64s and uint64s. In such cases we cannot use floating-point numbers because they are represented by ap- proximations. Fortunately, Go’s standardlibrary providestwo unlimited accura- cy integer types: big.Int for integers and big.Rat for rationals (i.e., for numbers than can be represented as fractionssuch as 2 3 and 1.1496,but not irrationalslike e or π). These integer types can hold an arbitrary number of digits—providing only that the machine has sufficient memory—but are potentially a lot slower to process than built-in integers. Since Go—like C and Java—does not support operator overloading,the methods provided for big.Ints and big.Rats have names—for example, Add() and Mul().In most cases the methods modify their receiver (i.e.,the big integer they are called on), and also return their receiver as their result to support the chaining of ptg7913109 62 Chapter 2. Booleans and Numbers operations. We won’t list all the functions and methods provided by the math/big package since they can easily be looked up in the documentation and may have been added to since this was written; however, we will look at a representative example to get a flavor of how big.Ints are used. Using Go’s float64 type allows us to accurately compute to about 15 decimal digits—which is more than enough for most situations. However, if we want to compute to a large number of decimal places, say, tens or hundreds of places, as we might want to when computing π, no built-in type is sufficient. In 1706 John Machin developed a formula for calculating π to an arbitrary number of decimal places, and we can adapt this formula in conjunction with the Go standard library’s big.Ints to compute π to any number of decimal places. The pure formula, and the arccot() function it relies on, are shown in Figure 2.1. (No understanding of Machin’s formula is required to understand the use of the big.Int package introduced here.) Our implementation of the arccot() function accepts an additional argument to limit the precision of the calculation so that we don’t go beyond the number of digits required. π = 4 ×(4 × arccot(5)−arccot(239)) arccot(x)=1 x − 1 33x + 1 55x − 1 77x + … Figure 2.1 Machin’s formula The entire program is less than 80 lines and is in the file pi_by_digits/pi_by_dig- its.go; here is its main() function.★ func main() { places := handleCommandLine(1000) scaledPi := fmt.Sprint(π(places)) fmt.Printf("3.%s\n", scaledPi[1:]) } The program assumes a default value of 1000decimal places, although the user can choose any number they like by entering a value on the command line. The handleCommandLine() function (not shown) returns the value it is passed or the number the user entered on the command line (if any, and if it is valid). The π() function returns π as a big.Int of value 314159…; we print this to a string, and then print the string on the console properly formatted so that the output appears as, say, 3.14159265358979323846264338327950288419716939937510 (here we have used a mere 50 digits). ★ The implementation used here is based on's_for- mula_(Python). ptg7913109 2.3. Numeric Types 63 func π(places int) *big.Int { digits := big.NewInt(int64(places)) unity := big.NewInt(0) ten := big.NewInt(10) exponent := big.NewInt(0) unity.Exp(ten, exponent.Add(digits, ten), nil) ➊ pi := big.NewInt(4) left := arccot(big.NewInt(5), unity) left.Mul(left, big.NewInt(4)) ➋ right := arccot(big.NewInt(239), unity) left.Sub(left, right) pi.Mul(pi, left) ➌ return pi.Div(pi, big.NewInt(0).Exp(ten, ten, nil)) ➍ } The π() function begins by computing a value for the unity variable ( digits+1010 ) which we use as a scale factor so that we can do all our calculations using inte- gers. The +10 adds an extra ten digits to those given by the user, to avoid round- ing errors. We then use Machin’s formula with our modified arccot() function (not shown) that takes the unity variable as its second argument. Finally, we return the result divided by 1010 to reverse the effects of the unity scale factor. To get the unity variable to hold the correct value we begin by creating four variables, all of type *big.Int (i.e., pointer to big.Int; see §4.1, ➤ 140). The unity and exponent variables are initialized to 0, the ten variable to 10, and the digits variable to the number of digits requested by the user. The unity computation is performed in a single line (➊). The big.Int.Add() method adds 10 to the number of digits. Then the big.Int.Exp() method is used to raise 10 to the power of its second argument (digits + 10). When used with a nil third argument—as here—big.Int.Exp(x, y, nil) performs the computation yx ; with three non-nil arguments, big.Int.Exp(x, y, z) computes yx mod z. Notice that we did not need to assign to unity; this is because most big.Int methods modify their receiver as well as return it, so here, unity is modified to have the resultant value. The rest of the computation follows a similar pattern. We set an initial value of pi to 4 and then compute the inner left-hand part of Machin’s formula. We don’t need to assign to left after creating it (➋), since the big.Int.Mul() method stores the result in its receiver (i.e., in this case in variable left) as well as returning the result (which we can safely ignore). Next we compute the inner right-hand part of the formula and subtract the right from the left (leaving the result in left).Now we multiply pi (of value 4) by left (which holds the result of Machin’s formula). This produces the result but scaled by unity. So in the final line (➍)we reverse the scaling by dividing the result (in pi)by 1010 . Using the big.Int type takessome care since most methodsmodify their receiver (this is done for efficiency to save creating lots of temporary big.Ints). Compare ptg7913109 64 Chapter 2. Booleans and Numbers the line where we perform the computation pi × left with the result being stored in pi (63 ➤ , ➌) to the line where we compute pi ÷ 1010 and return the result (63 ➤ , ➍)—not caring that the value of pi has been overwritten by the result. Wherever possible it is best to use plain ints, falling back to int64s if the int range isn’t sufficient, or using float32sorfloat64s if the fact that they are ap- proximations is not a concern. However, if computations of perfect accuracy are required and we are prepared to pay the price in memory use and processing overhead,then we can use big.Intsorbig.Rats—the latter particularly useful for financial calculations—scaling if necessary as we did here, when floating-point computations are required. 2.3.2. Floating-Point Types Go provides two types of floating-point numbers and two types of complex numbers—their names and ranges are shown in Table 2.7. Floating-point numbers in Go are held in the widely used IEEE-754 format ( This format is also the native format used by many microprocessorsand floating-point units,so in most casesGo isable to take direct advantage of the hardware’s floating-point support. Table 2.7 Go’s Floating-Point Types Type Range float32 ±3.402823466385288598117 041834 845169254 40 × 3810 The mantissa is reliably accurate to about 7 decimal places. float64 ±1.797693134 862315708145274 237 317 043567 981 × 30810 The mantissa is reliably accurate to about 15 decimal places. complex64 The real and imaginary parts are both of type float32. complex128 The real and imaginary parts are both of type float64. Go floating-point numbers support all the arithmetic operations listed in Table 2.4 (59 ➤ ). Most of the math package’s constants and all of its functions are listed in Tables 2.8 to 2.10 (➤ 65–67). Floating-point numbers are written with a decimal point, or using exponential notation, for example, 0.0, 3., 8.2, −7.4, −6e4, .1, 5.9E−3. Computers commonly represent floating-point numbersinternally using base 2—thismeansthat some decimals can be represented exactly (such as 0.5),but others only approximately (such as 0.1 and 0.2). Furthermore, the representation uses a fixed number of bits, so there is a limit to the number of digits that can be held. This is not a Go- specific problem, but one that afflicts floating-point numbers in all mainstream programming languages. However, the imprecision isn’t always apparent, be- cause Go usesa smart algorithm for outputting floating-point numbersthat uses the fewest possible digits consistent with maintaining accuracy. ptg7913109 2.3. Numeric Types 65 Table 2.8 The Math Package’s Constants and Functions #1 All the math package’s functionsaccept and return float64s unless specified otherwise. All the constants are shown truncated to 15 decimal digits to fit neatly into the tables. Syntax Description/result math.Abs(x) |x|, i.e., the absolute value of x math.Acos(x) The arc cosine of x in radians math.Acosh(x) The arc hyperbolic cosine of x in radians math.Asin(x) The arc sine of x in radians math.Asinh(x) The arc hyperbolic sine of x in radians math.Atan(x) The arc tangent of x in radians math.Atan2(y, x) The arc tangent of y x in radians math.Atanh(x) The arc hyperbolic tangent of x in radians math.Cbrt(x) 3√ x, the cube root of x math.Ceil(x) ⎡x⎤ , i.e., the smallest integer greater than or equal to x; e.g., math.Ceil(5.4) == 6.0 math.Copysign(x, y)A value with x’s magnitude and y’s sign math.Cos(x) The cosine of x in radians math.Cosh(x) The hyperbolic cosine of x in radians math.Dim(x, y) In effect, math.Max(x - y, 0.0) math.E The constant e; approximately 2.718 281828459045 math.Erf(x) erf (x); x’s Gauss error function math.Erfc(x) erfc(x); x’s complementary Gauss error function math.Exp(x) xe math.Exp2(x) x2 math.Expm1(x) xe − 1; this is more accurate than using math.Exp(x)-1 when x is close to 0 math.Float32bits(f) The IEEE-754 binary representation of f (of type float32)asauint32 math. Float32frombits(u) The float32 represented by the IEEE-754 bits in u (of type uint32) math.Float64bits(x) The IEEE-754 binary representation of x (of type float64)asauint64 math. Float64frombits(u) The float64 represented by the IEEE-754 bits in u (of type uint64) ptg7913109 66 Chapter 2. Booleans and Numbers Table 2.9 The Math Package’s Constants and Functions #2 Syntax Description/result math.Floor(x) ⎣x⎦ , i.e., the largest integer less than or equal to x; e.g., math.Floor(5.4) == 5.0 math.Frexp(x) frac of type float64 and exp of type int such that x = frac exp× 2 ; the inverse function is math.Ldexp() math.Gamma(x) Γ(x), i.e., (x − 1)! math.Hypot(x, y) math.Sqrt(x * x, y * y) math.Ilogb(x) The binary exponent of x as an int; see also math.Logb() math.Inf(n)A float64 of value +∞ if n of type int is ≥ 0; otherwise −∞ math.IsInf(x, n) true if x of type float64 is +∞ and n of type int is > 0, or if x is −∞ and n is < 0, or if x is either infinity and n is 0; otherwise false math.IsNaN(x) true if x has the IEEE-754 “not a number” value math.J0(x)J0(x), the Bessel function of the first kind math.J1(x)J1(x), the Bessel function of the first kind math.Jn(n, x)Jn(x), the order-n (where n is of type int) Bessel function of the first kind math.Ldexp(x, n) nx × 2 where x is of type float64 and n is of type int; the inverse function is math.Frexp() math.Lgamma(x) loge(Γ(x)) as a float64 and the sign of Γ(x) as an int (−1 or +1) math.Ln2 loge(2); approximately 0.693147 180559945 math.Ln10 loge(10); approximately 2.302585092994 045 math.Log(x) loge(x) math.Log2E 1 loge(2); approximately 1.442695021629333 math.Log10(x) log10(x) math.Log10E 1 loge(10); approximately 0.434294 492006301 math.Log1p(x) loge(1 + x) but is more accurate than using math.Log() when x is near zero math.Log2(x) log2(x) math.Logb(x) The binary exponent of x; see also math.Ilogb() math.Max(x, y) The larger of x and y math.Min(x, y) The smaller of x and y math.Mod(x, y) The remainder of x y; see also math.Remainder() ptg7913109 2.3. Numeric Types 67 Table 2.10 The Math Package’s Constants and Functions #3 Syntax Description/result math.Modf(x) The whole and fractional parts of x as float64s math.NaN(x) An IEEE-754 “not a number” value math.Nextafter( x, y) The next representable value after x going toward y math.Pi The constant π; approximately 3.141592653589793 math.Phi The constant φ; approximately 1.618 033988749984 math.Pow(x, y) yx math.Pow10(n) n10 as a float64; n is of type int math.Remainder( x, y) the IEEE-754-compliant remainder of x y; see also math.Mod() math.Signbit(x) Returns a bool; true if x is negative (including −0.0) math.Sin(x) The sine of x in radians math.SinCos(x) The sine and cosine of x in radians math.Sinh(x) The hyperbolic sine of x in radians math.Sqrt(x) √x math.Sqrt2 √2; approximately 1.414213562373095 math.SqrtE √e; approximately 1.648 721270700128 math.SqrtPi √π; approximately 1.772453850905516 math.SqrtPhi √φ; approximately 1.272019649514 068 math.Tan(x) The tangent of x in radians math.Tanh(x) The hyperbolic tangent of x in radians math.Trunc(x) x with its fractional part set to 0 math.Y0(x)Y0(x), the Bessel function of the second kind math.Y1(x)Y1(x), the Bessel function of the second kind math.Yn(n, x)Yn(x), the order-n (where n is of type int) Bessel function of the second kind ptg7913109 68 Chapter 2. Booleans and Numbers All the comparison operations listed in Table 2.3 (57 ➤ ) can be used with floating-point numbers. Unfortunately, due to the fact that floating-point num- bers are held as approximations,comparing them for equality or inequality does not always work intuitively. x, y := 0.0, 0.0 for i:=0;i<10; i++ { x += 0.1 if i%2 == 0 { y += 0.2 } else { fmt.Printf("%-5t %-5t %-5t %-5t",x==y, EqualFloat(x, y, -1), EqualFloat(x, y, 0.000000000001), EqualFloatPrec(x, y, 6)) fmt.Println(x, y) } } true true true true 0.2 0.2 true true true true 0.4 0.4 false false true true 0.6 0.6000000000000001 false false true true 0.7999999999999999 0.8 false false true true 0.9999999999999999 1 Here we start with two float64s with initial values of 0. We add ten 0.1s to the first one and five 0.2s to the second, so at the end both should be 1. However, as the output shown below the code snippet illustrates, perfect accuracy for some floating-point numbers is not possible. In view of this we must be very careful when comparing floating-point numbers for equality or inequality using == and !=. Of course, there are cases where it is sensible to compare floating-point numbers for equality or inequality using the built-in operators—for example, when trying to avoid division by zero, as in, say, if y != 0.0 { return x / y }. The "%-5t" format prints a bool left-aligned in a field five characters wide— string formatting is covered in the next chapter; §3.5, ➤ 93. func EqualFloat(x, y, limit float64) bool { if limit <= 0.0 { limit = math.SmallestNonzeroFloat64 } return math.Abs(x-y) <= (limit * math.Min(math.Abs(x), math.Abs(y))) } The EqualFloat() function compares two float64s to the given accuracy—or to the greatest accuracy the machine can achieve if a negative number (e.g., -1) ptg7913109 2.3. Numeric Types 69 is passed as the limit. It relies on functions (and a constant) from the standard library’s math package. An alternative (and slower) approach is to compare numbers as strings. func EqualFloatPrec(x, y float64, decimals int) bool { a := fmt.Sprintf("%.*f", decimals, x) b := fmt.Sprintf("%.*f", decimals, y) return len(a) == len(b) && a == b } For this function the accuracy is specified as the number of digits after the dec- imal point. The fmt.Sprintf() function’s % formatting argument can accept a * placeholder where it expects a number, so here we create two strings based on the two given float64s, formatting each with the specified number of decimal places. If the magnitudes of the numbers differ, then so will the lengths of the a and b strings (e.g., 12.32 vs. 592.85), which gives us a relatively fast short-cir- cuiting equality test. (String formatting is covered in §3.5, ➤ 93.) In most cases where floating-point numbers are needed the float64 type is the best choice—especially since all the functions in the math package work in terms of float64s. However, Go also provides the float32 type which may be useful when memory is at a premium and we either don’t need to use the math package, or are willing to put up with the minor inconvenience of converting to and from float64s when necessary. Since Go’s floating-point types are sized it is always safe to read or write them from or to external sources such as files or network connections. Floating-point numbers can be converted to integers using the standard Go syntax (e.g., int(float)), in which case the fractional part is simply discarded. Of course, if the floating-point value exceeds the range of the integer type converted to, the resultant integer will have an unpredictable value. We can address this problem using a safe conversion function. For example: func IntFromFloat64(x float64) int { if math.MinInt32 <= x && x <= math.MaxInt32 { whole, fraction := math.Modf(x) if fraction >= 0.5 { whole++ } return int(whole) } panic(fmt.Sprintf("%g is out of the int32 range", x)) } The Go Specification ( states that an int occupies the same number of bits as a uint and that a uint is always 32 or 64 bits. This ptg7913109 70 Chapter 2. Booleans and Numbers implies that an int is at least 32 bits which means that we can safely use the math.MinInt32 and math.MaxInt32 constants as the int range. We use the math.Modf() function to separate the whole and fractional parts of the given number (both as float64s), and rather than simply returning the whole part (i.e., truncating), we perform a very simple rounding if the fractional part is ≥ 0.5. Rather than return an error as we did for our custom Uint8FromInt() function (58 ➤ ), we have chosen to treat out-of-range values as important enough to stop the program, so we have used the built-in panic() function which will cause a runtime panic and stop the program unless the panic is caught by a recover() call (§5.5, ➤ 212). This means that if the program runs successfully we know that no out-of-range conversions were attempted. (Notice also that the function does not end with a return statement;the Go compiler is smart enough to realize that a call to panic() means that a normal return cannot occur at that point.) Complex Types The two complex types supported by Go are shown in Table 2.7 (64 ➤ ). Complex numbers can be created using the built-in complex() function or by using constant literals involving imaginary numbers. Complex numbers’ components can be retrieved using the built-in real() and imag() functions, both of which return a float64 (or a float32 for complex64s). Complex numbers support all the arithmetic operations listed in Table 2.4 (59 ➤ ). The only comparison operators that can be used with complex numbers are == and != (see Table 2.3, 57 ➤ ), but these suffer from the same issues as they do when comparing floating-point numbers. The standard library has a complex number-specific package, math/cmplx, whose functions are listed in Table 2.11. Here are some simple examples: f:=3.2e5 // type: float64 x:=-7.3 - 8.9i // type: complex128 (literal) y := complex64(-18.3 + 8.9i)// type: complex64 (conversion) ➊ z:=complex(f, 13.2)// type: complex128 (construction) ➋ fmt.Println(x, real(y), imag(z)) // Prints: (-7.3-8.9i) -18.3 13.2 Go signifies imaginary numbers using the suffix i as used in pure mathemat- ics.★ Here, the numbers x and z are of type complex128, so their real and imag- inary parts are of type float64; y is of type complex64 so its components are of type float32. One subtle point to notice is that using the complex64 type name (or any other built-in type name for that matter) as a function performs a type conversion. So here (➊),the complex number -18.3+8.9i (of type complex128—the ★ By contrast, in engineering and in Python, imaginary numbers are indicated using j. ptg7913109 2.3. Numeric Types 71 Table 2.11 The Complex Math Package’s Functions Import "math/cmplx".All the functions accept and return complex128s unless specified oth- erwise. Syntax Description/result cmplx.Abs(x) |x|, i.e., the absolute value of x as a float64 cmplx.Acos(x) The arc cosine of x in radians cmplx.Acosh(x) The arc hyperbolic cosine of x in radians cmplx.Asin(x) The arc sine of x in radians cmplx.Asinh(x) The arc hyperbolic sine of x in radians cmplx.Atan(x) The arc tangent of x in radians cmplx.Atanh(x) The arc hyperbolic tangent of x in radians cmplx.Conj(x) The complex conjugate of x cmplx.Cos(x) The cosine of x in radians cmplx.Cosh(x) The hyperbolic cosine of x in radians cmplx.Cot(x) The cotangent of x in radians cmplx.Exp(x) xe cmplx.Inf() complex(math.Inf(1), math.Inf(1)) cmplx.IsInf(x) true if real(x) or imag(x) is ±∞; otherwise false cmplx.IsNaN(x) true if real(x) or imag(x) is “not a number” and if neither is ±∞; otherwise false cmplx.Log(x) loge(x) cmplx.Log10(x) log10(x) cmplx.NaN() A complex “not a number” value cmplx.Phase(x) The phase of x as a float64 in the range [−π, +π] cmplx.Polar(x) The absolute value r and phase θ both of type float64, satisfying x = θir × e ; phase is in the range [−π, +π] cmplx.Pow(x, y) yx cmplx.Rect(r, θ)A complex128 with polar coordinates r and θ both of type float64 cmplx.Sin(x) The sine of x in radians cmplx.Sinh(x) The hyperbolic sine of x in radians cmplx.Sqrt(x) √x cmplx.Tan(x) The tangent of x in radians cmplx.Tanh(x) The hyperbolic tangent of x in radians ptg7913109 72 Chapter 2. Booleans and Numbers inferred complex type for complex literals) is converted to a complex64. However, complex() is a function (there is no type of that name) that takes two floats and returns the corresponding complex128 (70 ➤ , ➋). Another subtle point is that the fmt.Println() function can print complex numbers without formality. (As we will see in Chapter 6 we can make our own types seamlessly cooperate with Go’s print functions simply by providing them with a String() method.) In general the best complex type to use is complex128 since all the functions in the math/cmplx package work in terms of complex128s. However, Go also provides the complex64 type which may be useful when memory is very tight. Since Go’s complex types are sized it is always safe to read or write them from or to external sources such as files or network connections. In this chapter we have looked at Go’s Boolean and numeric types and presented tables showing the operators and functions that are available to query and manipulate them. The next chapter covers Go’s string type, including thorough coverage of Go’s print formatting functionality (§3.5, ➤ 93), which includes, of course,the printing of Booleans and numbers formatted as we want. We will see how to read and write Go data types—including Booleans and numbers—from and to files in Chapter 8. Before closing this chapter, though, we will review a small but complete working example program. 2.4. Example: Statistics The purpose of this example (and the exercises that follow) is to provide some context for (and practice of) Go programming. Just like in Chapter 1, the ex- ample makes use of some Go features that haven’t yet been fully covered. This shouldn’t cause problems since brief explanations and forward references are provided. The example also introduces some very simple usage of the Go stan- dard library’s net/http package—this package makes it incredibly easy to create HTTP servers. As appropriate to the main theme of the chapter, the example and the exercises are numeric in flavor. The statistics program (in file statistics/statistics.go) is a web application that asks the user to enter a list of numbers and then does some very simple statistical calculations. Figure 2.2 shows the program in action. We will review the program’s code in two parts, first the implementation of the mathematical functionality, and then the implementation of the application’s web page. We won’t show the whole program (e.g., we will skip the imports and most of the constants), since it is available for download, but we will cover enough to make it understandable. ptg7913109 2.4. Example: Statistics 73 Figure 2.2 The Statistics program on Linux and Windows 2.4.1. Implementing Simple Statistics Functions For convenience we have created an aggregate type that holds the numbers the user entered and the two statistics we plan to calculate. type statistics struct { numbers []float64 mean float64 median float64 } A Go struct is similar to a C struct or to a Java class that has public data fields and no methods—but not like a C++ struct since it isn’t a class. As we will see, Go structs provide excellent support for aggregation and embedding (§6.4, ➤ 275), and are also often central to Go’s object-oriented functionality (Chap- ter 6). func getStats(numbers []float64) (stats statistics) { stats.numbers = numbers sort.Float64s(stats.numbers) stats.mean = sum(numbers) / float64(len(numbers)) stats.median = median(numbers) return stats } This function accepts a slice of numbers (in this case as obtained by our process- Request() function ➤ 77),and populatesits stats result value (of type statistics) with appropriate values. To compute the median we need the numbers to be ptg7913109 74 Chapter 2. Booleans and Numbers sorted into ascending order; this is achieved using the sort package’s Float64s() function which sorts a []float64 in-place. This means that the getStats() func- tion modifies its argument—something that is quite common when slices, refer- ences, or pointers are passed to functions. If we wanted to preserve the original slice of numbers, we could copy it to a temporary slice using the built-in copy() function (§4.2.3, ➤ 156), and work on the copy. The mean (or average) is simply the sum of a sequence of values divided by the number of values in the sequence. Here we have used a separate helper function to sum the numbers, and converted the length (count of numbers) to a float64 to make the types compatible (since sum() returns a float64). This also ensures that we get floating-point division and avoid the truncation that would occur if we used integers. The median is the middle value; we compute this separately using the median() function. We haven’t checked for division by zero since our program’s logic means that getStats() is only called when there is at least one number; so if we ever break the logic the program will terminate with a runtime panic. For a mission-critical application that should never terminate when problems occur we could use Go’s recover() function to catch panics, restore the application to a sane state, and continue to run (§5.5, ➤ 212). func sum(numbers []float64) (total float64) { for _, x := range numbers { total += x } return total } Thisfunction usesa for …range loop to iterate over all the numbers(and discard- ing their index positions) to produce their sum. Thanksto Go alwaysinitializing variables—including named return values—to their zero value, total correctly starts at zero. func median(numbers []float64) float64 { middle := len(numbers) / 2 result := numbers[middle] if len(numbers)%2 == 0 { result = (result + numbers[middle-1]) / 2 } return result } This function must be called with a sorted slice of float64s. It initially takes the median to be the middle value, but if the number of numbers is even there are actually two middle values, so in that case we sum those values and divide ptg7913109 2.4. Example: Statistics 75 by two to get the mean of the two middle values. And at the end we return the result. In this subsection we have covered the application-specific processing. In the next subsection we will look at the basics of implementing the infrastructure to support a web application that has a single web page. (Readers who aren’t interested in web programming might prefer to skip to the exercises or to the next chapter.) 2.4.2. Implementing a Basic HTTP Server The statistics program provides a single web page on the local host. Here is its main() function. func main() { http.HandleFunc("/", homePage) if err := http.ListenAndServe(":9001", nil); err != nil { log.Fatal("failed to start server", err) } } The http.HandleFunc() function takes two arguments: a path and a reference to a function to call when that path is requested. The function must have the signature func(http.ResponseWriter, *http.Request). We can register as many path–function pairs as we like. Here we have registered the / path (i.e., the web application’s home page) with a custom homePage() function. The http.ListenAndServe() function starts up a web server at the given TCP network address; here we have used localhost and port number 9001. The local host is assumed if only the port number is given—we could just as easily have used an address of "localhost:9001" or "".(The port number we have chosen is arbitrary—simply change the code to use a different one if it conflicts with an existing server.) The second argument is used to specify which kind of server to use—normally we pass nil to indicate that we want to use the default kind. The program has several string constants but we will only show one of them here. form = `

` The form string constant contains a
element which itself contains the text and submit button elements. ptg7913109 76 Chapter 2. Booleans and Numbers func homePage(writer http.ResponseWriter, request *http.Request) { err := request.ParseForm() // Must be called before writing response fmt.Fprint(writer, pageTop, form) if err != nil { fmt.Fprintf(writer, anError, err) } else { if numbers, message, ok := processRequest(request); ok { stats := getStats(numbers) fmt.Fprint(writer, formatStats(stats)) } else if message != "" { fmt.Fprintf(writer, anError, message) } } fmt.Fprint(writer, pageBottom) } This function is called whenever the statistics web site is visited. The writer ar- gument is where we write our response to (in HTML) and the request argument contains details of the request. We begin by parsing the form (which will initially have an empty text element. We have called the text element “numbers” so that we can refer to it when we process the form later on. Also, the form’s action is set to /, so when the user presses the Calculate button the same page is requested again. This means that the homePage() function is called in all cases, so it must handle the initial case where no numbers have been entered, and subsequent cases where numbers have been entered or where an error has occurred. In fact, all the work is passed on to a custom processRequest() function, so it is in that function that each case is dealt with. After the parse, we write the pageTop (not shown) and form string constants. If the parse fails for any reason we write an error message; anError is a format string and err is the error value to be formatted. (Format strings are covered later; §3.5, ➤ 93.) anError = `


` If the parse succeeds (as it should),we call a custom processRequest() function to retrieve the numbers entered by the user ready for processing. If the numbers are valid we compute the statistics using the getStats() function we saw earlier (73 ➤ ) and write the formatted results; otherwise we write an error message if we are given one. (When the form is shown for the first time it has no numbers, yet no error has occurred, in which case ok is false and message is empty.) And at the end we print the pageBottom constant string (not shown) which just closes the and tags. ptg7913109 2.4. Example: Statistics 77 func processRequest(request *http.Request) ([]float64, string, bool) { var numbers []float64 if slice, found := request.Form["numbers"]; found && len(slice) > 0 { text := strings.Replace(slice[0], ",","",-1) for _, field := range strings.Fields(text) { if x, err := strconv.ParseFloat(field, 64); err != nil { return numbers, "'" + field + "' is invalid", false } else { numbers = append(numbers, x) } } } if len(numbers) == 0 { return numbers, "", false // no data first time form is shown } return numbers, "", true } This function reads the form’s data from the request value. If the form is being shown for the first time the “numbers” text element is empty. This isn’t an error so we return an empty slice of float64s, an empty error message, and false to indicate that there are no statistics to gather—this results in the empty form being shown. If the user has entered some numbers we return either a slice of float64s, an empty error message, and true; or, if one or more numbers is invalid, a possibly empty slice, an error message, and false. The request value has a Form field of type map[string][]string (§4.3, ➤ 164). This means that the map’s keys are strings and its values are slices of strings. So any one key may have any number of strings as its value. For example, if the user has entered the numbers “5 8.2 7 13 6”, the Form map will have a "numbers" key with a value of []string{"5 8.2 7 13 6"}, that is, its value will be a slice of strings that actually has only one string. (For comparison, here is an example of a slice of two strings: []string{"1 2 3", "a b c"}.) We check to see if the "numbers" key is present (it ought to be), and if it is—and if its value has at least one string—we know that we have numbers to read. We use the strings.Replace() function to obtain the string of numbers that the user entered but with any commas replaced by spaces. (The third argument is the number of replacements to perform; -1 means do as many as possible.) Having obtained the string of whitespace-separated numbers we then use the strings.Fields() function to split the string (on any amount of whitespace) into a slice of strings which we iterate over straight away using a for … range loop. (The strings package’s functions are covered in §3.6, ➤ 106; the for … range loop is covered in §5.3, ➤ 203.) For each string (“5”, “8.2”, etc.) we attempt to convert it to a float64 using the strconv.ParseFloat() function which takes a string to parse and a bit size of 32 or 64 (§3.6, ➤ 106). If the conversion fails we imme- ptg7913109 78 Chapter 2. Booleans and Numbers diately return with whatever float64s we have, a nonempty error message, and false. If the conversion succeeds we append the float64 to the numbers slice. The built-in append() function takes a slice and one or more values and returns a slice that has all the items from the original slice plus the values—the function is smart enough to reuse the original slice if its capacity is greater than its length, so it is efficient to use. (We cover append() in §4.2.3, ➤ 156.) If we haven’t already returned due to an error (i.e., an invalid number), we return the numbers with an empty error message and true, unless there are no numbers to process (because the form has been shown for the first time) in which case we return false. func formatStats(stats statistics) string { return fmt.Sprintf(`
`, stats.numbers, len(stats.numbers), stats.mean, stats.median) } Once the statistics have been computed we must output them to the user, and since the program is a web application we need to produce HTML. (Go’s standard library has dedicated text/template and html/template packages for creating data-driven text and HTML, but our needs here are so simple that we have chosen to do it all by hand. A small text/template-based example is shown later; §9.4.2, ➤ 419.) The fmt.Sprintf() function takes a format string and one or more values and re- turns a string that is a copy of the format string but with the format verbs (e.g., %v,%d,%f) replaced with corresponding values. (String formatting is thoroughly covered in §3.5, ➤ 93.) We had no need to do any HTML escaping since all of our values are numbers. (If escaping is needed we can use the template.HTMLEscape() or html.EscapeString() functions.) As this example illustrates, Go makes it easy to create simple web applications —providing we know some basic HTML—and provides the html, net/http, html/ template, and text/template packages to make life easier. 2.5. Exercises There are two exercises for this chapter, both numeric in flavor. The first involves modifying the statistics program we have just reviewed; the second involves creating a simple mathematical web application from scratch. ptg7913109 2.5. Exercises 79 1. Copy the statistics directory to, say, my_statistics and modify my_statis- tics/statistics.go to produce two more statistical measures: the mode and the standard deviation. When the user clicks the Calculate button it should produce output similar to that shown in Figure 2.3. Figure 2.3 The Statistics solution on Mac OS X This involves adding a couple more items to the statistics struct and adding two new functions to perform the calculations. A solution is in the file statistics_ans/statistics.go; this needed about 40 extra lines and made use of the built-in append() function (§4.2.3, ➤ 156) to add numbers to a slice. The standard deviation function is the easiest to write—it just requires the use of some functions from the math package and can be done in fewer than ten lines. We used the formula σ = √∑(x−−x 2) n − 1 , where x is each number, −x is the mean, and n is the number of numbers. The mode is the most frequently occurring number—or numbers, if two or more are equally the most frequently occurring. However, we return no mode if all of the numbers occur with the same frequency. The mode is trickier than the standard deviation, and needs about 20 lines of code. 2. Create a web application for calculating the solution or solutionsto quadrat- ic equations using the standard formula x = −b±√ 2b − 4ac 2a . Use complex num- bers so that it is possible to find solutions even when the discriminant (the 2b − 4ac part) is negative. Initially, just get the math working, as shown in Figure 2.4’s left-hand screenshot (➤ 80). Then, modify your application to produce more intelligent output, as shown in Figure 2.4’s right-hand screenshot. ptg7913109 80 Chapter 2. Booleans and Numbers Figure 2.4 The Quadratic solutions on Linux One easy way to get started is to copy the statistics application’s main(), homePage(), and processRequest() functions, modifying homePage() to call three new custom functions—formatQuestion(), solve(), and formatSolu- tions()—and heavily modifying processRequest() to read in three separate floating-point numbers. The file quadratic_ans1/quadratic.go contains an initial application of about 120 lines. This version is smart enough to out- put just one solution if both solutions are approximately equal by making use of the EqualFloat() function discussed earlier in the chapter (68 ➤ ). A second application is in file quadratic_ans2/quadratic.go; this runs to about 160 lines and is much smarter about how it formats the output. For example, it replaces “+ -” with “-” and “1x” with “x”, suppresses zero com- ponents (e.g., eliminates “0x”), and formats a solution as a floating-point number if the imaginary part is approximately zero. It makes use of some math/cmplx package functions such as cmplx.IsNaN(), and more advanced string formatting (§3.5, ➤ 93). ptg7913109 3 Strings §3.1. Literals, Operators, and Escapes ➤ 83 §3.2. Comparing Strings ➤ 86 §3.3. Characters and Strings ➤ 87 §3.4. Indexing and Slicing Strings ➤ 90 §3.5. String Formatting with the Fmt Package ➤ 93 §3.5.1. Formatting Booleans ➤ 97 §3.5.2. Formatting Integers ➤ 98 §3.5.3. Formatting Characters ➤ 99 §3.5.4. Formatting Floating-Point Numbers ➤ 100 §3.5.5. Formatting Strings and Slices ➤ 101 §3.5.6. Formatting for Debugging ➤ 103 §3.6. Other String-Related Packages ➤ 106 §3.6.1. The Strings Package ➤ 107 §3.6.2. The Strconv Package ➤ 113 §3.6.3. The Utf8 Package ➤ 117 §3.6.4. The Unicode Package ➤ 118 §3.6.5. The Regexp Package ➤ 120 §3.7. Example: M3u2pls ➤ 130 This chapter covers Go’s string type and key string-related packages from the standard library. The chapter’s sections include coverage of how to write literal strings and use the string operators; how to index and slice (take substrings of) strings; and how to format the output of strings,numbers, and other built-in and custom types. Go’s high-level string-related functionality, such as its for … range loop which iterates over a string character by character, the functions from the strings and strconv packages,and Go’s ability to slice strings,are all that is needed for every- day programming. Nonetheless, this chapter covers Go strings in depth, includ- 81 ptg7913109 82 Chapter 3. Strings Unicode i Prior to Unicode it was not really possible to have plain text files that con- tained text in different languages—for example, English with some quoted sentences in Japanese and Russian—since separate encodings were used for separate languages and each text file used a single encoding. Unicode is designed to be able to represent the characters from all of the world’s writing systems, so a single text file using a Unicode encoding can contain text in any mixture of languages—as well as math, “dingbats”, and other special characters. Every Unicode character has a unique identifying number called a code point. There are more than 100000 Unicode characters defined, with code points ranging in value from 0x0 to 0x10FFFF (the latter defined in Go as the constant unicode.MaxRune), with some large gaps and various special cases. In Unicode documentation, code points are written using four or more hexadecimal digits in the form U+hhhh—for example, U+21D4 for the ⇔ character. In Go, an individual code point (i.e., a character) is represented by a rune in memory. (The rune type is a synonym for int32; see §2.3.1, 59 ➤ .) Unicode text—whether in files or in memory—must be represented using an encoding. The Unicode standard defines various Unicode Transformation Formats (encodings),such as UTF-8,UTF-16,and UTF-32.Go uses the UTF-8 encoding for strings. UTF-8 is the most widely used encoding; it is also the de facto standard encoding for text files and the default encoding for XML and JSON files. The UTF-8 encoding uses between one and four bytes to represent each code point. For strings that contain only 7-bit ASCII (US-ASCII) characters,there is a one-to-one relationship between bytes and characters because each 7-bit ASCII character is represented by a single byte (of the same value) in UTF-8. One consequence of this is that UTF-8 stores English text very compactly (one byte per character); another consequence is that a text file encoded using 7-bit ASCII is indistinguishable from a UTF-8-encoded text file. ing some low-level details such as how strings are represented internally. The low-level aspects are interesting and can be useful to know in some situations. A Go string is an immutable sequence of arbitrary bytes. In most casesa string’s bytes represent Unicode text using the UTF-8 encoding; (see the “Unicode” sidebar above). The use of Unicode means that Go strings can contain text in a mixture of any of the world’s languages, without any of the confusions and limitations of code pages. Go’s string type is fundamentally different from the equivalent type in many other languages. Java’s String,C++’s std::string, and Python 3’s str types are all ptg7913109 3.1. Literals,Operators,and Escapes 83 sequences of fixed-width characters (with some caveats), whereas a Go string is a sequence of variable-width characters where each character is represented by one or more bytes, normally using the UTF-8 encoding. At first sight it might appear that these other languages’ string types are more convenient than Go’s since individual characters in their strings can be direct- ly indexed—something only possible in Go if the string exclusively holds 7-bit ASCII characters (since these are all represented by a single UTF-8 byte). In practical terms this is never a problem for Go programmers:first, because direct indexing isn’t used much in Go because Go supports character-by-character iter- ation over strings; second, because the standard library provides a comprehen- sive range of string searching and manipulation functions; and third, because we can always convert a Go string into a slice of Unicode code points (of type []rune) which can be indexed directly. Go’s use of UTF-8 for its string type has several advantages compared with, say, Java or Python, both of which also have Unicode strings. Java represents strings as sequences of code points, each occupying 16 bits; Python versions 2.x to 3.2 use the same approach but using 16 or 32 bits depending on how Python is built. For English text this means that Go uses 8 bits per character compared to at least twice that for Java and Python. Another advantage of UTF-8 is that machine endianness doesn’t matter,whereas for UTF-16 and UTF-32 it is essen- tial to know the endianness (e.g., UTF-16 little-endian) to be able to decode the text correctly. In addition, since UTF-8 is the world’s de facto standard encoding for text files, while other languages must decode and encode such files to con- vert to and from their internal Unicode representations, Go can read and write such files directly. Furthermore, some major libraries (such as GTK+) use UTF- 8 strings natively, so Go can work with them without encoding or decoding. In practice,Go strings are just as convenient and easy to use as other languages’ string types—once we have learned the Go idioms for working with them. 3.1. Literals, Operators, and Escapes String literals are created using double quotes (") or backticks (`).Double quotes are used to create interpreted string literals—such strings support the escape sequences listed in Table 3.1 (➤ 84) but may not span multiple lines. Backticks are used to create raw string literals—these strings may span multiple lines; they do not support any escape sequences and may contain any character except for a backtick. Interpreted string literals are the most commonly used kind, but raw string literals are useful for writing multiline messages, HTML, and regular expressions. Here are a few examples. text1 := "\"what's that?\", he said" // Interpreted string literal text2 := `"what's that?", he said` // Raw string literal radicals := "√ \u221A \U0000221a" // radicals == "√ √ √" ptg7913109 84 Chapter 3. Strings Table 3.1 Go’s String and Character Escapes Escape Meaning \\ Backslash (\) \ooo Unicode character with the given 3-digit 8-bit octal code point \' Single quote ('); only allowed inside character literals \" Double quote ("); only allowed inside interpreted string literals \a ASCII bell (BEL) \b ASCII backspace (BS) \f ASCII formfeed (FF) \n ASCII linefeed (LF) \r ASCII carriage return (CR) \t ASCII tab (TAB) \uhhhh Unicode character with the given 4-digit 16-bit hex code point \Uhhhhhhhh Unicode character with the given 8-digit 32-bit hex code point \v ASCII vertical tab (VT) \xhh Unicode character with the given 2-digit 8-bit hex code point The three variables created here are of type string, and text1 and text2 contain exactly the same text. Since .go files use the UTF-8 encoding we can include Unicode characters in them without formality. However, we can still use Unicode escapes as we have done here for the second and third √ symbols. We could not use an octal or hexadecimal escape in this particular case, since their code point range is limited to U+0000 to U+00FF, far too small for the √ symbol’s U+221A code point value. If we want to create a long interpreted string literal without having an equally long line in our code we can create the literal in pieces, joining the pieces us- ing the + concatenation operator. Furthermore, although Go’s strings are im- mutable, they support the += append operator: This replaces the underlying string with itsconcatenation with the appended string,if the underlying string’s capacity isn’t large enough to accommodate the appended string. These opera- tors are listed in Table 3.2 (➤ 85). Strings can be compared using the compari- son operators (see Table 2.3, 57 ➤ ). Here is an example that uses some of these operators: book := "The Spirit Level" + // String concatenation " by Richard Wilkinson" book += " and Kate Pickett" // String append fmt.Println("Josey" < "José","Josey" == "José")// String comparisons ptg7913109 3.1. Literals,Operators,and Escapes 85 Table 3.2 String Operations All usesof the [] sliceoperator arefinefor stringscontaining only 7-bit ASCII characters; but care is needed for strings containing non-ASCII characters (see §3.4, ➤ 90). Strings can be compared using the standard comparison operators: <, <=, ==,!=, >=, >= (see Table 2.3, 57 ➤ and §3.2, ➤ 86.) Syntax Description/result s += t Appends string t to the end of string s s + t The concatenation of strings s and t s[n] The raw byte at index position n (of type uint8)ins s[n:m]A string taken from s from index positions n to m -1 s[n:] A string taken from s from index positions n to len(s)- 1 s[:m]A string taken from s from index positions 0 to m -1 len(s) The number of bytes in string s len([]rune(s)) The number of characters in string s—use the faster utf8. RuneCountInString() instead; see Table 3.10 (➤ 118) []rune(s) Converts string s into a slice of Unicode code points string(chars) Converts a []rune or []int32 into a string; assumes that the runesorint32s are Unicode code points★ []byte(s) Converts string s into a slice of raw bytes without copying; there’s no guarantee that the bytes are valid UTF-8 string(bytes) Converts a []byte or []uint8 into a string without copying; there’s no guarantee that the bytes are valid UTF-8 string(i) Converts i of any integer type into a string; assumes that i is a Unicode code point; e.g., if i is 65, it returns "A"★ strconv. Itoa(i) The string representation of i of type int and an error; e.g., if i is 65, it returns ("65", nil); see also Tables 3.8 and 3.9 (➤ 114–115) fmt.Sprint(x) The string representation of x of any type; e.g., if x is an integer of value 65, it returns "65"; see also Table 3.3 (➤ 94) This results in book containing the text “The Spirit Level by Richard Wilkinson and Kate Pickett”, and “true false” being output to os.Stdout. ★ The conversion always succeeds; invalid integers are converted as the Unicode replacement character U+FFFD which is often depicted as ?. ptg7913109 86 Chapter 3. Strings 3.2. Comparing Strings As we have noted, Go strings support the usual comparison operators (<, <=, ==, !=, >, >=); these are shown in Table 2.3 (57 ➤ ).The comparison operators compare strings byte by byte in memory. Comparisons are used directly—for example, to compare two strings for equality, and indirectly—for example, when < is used to compare the strings in a []string that is being sorted. Unfortunately, three problems can arise when performing comparisons—these problems afflict every programming language that uses Unicode strings; none of them is specific to Go. The first problem is that some Unicode characters can be represented by two or more different byte sequences. For example, the character Å could be the Ångström symbol or simply an A with a ring above—the two are often visually indistinguishable. The Ångström symbol’sUnicode code point is U+212B,but an A with a ring above can be represented by Unicode code point U+00C5 or by the two code points U+0041 (A) and U+030A (°; combining ring above). In terms of UTF-8 bytes the Ångström symbol (Å) is represented by the bytes [0xE2, 0x84, 0xAB], the Å character by the bytes [0xC3, 0x85],and an A with the ° combining character by the bytes [0x41, 0xCC, 0x81]. Of course, from a user’s point of view two Å charac- ters ought to compare and sort as equals no matter what the underlying bytes. This first problem isn’t necessarily as significant as we might imagine since all UTF-8 byte sequences (i.e., strings) in Go are produced using the same code point to bytes mappings. This means, for example, that an é character in a Go character or string literal will always be represented by the same bytes. And, of course, if we are only concerned with ASCII characters (i.e., English), the problem doesn’t occur at all. And even when we deal with non-ASCII characters, the problem only really arises when we have two different characters that look the same, or when we are reading UTF-8 bytes from outside our program from a source that has used code point to bytes mappings that are legal UTF-8 but which differ from Go’s mappings. If this really does turn out to be a problem it is always possible to write a custom normalization function that, for example, ensured that, say, é was always represented by the bytes [0xC3, 0xA9] (which Go uses natively) rather than, say, [0x65, 0xCC, 0x81] (i.e., an e and an ´ combining character).Normalizing Unicode charactersis explained in the Unicode Normal- ization Forms document ( At the time of this writing, the Go standard library has an experimental normalization package (exp/norm). Since this first problem can only really arise with strings coming from external sources—and then only if they use different code point to bytes mappings than Go—it is probably best handled by isolating the code that accepts external strings. The isolating code could then normalize the strings it receives before providing them to the rest of the program. ptg7913109 3.2. Comparing Strings 87 The second problem is that there are cases where our users might reasonably expect different characters to be considered equal. For example, we might write a program that providesa text search function and a user might type in the word “f ile”. Naturally, they would expect the search to find any occurrences of “f ile”; but they might also expect the search to match occurrences of “file” (i.e., an “fi” ligature followed by “le”). Similarly, users might expect a search for “5” to match “5”, “5”, 5“”, and maybe even “➄”. As with the first problem, this can be solved by using some form of normalization. The third problem is that the sorting of some characters is language-specific. One example is that in Swedish ä is sorted after z, whereas in German phone- books ä is sorted as though it were spelled ae and in German dictionaries as though it were spelled a. Another example is that although in English we sort ø as though it were o,in Danish and Norwegian it is sorted after z.Thereare lots of rules along these lines, and they can be complicated by the fact that sometimes the same application is used by people of different nationalities (who therefore expect different sorting orders), and sometimes strings are in a mixture of lan- guages (e.g., some Spanish, others English), and some characters (such as ar- rows, dingbats, and mathematical symbols) don’t really have meaningful sort positions at all. On the plus side, Go’s comparing of strings byte by byte produces an ASCII sort ordering for English. And if we lowercase or uppercase all the strings we want to compare, we can get a more natural English language ordering—as we will see in an example later (§4.2.4, ➤ 160). 3.3. Characters and Strings In Go,charactersare represented in two different (easy-to-interchange)ways. A single character can be represented by a single rune (or int32). From now on we will use the terms “character”, “code point”, “Unicode character”, and “Unicode code point” interchangeably to refer to a rune (or int32) that holds a single character. Go strings represent sequences of zero or more characters—within a string each character is represented by one or more UTF-8 encoded bytes. We can convert a single character into a one-character string using Go’s standard conversion syntax (string(char)). Here is an example. æs := "" for _, char := range []rune{'æ', 0xE6, 0346, 230,'\xE6','\u00E6'}{ fmt.Printf("[0x%X '%c'] ", char, char) æs += string(char) } ptg7913109 88 Chapter 3. Strings This will print a line containing the text “[0xE6 'æ']” repeated six times. And at the end the æs string will contain the text ææææææ.(We will see more efficient alternatives to using the string += operator in a loop in a moment.) An entire string can be converted to a slice of runes (i.e., code points) using the syntax chars := []rune(s) where s is of type string. The chars will have type []int32 since rune is a synonym for int32. This can sometimes be useful when we want to parse a string working character by character and at the same time be able to peek at characters before or after the current one. The reverse conversion is equally simple using the syntax s := string(chars) where chars is of type []rune or []int32; s will have type string. Neither conversion is free—but both are reasonably fast (O(n) where n is the number of bytes; see the sidebar “Big-O Notation”, ➤ 89). For more string conversions see Table 3.2 (85 ➤ ); for number↔string conversions see Tables 3.8 and 3.9 (➤ 114–115). Although convenient, using the string += operator is not the most efficient way to append to a string in a loop. A better approach (and one familiar to Python programmers) is to populate a slice of strings ([]string) one at a time and then concatenate them all in one go using the strings.Join() function. For Go, though, there is an even better way, similar to the way Java’s StringBuilder works. Here is an example. var buffer bytes.Buffer for { if piece, ok := getNextValidString(); ok { buffer.WriteString(piece) } else { break } } fmt.Print(buffer.String(), "\n") We begin by creating an empty bytes.Buffer. Then we write each string we want to concatenate into the buffer using its bytes.Buffer.WriteString() method. (We could, of course, write a separator between each string if we wanted to.) At the end, the bytes.Buffer.String() method can be used to retrieve the entire concatenated string. (We will see further uses of the powerful and versatile bytes.Buffer type later, e.g., ➤ 111 and ➤ 201.) Accumulating strings in a bytes.Buffer is potentially much more memory- and CPU-efficient than using the += operator, especially if the number of strings to concatenate is large. Go’s for … range loop (§5.3, ➤ 203) can be used to iterate over a string character by character, producing an index position and a code point at each iteration. Here is an example with its output beside it. ptg7913109 3.3. Characters and Strings 89 Big-O Notation i Big-O notation, O(…), is used in complexity theory to give approximate bounds for processing and for memory use for particular algorithms. Most of the measures are in proportion to n which is the number of items to process or the length of the item to process. They could be measures of memory con- sumption or of processing time. O(1) means constant time, that is, the fastest possible time no matter what n’s size. O(log n) means logarithmic time;this is very fast and in proportion to log n.O(n) means linear time; this is fast and in proportion to n.O( 2n ) means quadratic time; this is starting to get slow and is in proportion to 2n . O( mn ) means polynomial time which quickly becomes slow as n grows, especially if m ≥ 3. O(n!) means factorial time; even for small values of n this can become too slow to be practical. This book uses big-O notation in a few places to give a feel for the costs of processing, for example, the cost of converting from a string to a []rune. phrase := "vått og tørt" fmt.Printf("string: \"%s\"\n", phrase) fmt.Println("index rune char bytes") for index, char := range phrase { fmt.Printf("%-2d %U '%c' % X\n", index, char, char, []byte(string(char))) } string: "vått og tørt" index rune char bytes 0 U+0076 'v' 76 1 U+00E5 'å' C3 A5 3 U+0074 't' 74 4 U+0074 't' 74 5 U+0020 ' ' 20 6 U+006F 'o' 6F 7 U+0067 'g' 67 8 U+0020 ' ' 20 9 U+0074 't' 74 10 U+00F8 'ø' C3 B8 12 U+0072 'r' 72 13 U+0074 't' 74 We create the phrase string literal, and then we print it followed by a heading on the next line. Then we iterate over every character in the string—Go’s for … range loop decodes UTF-8 bytes into Unicode code points (runes) as it iterates, so we don’t have to concern ourselves with the underlying representation. For each character, we print its index position, its code point value (using Unicode notation), the character it represents, and the UTF-8 bytes used to encode the character. To get the list of bytes we convert the code point (char of type rune) into a string (which will contain a single character consisting of one or more UTF-8-encoded bytes). Then we convert this one-character string into a []byte, that is, a byte ptg7913109 90 Chapter 3. Strings slice, so that we can access the actual bytes. The []byte(string) conversion is very fast (O(1)) since under the hood the []byte can simply refer to the string’s underlying bytes with no copying required. The same is true of the reverse conversion, string([]byte); again the underlying bytes are not copied, so the conversion is O(1). Table 3.2 (85 ➤ ) lists Go’s string and byte conversions. The %-2d,%U,%c, and %Xformat specifiers are explained shortly (§3.5, ➤ 93). As we will see, when the %X format specifier is used for an integer it outputs the integer in hexadecimal, and when it is used for a []byte it outputs a sequence of two-digit hexadecimal numbers, one number per byte. Here we have specified that the bytes should be output space-separated by including a space in the format specifier. In practical programming using a for …range loop to iterate over the characters in a string, along with the functions from the strings and fmt packages (and to a lesser extent from the strconv, unicode, and unicode/utf8 packages), provides all the functionality needed for the powerful and convenient processing and manipulation of strings. However, in addition, the string type supports slicing (since under the hood a string is in effect an enhanced []byte), and this can be very useful—providing we are careful not to slice any multibyte characters in half! 3.4. Indexing and Slicing Strings As Table 3.2 (85 ➤ ) shows, Go supports string slicing using a subset of the syntax used by Python. This syntax can be used for slices of any type, as we will see in Chapter 4. Since Go strings store their text as UTF-8-encoded bytes we must be careful to only ever slice on character boundaries. This is easy if we have 7-bit ASCII text since every byte represents one character, but for non-ASCII text the situation is more challenging since such characters may be represented by one or more bytes. Usually we don’t need to slice strings at all but simply iterate over them character by character using a for …range loop, but in some situations we really do want to extract substrings using slicing. One way to be sure to use slice indexes that slice on character boundaries is to use functions from Go’s strings package, such as strings.Index() or strings.LastIndex(). The strings package’s functions are listed in Tables 3.6 and 3.7 (➤ 108–109). We will begin by looking at the different ways we can perceive a string. Index positions—which are the positions of the string’s UTF-8 bytes—begin at 0 and go up to the length of the string minus 1. It is also possible to index back from the end of the slice using indexes of the form len(s)-n where n is the number of bytes counting back from the end. For example, given the assignment s:= "naïve", Figure 3.1 shows string s as Unicode characters, code points, and bytes, as well as some valid index positions and a couple of slices. ptg7913109 3.4. Indexing and Slicing Strings 91 s[:2] s[2:] == s[len(s)-4:] Slices 'n' 'a' 'ï' 'v' 'e' Characters U+006E U+0061 U+00EF U+0076 U+0065 Code points 0x6E 0x61 0xC3 0xAF 0x76 0x65 Bytes 0 1 2 3 4 5 len(s)-2 len(s)-1 Indexes Figure 3.1 Anatomy of a string Each index position shown in Figure 3.1 can be used with the [] index operator to return the corresponding ASCII character (as a byte)—for example, s[0] == 'n' and s[len(s)- 1] == 'e'. The index position of the start of the ï character is 2, but if we used s[2] we would simply get the first of the UTF-8 bytes used to encode ï (0xC3); such bytes are rarely what we want. For strings that contain only 7-bit ASCII characters we can extract the first character (as a byte) using the syntax s[0], and the last character using s[len(s) -1].However,in general we should use utf8.DecodeRuneInString() to get the first character (as a rune, along with the number of UTF-8 bytes used to represent it), and utf8.DecodeLastRuneInString() to get the last character. (See Table 3.10, ➤ 118.) If we really need to index individual characters, a couple of options are open to us. For strings that contain only 7-bit ASCII we can simply use the [] index operator which gives us very fast (O(1)) lookups. For non-ASCII strings we can convert the string to a []rune and use the [] index operator. This delivers very fast (O(1)) lookup performance, but at the expense of the one-off conversion which costs both CPU and memory (O(n)). In the case of our example, if we wrote chars := []rune(s), the chars variable would be created as a rune (i.e., int32) slice with the five code points—compared with six bytes—shown in Figure 3.1. Recall that we can easily convert any rune (code point) back to a string—containing one character—using the string(char) syntax. For arbitrary strings (i.e., those that might contain non-ASCII characters), ex- tracting characters by index is rarely the right approach. Much better is to use string slicing—which also has the convenience of returning a string rather than a byte. To safely slice arbitrary strings, it is best to find the index position where we want to slice up to or from using one of the strings package’s functions—see Tables 3.6 and 3.7 (➤ 108–109). The following equality holds for string slices—and, in fact, for slices of every kind: s == s[:i] + s[i:] // s is a string; i is an int; 0 <= i <= len(s) ptg7913109 92 Chapter 3. Strings Now let’s look at a real slice example, one that takes a rather naïve approach. Suppose we have a line of text and want to extract the line’s first and last words. One simple way to write the code is like this: line := "røde og gule sløjfer" i := strings.Index(line, "")// Get the index of the first space firstWord := line[:i] // Slice up to the first space j := strings.LastIndex(line, "")// Get the index of the last space lastWord := line[j+1:] // Slice from after the last space fmt.Println(firstWord, lastWord) // Prints: røde sløjfer The firstWord (of type string) is assigned the bytes from the line from index po- sition 0 (the first byte) to index position i-1(i.e., up to the last byte before the space) since string slices go up to but exclude the end index position. Similarly, the lastWord is assigned the bytesfrom the line from index position j+1(the byte after the space), to the end of the line’s bytes (i.e., to index position len(line) - 1). Although this example’s approach is fine for spaces and would also work for oth- er 7-bit ASCII characters, it isn’t suitable for working with arbitrary Unicode whitespace characters such as U+2028 (Line Separator, L S) or U+2029 (Paragraph Separator, P S). Here is how to find the first and last wordsof a string no matter what whitespace characters are used to separate the words. line := "rå tørt\u2028vær" i := strings.IndexFunc(line, unicode.IsSpace) // i == 3 firstWord := line[:i] j := strings.LastIndexFunc(line, unicode.IsSpace) // j == 9 _, size := utf8.DecodeRuneInString(line[j:]) // size == 3 lastWord := line[j+size:] // j + size == 12 fmt.Println(firstWord, lastWord) // Prints: rå vær The line string is shown as characters, code points, and bytes in Figure 3.2; the figure also shows the byte index positions and the slices used in the code snippet. The strings.IndexFunc() function returns the first index position in the string given as its first argument where the function given as its second argument (with signature func(rune) bool) returns true. The strings.LastIndexFunc() does the same except that it works from the end of the string and returns the last in- dex position for which the function returns true. Here we pass the unicode pack- age’s IsSpace() function as the second argument;this function accepts a Unicode code point (of type rune) as its sole argument and returns true if the code point is of a whitespace character. (See Table 3.11, ➤ 119.) A function’s name is a ref- ptg7913109 3.4. Indexing and Slicing Strings 93 line[j:] line[:i] line[j+size:] Slices 'r' 'å' '''t' 'ø' 'r' 't' 'L S''v' 'æ' 'r' Charac- ters U+0072 U+00E5 U+0020 U+0074 U+00F8 U+0072 U+0074 U+2028 U+0076 U+00E6 U+0072 Code points 0x72 0xC3 0xA5 0x20 0x74 0xC3 0xB8 0x72 0x74 0xE2 0x80 0xA8 0x76 0xC3 0xA6 0x72 Bytes 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Indexes Figure 3.2 Anatomy of a string with whitespace erence to the function, and so can be passed wherever a function parameter is required—so long as the named (i.e., referred to) function’s signature matches that specified by the parameter. (See §4.1, ➤ 140.) Using the strings.IndexFunc() function to find the first whitespace character and slicing the string up to but excluding that character to get the first word is easy. But when searching for the last whitespace character we must be careful because some whitespace characters are encoded as more than a single UTF-8 byte. We solve this problem by using the utf8.DecodeRuneInString() function to give us the number of bytes occupied by the first character in the slice of the string that starts where the last whitespace character begins. We then add this number to the last whitespace character’s index position to jump over the last whitespace character—however many bytes are used to represent it—so that we slice only the last word. 3.5. String Formatting with the Fmt Package Go’s standard library’s fmt package provides print functions for writing data as strings to the console,to files and other values satisfying the io.Writer interface, and to other strings. These functions are listed in Table 3.3 (➤ 94). Some of the print functions return an error. It is not uncommon to ignore this return value when printing to the console,but the error should always be checked when printing to files, network connections, and so on.★ The fmt package also provides various scan functions (such as fmt.Scan(), fmt.Scanf(), and fmt.Scanln()) for reading data from the console, from files, and from strings. Some of these functions are used in Chapter 8 (§, ★ Go also has two built-in print functions, print() and println(). These should not be used; they exist purely for the convenience of Go compiler implementers and may be removed from the language. ptg7913109 94 Chapter 3. Strings ➤ 380)—see also, Table 8.2 (➤ 383). An alternative to using the scan functions is to split each string into fields using the strings.Fields() function and then convert those that aren’t strings to values (e.g., numbers) using functions from the strconv package—see Tables 3.8 and 3.9 (➤ 114–115). Recall from Chapter 1 that we can read input typed at the keyboard by creating a bufio.Reader to read from os.Stdin and use the bufio.Reader.ReadString() function to read each line entered (§1.7, 40 ➤ ). The easiest way to output values is to use the fmt.Print() and fmt.Println() functions (to print to os.Stdout, i.e., to the console), or the fmt.Fprint() and fmt.Fprintf() functions to output to a given io.Writer (e.g., to a file), or the fmt.Sprint() and fmt.Sprintln() functions to output to a string. Table 3.3 The Fmt Package’s Print Functions Syntax Description/result fmt.Errorf(format, args...) Returns an error value containing a string created with the format string and the args fmt.Fprint(writer, args...) Writes the args to the writer each using format %v and space-separating nonstrings; returns the number of bytes written, and an error or nil fmt.Fprintf(writer, format, args...) Writes the args to the writer using the format string; re- turns the number of bytes written, and an error or nil fmt.Fprintln( writer, args...) Writes the args to the writer each using format %v, space-separated and ending with a newline; returns the number of bytes written, and an error or nil fmt.Print(args...) Writes the args to os.Stdout each using format %v and space-separating nonstrings; returns the number of bytes written, and an error or nil fmt.Printf(format, args...) Writes the args to os.Stdout using the format string; re- turns the number of bytes written, and an error or nil fmt.Println( args...) Writes the args to os.Stdout each using format %v, space- separated and ending with a newline; returns the num- ber of bytes written, and an error or nil fmt.Sprint(args...) Returns a string of the args, each formatted using format %v and space-separating nonstrings fmt.Sprintf(format, args...) Returns a string of the args formatted using the format string fmt.Sprintln( args...) Returns a string of the args, each formatted using format %v, space-separated and ending with a newline ptg7913109 3.5. String Formatting with the Fmt Package 95 Table 3.4 The Fmt Package’s Verbs Verbs are mostly used to output single values. If a value is a slice the output is usually a square bracket enclosed sequence of space-separated values, with each value formatted as the verb specifies. If the value is a map only %v or %#v may be used—unless the key and value are of the same type,in which case type-compatible verbs may also be used. Verb Description/result %% A literal % character %b An integer value as a binary (base 2) number, or (advanced) a floating- point number in scientific notation with a power of 2 exponent %c An integer code point value as a Unicode character %d An integer value as a decimal (base 10) number %e A floating-point or complex value in scientific notation with e %E A floating-point or complex value in scientific notation with E %f A floating-point or complex value in standard notation %g A floating-point or complex value using %e or %f, whichever produces the most compact output %G A floating-point or complex value using %E or %f, whichever produces the most compact output %o An integer value as an octal (base 8) number %p A value’s address as a hexadecimal (base 16) number with a prefix of 0x and using lowercase for the digits a–f (for debugging) %q The string or []byte as a double-quoted string, or the integer as a single- quoted string, using Go syntax and using escapes where necessary %s The string or []byte as raw UTF-8 bytes; this will produce correct Unicode output for a text file or on a UTF-8-savvy console %t A bool value as true or false %T A value’s type using Go syntax %U An integer code point value using Unicode notation defaulting to four digits; e.g., fmt.Printf("%U", '¶') outputs U+00B6 %v A built-in or custom type’s value using a default format, or a custom value using its type’s String() method if it exists %x An integer value as a hexadecimal (base 16) number or a string or []byte value as hexadecimal digits (two per byte), using lowercase for the digits a–f %X An integer value as a hexadecimal (base 16) number or a string or []byte value as hexadecimal digits (two per byte), using uppercase for the digits A–F ptg7913109 96 Chapter 3. Strings Table 3.5 The Fmt Package’s Verb Modifiers Modifier Description/result space Makes the verb output “-” before negative numbers and a space before positive numbers or to put spaces between the bytes printed when using the %x or %X verbs; e.g., fmt.Printf("% X", "←") outputs E2 86 92 # Makes the verb use an “alternative” output format: %#o outputs octal with a leading 0 %#p outputs a pointer without the leading 0x %#q outputs a string or []byte as a raw string (using backticks) if possible—otherwise outputs a double-quoted string %#v outputs a value as itself using Go syntax %#x outputs hexadecimal with a leading 0x %#X outputs hexadecimal with a leading 0X + Makes the verb output + or - for numbers, ASCII characters (with others escaped) for strings, and field names for structs - Makes the verb left-justify the value (the default is to right-justify) 0 Makes the verb pad with leading 0s instead of spaces n.m n .m For numbers, makes the verb output a floating-point or complex value using n (of type int) characters (or more if necessary to avoid truncation) and with m (of type int) digits after the decimal point(s). For strings n specifies the minimum field width, and will result in space padding if the string has too few characters,and .m specifies the maximum number of the string’s characters to use (going from left to right), and will result in the string being truncated if it is too long. Either or both of m and n can be replaced with * in which case their values are taken from the arguments. Either n or .m may be omitted. type polar struct{ radius, θ float64 } p := polar{8.32,.49} fmt.Print(-18.5, 17,"Elephant",-8+.7i, 0x3C7,'\u03C7',"a","b",p) fmt.Println() fmt.Println(-18.5, 17,"Elephant",-8+.7i, 0x3C7,'\u03C7',"a","b",p) -18.5·17Elephant(-8+0.7i)·967·967ab{8.32·0.49} -18.5·17·Elephant·(-8+0.7i)·967·967·a·b·{8.32·0.49} For the sake of clarity,particularly when multiple consecutivespacesare output, we have put a light gray character (·) in the middle of every space shown. The way that fmt.Print() and fmt.Fprint() handle whitespace is subtly different from the fmt.Println() and fmt.Fprintln() functions. As a rule of thumb the former are most useful for printing a single value or for “converting” a value to a ptg7913109 3.5. String Formatting with the Fmt Package 97 string without error checking (use the strconv packages for proper conversions; ➤ 114–115),since they only output spaces between nonstring values. The latter are better for printing multiple values, since they output a space between each value and add a newline at the end. Under the hood these functions use the %v (general value) format specifier—and they can print any built-in or custom value without formality. For example, the print functions know nothing about the custom polar type but still manage to print a polar value successfully. In Chapter 6 we will see how to provide a String() method for custom types; this allows us to output them however we like. If we want to exercise similarly fine control over the printing of built-in types we can use the print functions that accept a format string as their first argument. The format string used by the fmt.Errorf(), fmt.Printf(), fmt.Fprintf(), and fmt.Sprintf() functions consists of one or more verbs—these are format speci- fiers of the form %ML where M stands for one or more optional verb modifiers and L stands for a particular verb letter. The verbs are listed in Table 3.4 (95 ➤ ).Some of the verbscan accept one or more modifiers;the modifiersare listed in Table 3.5 (96 ➤ ). We will now review some representative examples of format strings so that we can get a clear understanding of how they work. In each case we will show a tiny code snippet and then the output it produces.★ 3.5.1. Formatting Booleans Boolean values are output using the %t (truth value) verb. fmt.Printf("%t %t\n", true, false) true·false If we want to output Booleans as integers we must do the conversion our- selves: fmt.Printf("%d %d\n", IntForBool(true), IntForBool(false)) 1·0 This makes use of a tiny custom function. func IntForBool(b bool) int { if b{ ★ C, C++, and Python 2 programmers will find Go’s format strings familiar—but with some subtle differences. For example, Go’s %d can be used for any integer regardless of its size or signedness. ptg7913109 98 Chapter 3. Strings return 1 } return 0 } We can convert a string back to a Boolean using the strconv.ParseBool() func- tion. And, of course, there are similar functions for converting strings to num- bers. (See §3.6.2, ➤ 113.) 3.5.2. Formatting Integers Now we will look at the formatting of integers, starting with binary (base 2) output. fmt.Printf("|%b|%9b|%-9b|%09b|% 9b|\n", 37, 37, 37, 37, 37) |100101|···100101|100101···|000100101|···100101| The first format (%b) uses the %b (binary) verb and outputs an integer as a binary number using as few digits as possible. The second format (%9b) specifies a width of 9 characters (which will be exceeded if necessary to avoid truncation), and uses the default right justification. The third format (%-9b) uses the - modifier to get left justification. The fourth format (%09b) uses 0 padding and the fifth format (%9b) uses space padding. Octal formatting is similar to binary, but also supports an alternative format. It uses the %o (octal) verb. fmt.Printf("|%o|%#o|%# 8o|%#+ 8o|%+08o|\n", 41, 41, 41, 41,-41) |51|051|·····051|····+051|-0000051| The alternative format is switched on by using the # modifier and causes a leading 0 to be output. The + modifier forces the sign to be output—without it, positive numbers are output without a sign. Hexadecimal formatting uses the %x and %X (hexadecimal) verbs, the choice of which specifies whether to use lowercase or uppercase letters for the digits A–F. i:=3931 fmt.Printf("|%x|%X|%8x|%08x|%#04X|0x%04X|\n",i,i,i,i,i,i) |f5b|F5B|·····f5b|00000f5b|0X0F5B|0x0F5B| For hexadecimal numbers the alternate format modifier (#) causes a leading 0x or 0X to be output. As with all numbers, if we specify a width that is wider ptg7913109 3.5. String Formatting with the Fmt Package 99 than needed, extra spaces are output to right-justify the number in the given width—and if the width is too small the number is output in its entirety, so there’s no risk of digits being truncated. Decimal integers are output using the %d (decimal) verb. The only characters that can be used for padding are spaces and zeros,but it is easy to pad with other characters using a custom function. i=569 fmt.Printf("|$%d|$%06d|$%+06d|$%s|\n", i, i, i, Pad(i, 6,'*')) |$569|$000569|$+00569|$***569| For the last format we use the %s (string) verb to print a string since that’s what our Pad() function returns. func Pad(number, width int, pad rune) string { s := fmt.Sprint(number) gap := width - utf8.RuneCountInString(s) if gap > 0 { return strings.Repeat(string(pad), gap) + s } return s } The utf8.RuneCountInString() function returns the number of characters in the given string; this is always less than or equal to the number of bytes. The strings.Repeat() function takes a string and a count and returns a new string that contains the given string repeated count times. We chose to pass the padding character as a rune (i.e., as a Unicode code point) to avoid users of the function passing a string which might contain more than one character. 3.5.3. Formatting Characters Go characters are runes (i.e., int32s), and they can be output as numbers or as Unicode characters. fmt.Printf("%d %#04x %U '%c'\n", 0x3A6, 934,'\u03A6','\U000003A6') 934·0x03a6·U+03A6·'Φ' Here we have output the Greek capital letter Phi (‘Φ’) as decimal and hexadeci- mal integers, as a Unicode code point using the %U (Unicode) verb, and as a Uni- code character using the %c (character or code point) verb. ptg7913109 100 Chapter 3. Strings 3.5.4. Formatting Floating-Point Numbers For floating-point numbers we can specify the overall width, the number of dig- its after the decimal place—and whether to use standard or scientific notation. for _, x := range []float64{-.258, 7194.84,-60897162.0218, 1.500089e-8}{ fmt.Printf("|%20.5e|%20.5f|%s|\n", x, x, Humanize(x, 20, 5,'*',',')) } |········-2.58000e-01|············-0.25800|************-0.25800| |·········7.19484e+03|··········7194.84000|*********7,194.84000| |········-6.08972e+07|·····-60897162.02180|***-60,897,162.02180| |·········1.50009e-08|·············0.00000|*************0.00000| Here we have used a for …range loop to iterate over the numbers in a slice literal of float64 items. The custom Humanize() function returns a string representation of the number it is given with grouping separators (for languages that use simple three-digit groups) and padding. func Humanize(amount float64, width, decimals int, pad, separator rune) string { dollars, cents := math.Modf(amount) whole := fmt.Sprintf("%+.0f", dollars)[1:] // Strip "±" fraction := "" if decimals > 0 { fraction = fmt.Sprintf("%+.*f", decimals, cents)[2:] // Strip "±0" } sep := string(separator) for i:=len(whole) - 3;i>0;i-=3 { whole = whole[:i] + sep + whole[i:] } if amount < 0.0 { whole = "-" + whole } number := whole + fraction gap := width - utf8.RuneCountInString(number) if gap > 0 { return strings.Repeat(string(pad), gap) + number } return number } The math.Modf() function returns the whole and fractional parts of a float64 as two float64s. To get the whole part as a string we use the fmt.Sprintf() function ptg7913109 3.5. String Formatting with the Fmt Package 101 with a format that forces the sign to be output and then we immediately slice the string to strip off the sign. We use a similar technique for the fractional part,only this time we use the .m verb modifier specifying the number of decimal digits to use with a * placeholder. (So in this case, if decimals has the value 2 the format effectively becomes %+.2f.) For the fractional part we strip off the leading -0 or +0. The grouping separators are inserted from right to left in the whole string and then a - sign is added if the number is negative. At the end we concatenate the whole and fractional parts and return the result—padding if required. The %e,%E,%f,%g, and %G verbs can be used with complex numbers as well as with floating-point numbers. The %e and %E are the scientific format (exponen- tial) verbs, %f is the floating-point verb, and %g and %G are the general floating- point verbs. One factor to keep in mind, though, is that the modifiers are applied to both the real and imaginary parts of complex numbers individually—for example, a format of %6f will produce a result occupying at least 20 characters if the argument is a complex number. for _, x := range []complex128{2 + 3i, 172.6 - 58.3019i, -.827e2 + 9.04831e-3i}{ fmt.Printf("|%15s|%9.3f|%.2f|%.1e|\n", fmt.Sprintf("%6.2f%+.3fi", real(x), imag(x)), x, x, x) } |····2.00+3.000i|(····2.000···+3.000i)|(2.00+3.00i)|(2.0e+00+3.0e+00i)| |·172.60-58.302i|(··172.600··-58.302i)|(172.60-58.30i)|(1.7e+02-5.8e+01i)| |··-82.70+0.009i|(··-82.700···+0.009i)|(-82.70+0.01i)|(-8.3e+01+9.0e-03i)| For the first column of complex numbers we wanted the components to have different numbers of digits after the decimal place. To achieve this we formatted the real and imaginary parts individually using fmt.Sprintf(), and then output the result formatted as a string using a format of %15s. For the other columns we used the %f and %e verbs directly—these always put parentheses around complex numbers. 3.5.5. Formatting Strings and Slices Strings can be output with a minimum field width (which the print functions will pad with spaces if the string is too short), and with a maximum number of characters(which will result in truncation for any string that’s too long). Strings can be output as Unicode (i.e., characters), or as a sequence of code points (i.e., runes) or as the UTF-8 bytes that represent them. ptg7913109 102 Chapter 3. Strings slogan := "End Óréttlæti♥" fmt.Printf("%s\n%q\n%+q\n%#q\n", slogan, slogan, slogan, slogan) End Óréttlæti♥ "End Óréttlæti♥" "End \u00d3r\u00e9ttl\u00e6ti\u2665" `End Óréttlæti♥` The %s verb is used to print strings; we will return to it in a moment. The %q (quoted string) verb is used to print a string as a Go double-quoted string with printable charactersprinted literally,and with all other charactersoutput using escapes (see Table 3.1, 84 ➤ ). If the + modifier is used, only ASCII characters (U+0020 to U+007E) are printed literally, with the rest output using escapes. If the # modifier is used the output is a Go raw string where possible, and a double- quoted string otherwise. Although normally the variable corresponding to a verb is a single value of a compatible type (e.g., an int for the %d verb or for the %x verb), the variable can also be a slice—or a map, providing the map’s key and value are both compatible with the verb (e.g., both strings or both numbers). chars := []rune(slogan) fmt.Printf("%x\n%#x\n%#X\n", chars, chars, chars) [45·6e·64·20·d3·72·e9·74·74·6c·e6·74·69·2665] [0x45·0x6e·0x64·0x20·0xd3·0x72·0xe9·0x74·0x74·0x6c·0xe6·0x74·0x69·0x2665] [0X45·0X6E·0X64·0X20·0XD3·0X72·0XE9·0X74·0X74·0X6C·0XE6·0X74·0X69·0X2665] Here we print a slice of runes—in this example, a slice of code points—as a se- quence of hexadecimal numbers,one per code point, using the %x and %X verbs. If the # modifier is used it forces a leading 0x or 0X to be output for each number. For most types, slices of the type are output as a square bracket enclosed sequence of space-separated items. An exception is []byte where no brackets or spaces are output unless we use the %v verb. bytes := []byte(slogan) fmt.Printf("%s\n%x\n%X\n% X\n%v\n", bytes, bytes, bytes, bytes, bytes) End·Óréttlæti♥ 456e6420c39372c3a974746cc3a67469e299a5 456E6420C39372C3A974746CC3A67469E299A5 45·6E·64·20·C3·93·72·C3·A9·74·74·6C·C3·A6·74·69·E2·99·A5 [69·110·100·32·195·147·114·195·169·116·116·108·195·166·116·105·226·153·165] ptg7913109 3.5. String Formatting with the Fmt Package 103 A slice of bytes—here, the UTF-8 bytes that represent a string—can be printed as a sequence of two-digit hexadecimal numbers, one per byte. If we use the %s verb the bytes are assumed to be UTF-8-encoded Unicode and are printed as a string. There is no alternative hexadecimal format for []bytes, but the numbers can be space-separated as the penultimate output line illustrates. The %v verb outputs []bytes as a square bracket enclosed sequence of space-separated decimal values. Go right-aligns by default; we can left-align using the - modifier. And, of course, we can specify a minimum field width and the maximum number of characters to output as the next two examples illustrate. s:="Dare to be naïve" fmt.Printf("|%22s|%-22s|%10s|\n",s,s,s) |······Dare·to·be·naïve|Dare·to·be·naïve······|Dare·to·be·naïve| In this snippet, the third format (%10s) specifies a minimum field width of 10 characters, but since the string is longer than this—and the field width is a minimum—the string is printed in full. i := strings.Index(s, "n") fmt.Printf("|%.10s|%.*s|%-22.10s|%s|\n",s,i,s,s,s) |Dare·to·be|Dare·to·be·|Dare·to·be············|Dare·to·be·naïve| Here, the first format (%.10s) specifies that a maximum of 10 characters from the string may be output, so in this case the string is truncated to the specified width. The second format (%.*s) expects to get two arguments—the maximum number of characters to print and a string;here we have used the index position of the string’s n character for the maximum which means that all the characters up to but excluding that character are printed. The third format (%-22.10s) specifies both a minimum field width of 22 characters and a maximum number of characters to print of 10 characters—this means that only the string’s first 10 characters are printed, but in a field that is 22 characters wide. Since the field width is greater than the number of characters to print, the field is padded with spaces—and left-justified because of the - modifier. 3.5.6. Formatting for Debugging The %T (type) verb is used to print a built-in or custom value’s type, and the %v verb is used to print a built-in value’s value. In fact, %v can also print the value of custom types, using a default format for types that do not have a String() method defined, or using the type’s String() method if it has one. ptg7913109 104 Chapter 3. Strings p := polar{-83.40, 71.60} fmt.Printf("|%T|%v|%#v|\n",p,p,p) fmt.Printf("|%T|%v|%t|\n", false, false, false) fmt.Printf("|%T|%v|%d|\n", 7607, 7607, 7607) fmt.Printf("|%T|%v|%f|\n", math.E, math.E, math.E) fmt.Printf("|%T|%v|%f|\n", 5+7i, 5+7i, 5+7i) s:="Relativity" fmt.Printf("|%T|\"%v\"|\"%s\"|%q|\n",s,s,s,s) |main.polar|{-83.4·71.6}|main.polar{radius:-83.4,·θ:71.6}| |bool|false|false| |int|7607|7607| |float64|2.718281828459045|2.718282| |complex128|(5+7i)|(5.000000+7.000000i)| |string|"Relativity"|"Relativity"|"Relativity"| This example shows how to output an arbitrary value’s type and value using %T and %v. If the %v verb’s formatting is satisfactory we can simply use fmt.Print() and similar functions since these use the %v verb’s format by default. Using the # alternative format verb modifier with %v affects only struct types and causes them to be output with their type name and field names. For floating-point values, %v formats like the %g verb rather than like the %f verb. The %T format is mostly useful for debugging and includes the package name (in this case main) for custom types. Using the %q verb for stringsputsthem in quotes which is often convenient when debugging. Two of Go’s types have synonyms: byte for uint8 and rune for int32. Use int32 when handling 32-bit signed integers where int won’t do (e.g., reading/writing binary files), and rune for Unicode code points (characters). s:="Alias↔Synonym" chars := []rune(s) bytes := []byte(s) fmt.Printf("%T: %v\n%T: %v\n", chars, chars, bytes, bytes) []int32: [65 108 105 97 115 8596 83 121 110 111 110 121 109] []uint8: [65 108 105 97 115 226 134 148 83 121 110 111 110 121 109] As the code snippet illustrates, the %T verb always prints the original type name, not the synonym. Since the string has a non-ASCII character it is clear that we have a slice of runes (code points) and a slice of UTF-8-encoded bytes. Go can also output any value’s address in memory using the %p (pointer) verb. i:=5 f:=-48.3124 ptg7913109 3.5. String Formatting with the Fmt Package 105 s:="Tomás Bretón" fmt.Printf("|%p → %d|%p → %f|%#p → %s|\n",&i, i, &f, f, &s, s) |0xf840000300·→·5|0xf840000308·→·-48.312400|f840001990·→·Tomás·Bretón| The & address of operator is explained in the next chapter (§4.1, ➤ 140). If we use the %p verb with the # modifier, the address’s leading 0x is dropped. Outputting memory addresses like this can be useful when debugging. Go’s ability to output slices and maps is also useful for debugging, as is the ability to output channels—that is, the type that can be sent and received through the channel and the channel’s memory address. fmt.Println([]float64{math.E, math.Pi, math.Phi}) fmt.Printf("%v\n",[]float64{math.E, math.Pi, math.Phi}) fmt.Printf("%#v\n",[]float64{math.E, math.Pi, math.Phi}) fmt.Printf("%.5f\n",[]float64{math.E, math.Pi, math.Phi}) [2.718281828459045·3.141592653589793·1.618033988749895] [2.718281828459045·3.141592653589793·1.618033988749895] []float64{2.718281828459045,·3.141592653589793,·1.618033988749895} [2.71828·3.14159·1.61803] Using the unmodified %v verb, slices are output as square bracket enclosed sequences of space-separated items. Usually we output them using functions like fmt.Print() or fmt.Sprint(), but if we use a formatting output function then the usual verb to use is %v or %#v. However, we can also use a type-compatible verb such as %f for floating-point numbers or %s for strings. fmt.Printf("%q\n",[]string{"Software patents","kill","innovation"}) fmt.Printf("%v\n",[]string{"Software patents","kill","innovation"}) fmt.Printf("%#v\n",[]string{"Software patents","kill","innovation"}) fmt.Printf("%17s\n",[]string{"Software patents","kill","innovation"}) ["Software·patents"·"kill"·"innovation"] [Software·patents·kill·innovation] []string{"Software·patents",·"kill",·"innovation"} [·Software·patents··············kill········innovation] Using the %q verb for outputting slices of strings is particularly useful when the strings contain spaces since it makes each individual string identifiable— something that doesn’t happen if we use the %v verb. The last output might look wrong at first sight since it occupies 53 characters (not including the enclosing square brackets) rather than 51 (three strings of ptg7913109 106 Chapter 3. Strings 17 characters, none of which is too big). The apparent discrepancy is due to the space separator that is output between each slice item. In addition to debugging, the %#v verb may be useful when generating Go code programmatically. fmt.Printf("%v\n", map[int]string{1:"A", 2:"B", 3:"C", 4:"D"}) fmt.Printf("%#v\n", map[int]string{1:"A", 2:"B", 3:"C", 4:"D"}) fmt.Printf("%v\n", map[int]int{1: 1, 2: 2, 3: 4, 4: 8}) fmt.Printf("%#v\n", map[int]int{1: 1, 2: 2, 3: 4, 4: 8}) fmt.Printf("%04b\n", map[int]int{1: 1, 2: 2, 3: 4, 4: 8}) map[4:D·1:A·2:B·3:C] map[int]·string{4:"D",·1:"A",·2:"B",·3:"C"} map[4:8·1:1·2:2·3:4] map[int]·int{4:8,·1:1,·2:2,·3:4} map[0100:1000·0001:0001·0010:0010·0011:0100] Maps are output as the word “map”, and then the map’s key–value pairs (in an arbitrary order since maps are unordered). Just as with slices it is possible to use verbs other than %v—but only if both the key and value are compatible with the verb used, as in the example’s last statement. (Maps and slices are covered in detail in Chapter 4.) The fmt package’s print functions are very versatile and can be used to print whatever output we need. The only feature not offered by the package’s func- tions is padding with a particular character (other than zeros or spaces), but as we saw in the custom Pad() (99 ➤ ) and Humanize() (100 ➤ ) functions, this is very easy to do. 3.6. Other String-Related Packages Go’s considerable support for stringsdoesn’t stop at indexing and slicing,or with the versatile fmt package’s functions. The strings package in particular provides very rich functionality, and the strconv, unicode/utf8, and unicode packages also provide lots of useful functions. Examples that make use of functionality from all these packages are presented in this section. Regular expressions—provided by the powerful regexp package introduced later in this section—are used in several examples throughout the book. There are other packages in the standard library that provide string-related functionality, and some of them are covered elsewhere in the book either in examples or in exercises. ptg7913109 3.6. Other String-Related Packages 107 3.6.1. The Strings Package A common requirement in string processing is to be able to split a string into a slice of separate strings and then do further processing—for example, convert strings to numbers or trim whitespace. To get a flavor of how to use some of the strings package’s functions we will review some tiny examples that show some of the functions in use. All the package’s functions are listed in Tables 3.6 and 3.7 (➤ 108–109). Let’s start with splitting strings. names := "Niccolò•Noël•Geoffrey•Amélie••Turlough•José" fmt.Print("|") for _, name := range strings.Split(names, "•"){ fmt.Printf("%s|", name) } fmt.Println() |Niccolò|Noël|Geoffrey|Amélie||Turlough|José| Here we have a bullet-separated list of names (including one blank field) which we split using the strings.Split() function. This function takes a string to split and a separator string to split on and does as many splits as possible. (If we want to limit the number of splits we can use the strings.SplitN() function instead.) If we used the strings.SplitAfter() function the output would look like this: |Niccolò•|Noël•|Geoffrey•|Amélie•|•|Turlough•|José| The strings.SplitAfter() function performs the same splits as the strings. Split() function but keeps the separator. There is also a strings.SplitAfterN() function for when we want to split a specific number of times. If we need to be able to split on any of two or more different characters we can use the strings.FieldsFunc() function. for _, record := range []string{"László Lajtha*1892*1963", "Édouard Lalo\t1823\t1892","José Ángel Lamas|1775|1814"}{ fmt.Println(strings.FieldsFunc(record, func(char rune) bool { switch char { case '\t','*','|': return true } return false })) } ptg7913109 108 Chapter 3. Strings Table 3.6 The Strings Package’s Functions #1 Variables s and t are of type string, xs is of type []string, i is of type int, and f is a function with the signature func(rune) bool.Indexpositions are of the first UTF-8 byte of the matching Unicode code point (character) or string,or -1 when there isn’t a match. Syntax Description/result strings.Contains(s, t) true if t occurs in s strings.Count(s, t) How many (nonoverlapping) times t occurs in s strings.EqualFold(s, t) true if the strings are case-insensitively equal strings.Fields(s) The []string that results in splitting s on white- space strings. FieldsFunc(s, f) The []string that results in splitting s at every character where f returns true strings.HasPrefix(s, t) true if s starts with t strings.HasSuffix(s, t) true if s ends with t strings.Index(s, t) The index of the first occurrence of t in s strings.IndexAny(s, t) The first index in s of any character that is in t strings. IndexFunc(s, f) The index of the first character in s for which f returns true strings. IndexRune(s, char) The index of the first occurrence of character char of type rune in s strings.Join(xs, t) A string containing the concatenation of all the strings in xs, each separated by t (which can be "") strings.LastIndex(s, t) The index of the last occurrence of t in s strings. LastIndexAny(s, t) The last index in s of any character that is in t strings. LastIndexFunc(s, f) The index of the last character in s for which f returns true strings.Map(mf, t) A copy of t with every character replaced or delet- ed according to the mapping function mf with the signature func(rune) rune (see text) strings.NewReader(s)A pointer to a value that provides Read(), Read- Byte(), and ReadRune() methods that operate on s strings.NewReplacer(...) A pointer to a value that has methods for replacing each pair of old, new strings it is given strings.Repeat(s, i)A string consisting of i concatenations of s ptg7913109 3.6. Other String-Related Packages 109 Table 3.7 The Strings Package’s Functions #2 Variable r of type unicode.SpecialCase is used to specify Unicode rules (advanced). Syntax Description/result strings.Replace(s, old, new, i) A copy of s with every nonoverlapping occurrence of string old replaced by string new if i is -1, or with at most i replacements otherwise strings.Split(s, t) The []string that results in splitting s on t as many times as t occurs in s strings. SplitAfter(s, t) Works like strings.Split() only the separator is kept in the resultant strings (see text) strings. SplitAfterN(s, t, i) Works like strings.SplitN() only the separator is kept in the resultant strings strings.SplitN(s, t, i) The []string that results in splitting s on t, i - 1 times strings.Title(s)A copy of s with the first letter of every word title- cased strings.ToLower(s)A lowercased copy of s strings. ToLowerSpecial(r, s) A lowercased copy of s, prioritizing the rules in r (advanced) strings.ToTitle(s)A title-cased copy of s strings. ToTitleSpecial(r, s) A title-cased copy of s, prioritizing the rules in r (advanced) strings.ToUpper(s) An uppercased copy of s strings. ToUpperSpecial(r, s) An uppercased copy of s, prioritizing the rules in r (advanced) strings.Trim(s, t)A copy of s with the characters in t removed from both ends strings. TrimFunc(s, f) A copy of s with the characters for which f returns true removed from both ends strings. TrimLeft(s, t) A copy of s with the characters in t removed from the start strings. TrimLeftFunc(s, f) A copy of s with the characters for which f returns true removed from the start strings. TrimRight(s, t) A copy of s with the characters in t removed from the end strings. TrimRightFunc(s, f) A copy of s with the characters for which f returns true removed from the end strings. TrimSpace(s) A copy of s with whitespace removed from both ends ptg7913109 110 Chapter 3. Strings [László·Lajtha·1892·1963] [Édouard·Lalo·1823·1892] [José·Ángel·Lamas·1775·1814] The strings.FieldsFunc() function takes a string (the record variable in this ex- ample) and a reference to a function with the signature func(rune) bool. Since the function is so tiny and is used only in one place, we have created it as an anonymous function at the point it is needed. (Functions created this way are closures, although in this particular case we make no use of the enclosed state; see §5.6.3, ➤ 225.) The strings.FieldsFunc() function iterates over every char- acter in the string it is given and calls the function it is passed as its second argument with each character. If the called function returns true a split is per- formed. Here we have said that the string should be split on tabs, stars, and vertical bars. (Go’s switch statement is covered in §5.2.2, ➤ 195.) We can replace all occurrences of a string within a string using the strings. Replace() function. For example: names = " Antônio\tAndré\tFriedrich\t\t\tJean\t\tÉlisabeth\tIsabella \t" names = strings.Replace(names, "\t","",-1) fmt.Printf("|%s|\n", names) |·Antônio·André··Friedrich···Jean··Élisabeth·Isabella··| The strings.Replace() function takes a string to work on, a substring to find, a replacement string, and the number of replacements to make (-1 meaning as many as possible), and returns a string with all the (nonoverlapping) replace- ments performed. When reading a string that has been entered by a human or that has come from an external source we often want to normalize its whitespace: that is, to get rid of any leading and trailing whitespace and replace each internal sequence of one or more whitespace characters with a single space. fmt.Printf("|%s|\n", SimpleSimplifyWhitespace(names)) |Antônio·André·Friedrich·Jean·Élisabeth·Isabella| Here is a one-line SimpleSimplifyWhitespace() function. func SimpleSimplifyWhitespace(s string) string { return strings.Join(strings.Fields(strings.TrimSpace(s)), "") } ptg7913109 3.6. Other String-Related Packages 111 The strings.TrimSpace() function returns a copy of the string it is passed with any leading and trailing whitespace stripped off. The strings.Fields() function splits a string on any amount of whitespace and returns a []string. And the strings.Join() function takes a []string and a separator (which could be an empty string, although here we have used a space), and returns a single string with all the []string’s strings joined by the separator. By using these three functions in this combination we get whitespace normalization. Of course, we can more efficiently simplify whitespace doing a single pass using a bytes.Buffer. func SimplifyWhitespace(s string) string { var buffer bytes.Buffer skip := true for _, char := range s{ if unicode.IsSpace(char) { if !skip { buffer.WriteRune('') skip = true } } else { buffer.WriteRune(char) skip = false } } s = buffer.String() if skip && len(s) > 0 { s = s[:len(s)-1] } return s } The SimplifyWhitespace() function iterates over the characters in the string it receives, skipping any leading whitespace using the unicode.IsSpace() function (Table 3.11, ➤ 119). Then, it accumulates characters by writing them into a bytes.Buffer and for any sequence of one or more internal whitespaces it writes a single space. At the end any trailing space is stripped off (the algorithm allows at most one),and the resultant string is returned. A much simpler version using regular expressions is shown later (➤ 128). The strings.Map() function can be used to replace or remove characters from strings. It takes two arguments,the first a mapping function with the signature func(rune) rune and the second a string. The mapping function is called for every character in the string and each character is replaced by the character returned by the function—or deleted if the mapping function returns a negative number. ptg7913109 112 Chapter 3. Strings asciiOnly := func(char rune) rune { if char > 127 { return '?' } return char } fmt.Println(strings.Map(asciiOnly, "Jérôme Österreich")) J?r?me·?sterreich Here, instead of creating the mapping function at the call site as we did with the strings.FieldsFunc() example shown earlier (107 ➤ ), we have created an anony- mous mapping function and assigned (a reference to) it to a variable (asciiOn- ly). We have then used the strings.Map() function, passing it the variable that refers to the mapping function and the string we want to process and printing the result—a string with all non-ASCII characters replaced by “?”. We could, of course, have created the mapping function at the call site,but doing it separately as we have done here is more convenient if the function being passed is long, or if we will need to use it more than once. It is easy to use this approach to delete non-ASCII characters and produce: Jrme·sterreich This is achieved by changing the mapping function to return -1 instead of ? for non-ASCII characters. We have mentioned previously that it is possible to iterate over every character (Unicode code point) in a string using a for … range loop (§5.3, ➤ 203). A similar effect can be achieved when reading data from types that implement the ReadRune() function, such as the bufio.Reader. for { char, size, err := reader.ReadRune() if err != nil {// might occur if the reader is reading a file if err == io.EOF {// finished without incident break } panic(err) // a problem occurred } fmt.Printf("%U '%c' %d: % X\n", char, char, size, []byte(string(char))) } ptg7913109 3.6. Other String-Related Packages 113 U+0043·'C'·1:·43 U+0061·'a'·1:·61 U+0066·'f'·1:·66 U+00E9·'é'·2:·C3·A9 This code snippet reads a string and outputs each character’s code point, the character itself, how many UTF-8 bytes the character occupies, and the bytes used to represent the character. In most cases readers operate on files, so here we might imagine that the reader variable was created by calling bufio.New- Reader() on the reader returned by an os.Open() call—something we saw in the first chapter’s americanise example (§1.6, 29 ➤ ). However, in this case the reader was created to operate on a string: reader := strings.NewReader("Café") The *strings.Reader returned by strings.NewReader() offers a subset of the func- tionality of a bufio.Reader; in particular it provides the strings.Reader.Read(), strings.Reader.ReadByte(), strings.Reader.ReadRune(), strings.Reader.Unread- Byte(), and strings.Reader.UnreadRune() methods. The ability to operate on val- ues that have a particular interface (e.g., provide a ReadRune() method), rather than on values of particular types, is a very powerful and flexible feature of Go, and is covered much more fully in Chapter 6. 3.6.2. The Strconv Package The strconv package provides many functions for converting strings into oth- er types and other types into strings. The package’s functions are listed in Ta- bles 3.8 and 3.9 (➤ 114–115; see also the fmt package’s print and scan functions, §3.5, 93 ➤ and §8.2, ➤ 383.) Here we will review a few illustrative examples. One common requirement is to convert a string representation of a truth value into a bool. This can be done using the strconv.ParseBool() function. for _, truth := range []string{"1","t","TRUE","false","F","0","5"}{ if b, err := strconv.ParseBool(truth); err != nil { fmt.Printf("\n{%v}", err) } else { fmt.Print(b, "") } } fmt.Println() true·true·true·false·false·false {strconv.ParseBool:·parsing·"5":·invalid·syntax} ptg7913109 114 Chapter 3. Strings Table 3.8 The Strconv Package’s Functions #1 Parameter bs is a []byte, base is a number base (2–36), bits is the bit size the result must fit into (8,16, 32, 64—or 0 for int’s size for ints; 32 or 64 for float64s),and s is a string. Syntax Description/result strconv.AppendBool(bs, b) bs with "true" or "false" appended depending on bool b strconv.AppendFloat(bs, f, fmt, prec, bits) bs with float64 f appended; see strconv.Format- Float() for the other parameters strconv.AppendInt(bs, i, base) bs with int64 i appended using the given base strconv.AppendQuote(bs, s) bs with s appended using strconv.Quote() strconv.AppendQuote- Rune(bs, char) bs with rune char appended using strconv. QuoteRune() strconv.AppendQuote- RuneToASCII(bs, char) bs with rune char appended using strconv. QuoteRuneToASCII() strconv.AppendQuote- ToASCII(bs, s) bs with s appended using strconv.QuoteTo- ASCII() strconv.AppendUInt(bs, u, base) bs with uint64 u appended using the given base strconv.Atoi(s) string s converted to an int, and an error or nil; see also strconv.ParseInt() strconv.CanBackquote(s) true if s can be represented in Go syntax using backticks strconv.FormatBool(tf)"true" or "false" depending on bool tf strconv.FormatFloat(f, fmt, prec, bits) float64 f as a string. The fmt is a byte corre- sponding to an fmt.Print() verb, 'b' for %b,'e' for %e, etc. (see Table 3.4, 95 ➤ ). The prec is the number of digits after the decimal point for an fmt of 'e','E', and 'f'; or the total number of digits for a fmt of 'g' and 'G'—use -1 to request the smallest number of digits that can be used while preserving accuracy going the other way (e.g., using strconv.ParseFloat()). The bits af- fects rounding and is usually 64. strconv.FormatInt(i, base) int64 i as a string in base base strconv.FormatUInt(u, base) uint64 u as a string in base base strconv.IsPrint(c) true if rune c is a printable character strconv.Itoa(i) int i as a string using base 10; see also strconv. FormatInt() ptg7913109 3.6. Other String-Related Packages 115 Table 3.9 The Strconv Package’s Functions #2 Syntax Description/result strconv.ParseBool(s) true and nil if s is "1","t","T","true","True",or"TRUE"; false and nil if s is "0","f","F","false","False",or "FALSE"; false and an error otherwise strconv.ParseFloat( s, bits) A float64 and nil if s is parseable as a floating-point number, or 0 and an error; bits should be 64; but use 32 if converting to a float32 strconv.ParseInt( s, base, bits) An int64 and nil if s is parseable as an integer, or 0 and an error;abase of 0 means the base will be deduced from s (a leading "0x" or "0X" means base 16, a leading "0" means base 8; otherwise base 10), or a specific base (2–36) can be given; bits should be 0 if converting to an int or the bit size if converting to a sized integer (e.g., 16 for an int16) strconv.ParseUint( s, base, bits) A uint64 and nil or 0 and an error—just the same as strconv.ParseInt() apart from being unsigned strconv.Quote(s)A string using Go’s double-quoted string syntax to represent string s; see also Table 3.1 (83 ➤ ) strconv.QuoteRune( char) A string using Go’s single-quoted string syntax to represent Unicode code point char of type rune strconv.QuoteRune- ToASCII(char) A string using Go’s single-quoted string syntax to represent Unicode code point char of type rune, using an escape sequence for a non-ASCII character strconv. QuoteToASCII(s) A string using Go’s double-quoted string syntax to rep- resent string s, using escape sequences for non-ASCII characters strconv.Unquote(s) A string that contains the Go syntax single-quoted character or double-quoted or backtick-quoted string in string s and an error strconv. UnquoteChar(s, b) A rune (the first character), a bool (whether the first character’s UTF-8 representation needs more than one byte), a string (the rest of the string), and an error; if b is set to a single or double quote that quote must be escaped ptg7913109 116 Chapter 3. Strings All the strconv conversion functions return the converted value and an error, with the latter being nil if the conversion succeeded. x, err := strconv.ParseFloat("-99.7", 64) fmt.Printf("%8T %6v %v\n", x, x, err) y, err := strconv.ParseInt("71309", 10, 0) fmt.Printf("%8T %6v %v\n", y, y, err) z, err := strconv.Atoi("71309") fmt.Printf("%8T %6v %v\n", z, z, err) ·float64··-99.7· ···int64··71309· ·····int··71309· The strconv.ParseFloat(), strconv.ParseInt(), and strconv.Atoi() (“ASCII to int”) functions shown here work much as we would expect. The call strconv. Atoi(s) is almost the same as strconv.ParseInt(s, 10, 0), that is, parse the given string as a base-ten integer and return an integer, only Atoi() returns an int and ParseInt() returns an int64. As we would expect, the strconv.ParseUint() function converts to an unsigned integer type and will fail if there’s a lead- ing minus sign in the string it is given. These functions will fail if there is any leading or trailing whitespace, but we can easily eliminate this with the strings.TrimSpace() function or by using the fmt package’s scan functions (Ta- ble 8.2, ➤ 383). Naturally, the floating-point conversions will accept strings that use standard or exponential notation, such as "984","424.019", and "3.916e-12". s := strconv.FormatBool(z > 100) fmt.Println(s) i, err := strconv.ParseInt("0xDEED", 0, 32) fmt.Println(i, err) j, err := strconv.ParseInt("0707", 0, 32) fmt.Println(j, err) k, err := strconv.ParseInt("10111010001", 2, 32) true 57069· 455· 1489· The strconv.FormatBool() function returns a string representing the Boolean ex- pression it is given as "true" or "false".Thestrconv.ParseInt() function converts an integer in string form into an int64. The second argument is the base to use, with 0 meaning use the base implied by the string’s prefix: "0x" or "0X" for hex- adecimal,"0" for octal,and decimal otherwise. In thissnippet we have converted a hexadecimal and an octal number using their implied base and a binary num- ber by specifying an explicit base of 2.Valid basesare 2to 36 inclusive with bases ptg7913109 3.6. Other String-Related Packages 117 higher than 10 representing 10 with A(or a) and so on. The third argument is the bit size (with 0 signifying the size of an int), so although the function always returns an int64, the conversion will only succeed if it can be converted perfectly to an integer of the given bit size. i:=16769023 fmt.Println(strconv.Itoa(i)) fmt.Println(strconv.FormatInt(int64(i), 10)) fmt.Println(strconv.FormatInt(int64(i), 2)) fmt.Println(strconv.FormatInt(int64(i), 16)) 16769023 16769023 111111111101111111111111 ffdfff The strconv.Itoa() (“Integer to ASCII”) function returns a string representing its int argument in base 10. The strconv.FormatInt() function formats an int64 as a string using the given base (which must be specified, and must be between 2 and 36 inclusive). s="Alle ønsker å være fri." quoted := strconv.Quote(s) fmt.Println(quoted) fmt.Println(strconv.Unquote(quoted)) "Alle·\u00f8nsker·\u00e5·v\u00e6re·fri." Alle·ønsker·å·være·fri.· The strconv.Quote() function returns the string it is given as a Go double-quoted string and with any nonprintable ASCII characters and any non-ASCII char- acters represented using escapes. (Go’s string escapes are shown in Table 3.1, 84 ➤ .) The strconv.Unquote() function takes a string containing a Go double- quoted string or a raw (backtick-quoted) string or a single-quoted character,and returns the unquoted string equivalent and an error (or nil). 3.6.3. The Utf8 Package The unicode/utf8 package provides several useful functions for querying and manipulating strings and []bytes which hold UTF-8 bytes—many of these are shown in Table 3.10. Earlier we saw how to use the utf8.DecodeRuneInString() and utf.DecodeLastRuneInString() functions (91 ➤ ) to get the first and last characters in a string. ptg7913109 118 Chapter 3. Strings Table 3.10 The Utf8 Package’s Functions Import "unicode/utf8".Variableb is type []byte,s is of type string,andc is a Unicodecode point of type rune. Syntax Description/result utf8. DecodeLastRune(b) The last rune in b and the number of bytes it occupies, or U+FFFD (the Unicode replacement character, ?) and 0,if b doesn’t end with a valid rune utf8.DecodeLast- RuneInString(s) The same as utf8.DecodeLastRune(), only it takes a string as input utf8.DecodeRune(b) The first rune in b and the number of bytes it occupies, or U+FFFD (the Unicode replacement character, ?) and 0,if b doesn’t start with a valid rune utf8.DecodeRune- InString(s) The same as utf8.DecodeRune(), only it takes a string as input utf8.EncodeRune( b, c) Writes c into b as UTF-8 bytes and returns the number of bytes written (b must have enough space) utf8.FullRune(b) true if b begins with a UTF-8-encoded rune utf8.FullRune- InString(b) true if s begins with a UTF-8-encoded rune utf8.RuneCount(b) Same as utf8.RuneCountInString() but works on a []byte utf8.RuneCount- InString(s) The number of runesins; this may be less than len(s) if s contains non-ASCII characters utf8.RuneLen(c) The number of bytes needed to encode c utf8.RuneStart(x) true if byte x could be the first byte of a rune utf8.Valid(b) true if b’s bytes represent valid UTF-8-encoded runes utf8.ValidString(s) true if s’s bytes represent valid UTF-8-encoded runes 3.6.4. The Unicode Package The unicode package provides functions for querying Unicode code points to determine if they meet certain criteria—for example, whether the character they represent is a digit or a lowercase letter. Table 3.11 shows the most com- monly used functions. In addition to those functions we would expect, such as unicode.ToLower() and unicode.IsUpper(), a generic unicode.Is() function is pro- vided so that we can check whether a character is in a particular Unicode cate- gory. ptg7913109 3.6. Other String-Related Packages 119 Table 3.11 The Unicode Package’s Functions Variable c is of type rune and represents a Unicode code point. Syntax Description/result unicode.Is( table, c) true if c is in the table (see text) unicode.IsControl(c) true if c is a control character unicode.IsDigit(c) true if c is a decimal digit unicode.IsGraphic(c) true if c is a “graphic” character such as a letter, num- ber, punctuation mark, symbol, or space unicode.IsLetter(c) true if c is a letter unicode.IsLower(c) true if c is a lowercase letter unicode.IsMark(c) true if c is a mark character unicode.IsOneOf( tables, c) true if c is in one of the tables unicode.IsPrint(c) true if c is a printable character unicode.IsPunct(c) true if c is a punctuation character unicode.IsSpace(c) true if c is a whitespace character unicode.IsSymbol(c) true if c is a symbol character unicode.IsTitle(c) true if c is a title-case letter unicode.IsUpper(c) true if c is an uppercase letter unicode.SimpleFold(c)A case-folded copy of the given c unicode.To( case, c) The case version of c where case is unicode.LowerCase, unicode.TitleCase,orunicode.UpperCase unicode.ToLower(c) The lowercase version of c unicode.ToTitle(c) The title-case version of c unicode.ToUpper(c) The uppercase version of c fmt.Println(IsHexDigit('8'), IsHexDigit('x'), IsHexDigit('X'), IsHexDigit('b'), IsHexDigit('B')) true·false·false·true·true The unicode package provides the unicode.IsDigit() function to check whether a character is a decimal digit, but there is no similar function to check for hex- adecimal digits, so here we have used our own custom IsHexDigit() function. ptg7913109 120 Chapter 3. Strings func IsHexDigit(char rune) bool { return unicode.Is(unicode.ASCII_Hex_Digit, char) } This tiny function uses the generic unicode.Is() function in conjunction with the unicode.ASCII_Hex_Digit range to determine whether the given character is a hexadecimal digit. We could easily create similar functions to test for other Unicode characteristics. 3.6.5. The Regexp Package This subsection presents tables listing the regexp package’s functions and the regular expression syntax the package supports, and includes a few illustrative examples. Here and elsewhere in this book, we assume prior knowledge of regular expressions, or “regexeps”.★ The regexp package is a Go implementation of Russ Cox’s RE2 regular expres- sion engine.✪ This engine is fast and thread-safe. The RE2 engine doesn’t use backtracking, so guarantees linear time execution O(n) where n is the length of the matched string, whereas backtracking engines can easily take exponential time O( n2 )(see the sidebar “Big-O Notation”, 89 ➤ ). The superior performance is gained at the expense of having no support for backreferences in searches. However,it is usually straightforward to work around this constraint by making good use of the regexp API. Table 3.12 lists the regexp package’s functions, including four functions for creating *regexp.Regexp values. These values provide the methods shown in Tables 3.18 and 3.19 (➤ 124–125). The RE2 engine’s syntax supports the escape sequences listed in Table 3.13 (➤ 121), the character classes listed in Table 3.14 (➤ 122), the zero-width assertions listed in Table 3.15 (➤ 122), the quantifiers listed in Table 3.16 (➤ 123), and the flags listed in Table 3.17 (➤ 123). The regexp.Regexp.ReplaceAll() and regexp.Regexp.ReplaceAllString() methods support both numbered and named replacements. Numbered replacements start at $1 for the first capturing parenthesized match. Named replacements refer to named capture groups. Although replacements can be referred to by number or by name (e.g., $2, $filename), it is safest to use braces as delimiters (e.g., ${2}, ${filename}). Use $$ to include a literal $ in a replacement string. ★ A good textbook that teaches regexeps is Mastering Regular Expressions; see Appendix C. The author’s book, Programming in Python 3,has a chapter that teaches Python regexeps (these support a subset of regexp syntax).This chapter is available as a free download from 9780321680563 (click the “Sample Content” link and download Chapter 13). ✪ Information on RE2, including links to documents covering its rationale, performance, and implementation, is available from . ptg7913109 3.6. Other String-Related Packages 121 Table 3.12 The Regexp Package’s Functions Variables p and s are of type string, with p being a regexp pattern. Syntax Description/result regexp.Match(p, b) true and nil if p matches b of type []byte regexp.Match- Reader(p, r) true and nil if p matches the text read by r of type io.RuneReader regexp.Match- String(p, s) true and nil if p matches s regexp.QuoteMeta(s)A string with all regexp metacharacters safely quoted regexp.Compile(p)A*regexp.Regexp and nil if p compiles successfully; see Tables 3.18 and 3.19 (➤ 124–125) regexp.Compile- POSIX(p) A*regexp.Regexp and nil if p compiles successfully; see Tables 3.18 and 3.19 (➤ 124–125) regexp.Must- Compile(p) A*regexp.Regexp if p compiles successfully, otherwise panics; see Tables 3.18 and 3.19 (➤ 124–125) regexp.Must- CompilePOSIX(p) A*regexp.Regexp if p compiles successfully, otherwise panics; see Tables 3.18 and 3.19 (➤ 124–125) Table 3.13 The Regexp Package’s Escape Sequences Syntax Description \c Literal character c; e.g., \* is a literal * rather than a quantifier \000 Character with the given octal code point \xHH Character with the given 2-digit hexadecimal code point \x{HHHH} Character with the given 1–6-digit hexadecimal code point \a ASCII bell (BEL) ≡ \007 \f ASCII formfeed (FF) ≡ \014 \n ASCII linefeed (LF) ≡ \012 \r ASCII carriage return (CR) ≡ \015 \t ASCII tab (TAB) ≡ \011 \v ASCII vertical tab (VT) ≡ \013 \Q...\E Matches the ... text literally even if it contains characters like * ptg7913109 122 Chapter 3. Strings Table 3.14 The Regexp Package’s Character Classes Syntax Description [chars] Any character in chars [^chars] Any character not in chars [:name:] Any ASCII character in the name character class: [[:alnum:]] ≡ [0-9A-Za-z] [[:lower:]] ≡ [a-z] [[:alpha:]] ≡ [A-Za-z] [[:print:]] ≡ [-~] [[:ascii:]] ≡ [\x00-\x7F] [[:punct:]] ≡ [!-/:-@[-`{-~] [[:blank:]] ≡ [ \t] [[:space:]] ≡ [ \t\n\v\f\r] [[:cntrl:]] ≡ [\x00-\x1F\x7F] [[:upper:]] ≡ [A-Z] [[:digit:]] ≡ [0-9] [[:word:]] ≡ [0-9A-Za-z_] [[:graph:]] ≡ [!-~] [[:xdigit:]] ≡ [0-9A-Fa-z] [:^name:] Any ASCII character not in the name character class . Any character (including newline if flag s is set) \d Any ASCII digit: [0-9] \D Any ASCII nondigit: [^0-9] \s Any ASCII whitespace: [\t\n\f\r] \S Any ASCII nonwhitespace: [^ \t\n\f\r] \w Any ASCII “word” character: [0-9A-Za-z_] \W Any ASCII non-“word” character: [^0-9A-Za-z_] \pN Any Unicode character in the N one-letter character class; e.g., \pL to match a Unicode letter \PN Any Unicode character not in the N one-letter character class; e.g., \PL to match a Unicode nonletter \p{Name} Any Unicode character in the Name character class; e.g., \p{Ll} matches lowercase letters, \p{Lu} matches uppercase letters, and \p{Greek} matches Greek characters \P{Name} Any Unicode character not in the Name character class Table 3.15 The Regexp Package’s Zero-Width Assertions Syntax Description/result ^ Start of text (or start of line if flag m is set) $ End of text (or end of line if flag m is set) \A Start of text \z End of text \b Word boundary (\w followed by \W or \A or \z; or vice versa) \B Not a word boundary ptg7913109 3.6. Other String-Related Packages 123 Table 3.16 The Regexp Package’s Quantifiers Syntax Description e? or e{0,1} Greedily match zero or one occurrence of expression e e+ or e{1,} Greedily match one or more occurrences of expression e e* or e{0,} Greedily match zero or more occurrences of expression e e{m,} Greedily match at least m occurrences of expression e e{,n} Greedily match at most n occurrences of expression e e{m,n} Greedily match at least m and at most n occurrencesof expres- sion e e{m} or e{m}? Match exactly m occurrences of expression e e?? or e{0,1}? Nongreedily match zero or one occurrence of expression e e+? or e{1,}? Nongreedily match one or more occurrences of expression e e*? or e{0,}? Nongreedily match zero or more occurrences of expression e e{m,}? Nongreedily match at least m occurrences of expression e e{,n}? Nongreedily match at most n occurrences of expression e e{m,n}? Nongreedily match at least m and at most n occurrences of expression e Table 3.17 The Regexp Package’s Flags and Groups Syntax Description i Match case-insensitively (the default is case-sensitive matching) m Multiline mode makes ^ and $ match at the start and end of every line (the default is single-line mode) s Make . match any character including newlines (the default is for . to match any character except newlines) U Make greedy matches nongreedy and vice versa; i.e., swap the meaning of ? after a quantifier (the default is for matches to be greedy unless their quantifier is followed by ? to make them nongreedy) (?flags) Apply the given flags from this point on (precede the flag or flags with - to negate) (?flags:e) Apply the given flags to expression e (precede the flag or flags with - to negate) (e) Group and capture the match for expression e (?Pe) Group and capture the match for expression e using the capture name name (?:e) Group but don’t capture the match for expression e ptg7913109 124 Chapter 3. Strings Table 3.18 The *regexp.Regexp Type’s Methods #1 Variable rx is of type *regexp.Regexp; s is the string to match; b is the []byte to match; r is the io.RuneReader to match; and n is the maximum number of matches(-1 means as many as possible).A nil return means no match(es). Syntax Description/result rx.Expand(...) Performs the $ replacements done by the Replace- All() method—rarely used directly (advanced) rx.ExpandString(...) Performs the $ replacements done by the ReplaceAll- String() method—rarely used directly (advanced) rx.Find(b)A[]byte with the leftmost match or nil rx.FindAll(b, n)A[][]byte of all nonoverlapping matches or nil rx.FindAllIndex(b, n) An [][]int (a slice of 2-item slices) each identifying a match or nil; e.g., b[pos[0]:pos[1]] where pos is one of the 2-item slices rx.FindAllString(s, n)A[]string of all nonoverlapping matches or nil rx.FindAllString- Index(s, n) An [][]int (a slice of 2-item slices) each identifying a match or nil; e.g., s[pos[0]:pos[1]] where pos is one of the 2-item slices rx.FindAllStringSub- match(s, n) A[][]string (a slice of string slices where each string corresponds to a capture) or nil rx.FindAllStringSub- matchIndex(s, n) An [][]int (a slice of 2-item int slices that corre- spond to captures) or nil rx.FindAllSub- match(b, n) A[][][]byte (a slice of slices of []bytes where each []byte corresponds to a capture) or nil rx.FindAllSubmatch- Index(b, n) An [][]int (a slice of 2-item int slices that corre- spond to captures) or nil rx.FindIndex(b)A 2-item []int identifying the leftmost match; e.g., b[pos[0]:pos[1]] where pos is the 2-item slice, or nil rx.FindReaderIndex(r)A 2-item []int identifying the leftmost match or nil rx.FindReaderSub- matchIndex(r) An []int identifying the leftmost match and captures or nil rx.FindString(s) The leftmost match or an empty string rx.FindString- Index(s)A 2-item []int identifying the leftmost match or nil rx.FindStringSub- match(s) A[]string with the leftmost match and captures or nil rx.FindStringSub- matchIndex(s) An []int identifying the leftmost match and captures or nil ptg7913109 3.6. Other String-Related Packages 125 Table 3.19 The *regexp.Regexp Type’s Methods #2 Variable rx is of type *regexp.Regexp; s is the string to match; b is the []byte to match. Syntax Description/result rx.FindSubmatch(b)A[][]byte with the leftmost match and captures or nil rx.FindSubmatch- Index(b) A[][]byte with the leftmost match and captures or nil rx.Literal- Prefix() The possibly empty prefix string that the regexp must begin with and a bool indicating whether the whole regexp is a literal string match rx.Match(b) true if the regexp matches b rx.MatchReader(r) true if the regexp matches r of type io.RuneReader rx.MatchString(s) true if the regexp matches s rx.NumSubexp() How many parenthesized groups the regexp has rx.Replace- All(b, br) A[]byte that is a copy of b with every match replaced with br of type []byte with $ replacements (see text) rx.ReplaceAll- Func(b, f) A[]byte that is a copy of b with every match replaced with the return value of a call to function f of type func([]byte) []byte and whose argument is a match rx.ReplaceAll- Literal(b, br) A[]byte that is a copy of b with every match replaced with br of type []byte rx.ReplaceAll- LiteralString(s, sr) A string that is a copy of s with every match replaced with sr of type string replacements rx.ReplaceAll- String(s, sr) A string that is a copy of s with every match replaced with sr of type string with $ replacements (see text) rx.ReplaceAll- StringFunc(s, f) A string that is a copy of s with every match replaced with the return value of a call to function f of type func(string) string and whose argument is a match rx.String() A string containing the regexp pattern rx.Subexp- Names() A[]string (which must not be modified), containing the names of all the named subexpressions ptg7913109 126 Chapter 3. Strings An example that typically involves the use of replacements is where we have a list of names of the form forename1 … forenameN surname and want to change the list to have the form surname, forename1 … forenameN. Here is how we can achieve this using the regexp package, and with correct handling of accented and other non-English characters. nameRx := regexp.MustCompile(`(\pL+\.?(?:\s+\pL+\.?)*)\s+(\pL+)`) for i:=0;i\pL+\.?(?:\s+\pL+\.?)*)\s+(?P\pL+)`) for i:=0;i Bowie-Singles.pls Here we tell the program to read the Bowie-Singles.m3u file and use console redirection to write the .pls format version to the Bowie-Singles.pls file. (Of course, it would be nice to be able to convert the other way too—and this is precisely what the exercise that follows this section involves.) We will review almost the entire program, skipping only the imports. func main() { if len(os.Args) == 1 || !strings.HasSuffix(os.Args[1], ".m3u"){ fmt.Printf("usage: %s \n", filepath.Base(os.Args[0])) os.Exit(1) } if rawBytes, err := ioutil.ReadFile(os.Args[1]); err != nil { log.Fatal(err) } else { songs := readM3uPlaylist(string(rawBytes)) writePlsPlaylist(songs) } } The main() function begins by checking to see if the program has been invoked with an .m3u file specified on the command line. The strings.HasSuffix() func- tion takes two strings and returns true if the first string ends with the second string. If no .m3u file has been specified a usage message is output and the pro- gram is terminated. The filepath.Base() function returns the basename (i.e., the filename)of the given path and the os.Exit() function cleanly terminatesthe ptg7913109 132 Chapter 3. Strings program—for example, stopping all goroutines and closing any open files—and returns its argument to the operating system. If an .m3u file has been specified we attempt to read the entire file using the ioutil.ReadFile() function. This function returns all the file’s bytes (as a []byte) and an error which will be nil if the file was read without incident. If a problem occurred (e.g.,the file doesn’t exist or is unreadable),we use the log.Fatal() func- tion to output the error to the console (actually to os.Stderr), and to terminate the program with an exit code of 1. If the file is successfully read we convert its raw bytes to a string—this assumes that the bytes represent 7-bit ASCII or UTF-8 Unicode—and immediately pass the string to a custom readM3uPlaylist() function for parsing. The function returns a slice of Songs (i.e., a []Song). We then write the song data using a custom writePlsPlaylist() function. type Song struct { Title string Filename string Seconds int } Here we have defined a custom Song type using a struct (§6.4, ➤ 275) to provide convenient file-format-independent storage for the information about each song. func readM3uPlaylist(data string) (songs []Song) { var song Song for _, line := range strings.Split(data, "\n"){ line = strings.TrimSpace(line) if line == "" || strings.HasPrefix(line, "#EXTM3U"){ continue } if strings.HasPrefix(line, "#EXTINF:"){ song.Title, song.Seconds = parseExtinfLine(line) } else { song.Filename = strings.Map(mapPlatformDirSeparator, line) } if song.Filename != "" && song.Title != "" && song.Seconds != 0 { songs = append(songs, song) song = Song{} } } return songs } ptg7913109 3.7. Example: M3u2pls 133 This function accepts the entire contents of an .m3u file as a single string and returns a slice of all the songs it is able to parse from the string. It begins by declaring an empty Song variable called song. Thanks to Go’s practice of always initializing things to their zero value, song’s initial contents are two empty strings and a Song.Seconds value of 0. At the heart of the function is a for … range loop (§5.3, ➤ 203). The strings.Split() function is used to split the single string that holds the entire .m3u file’s data into separate lines, and the for loop iterates over each of these lines. If a line is empty or is the first line (i.e., starts with the string literal "#EXTM3U"), the continue statement is reached; this simply passes control back to the for loop to force the next iteration—or the end of the loop if there are no more lines. If the line begins with the "#EXTINF:" string literal, the line is passed to a custom parseExtinfLine() function for parsing: This function returns a string and an int which are immediately assigned to the current song’s Song.Title and Song.Seconds fields. Otherwise, it is assumed that the line holds the filename (including the path) of the current song. Rather than storing the filename as is, the strings.Map() function is called with a custom mapPlatformDirSeparator() function to convert directory separators into those native for the platform the program is running on, and the resultant string is stored as the current song’s Song.Filename. The strings.Map() function is passed a mapping function with signature func(rune) rune and a string.For every character in the string the mapping function is called with the character replaced by the character returned by the passed-in function—which may be the same as the original one, of course. As usual with Go, a character is a rune whose value is the character’s Unicode code point. If the current song’s filename and title are both nonempty, and if the song’s duration isn’t zero, the current song is appended to the songs return value (of type []Song) and the current song is set to its zero value (two empty strings and 0) by assigning an empty Song to it. func parseExtinfLine(line string) (title string, seconds int) { if i := strings.IndexAny(line, "-0123456789"); i > -1 { const separator = "," line = line[i:] if j := strings.Index(line, separator); j > -1 { title = line[j+len(separator):] var err error if seconds, err = strconv.Atoi(line[:j]); err != nil { log.Printf("failed to read the duration for '%s': %v\n", title, err) seconds = -1 } ptg7913109 134 Chapter 3. Strings } } return title, seconds } This function is used to parse lines of the form: #EXTINF:duration,title and where the duration is expected to be an integer, either -1 or greater than zero. The strings.IndexAny() function is used to find the position of the first digit or the minus sign. An index position of -1 means not found; any other value is the index position of the first occurrence of any of the characters in the string given as the strings.IndexAny() function’s second argument, in which case variable i holds the position of the first digit of the duration (or of -). Once we know where the digits begin we slice the line to start at the digits. This effectively discards the "#EXTINF:" that was at the start of the string, so now the line has the form: duration,title. The second if statement uses the strings.Index() function to get the index position of the first occurrence of the "," string in the line—or -1 if there is no such occurrence. The title is the text from after the comma to the end of the line. To slice from after the comma we need the comma’s starting position (j) and must add to this the number of bytes the comma occupies (len(separator)). Of course, we know that a comma is a 7-bit ASCII character and so has a length of one, but the approach shown here will work with any Unicode character, no matter how many bytes are used to represent it. The duration is the number whose digits go from the start of the line up to but excluding the j-th byte (where the comma is). We convert the number into an int using the strconv.Atoi() function—and if the conversion fails we simply set the duration to -1 which is an acceptable “unknown duration” value, and log the problem so that the user is aware of it. func mapPlatformDirSeparator(char rune) rune { if char == '/' || char == '\\' { return filepath.Separator } return char } This function is called by the strings.Map() function (inside the readM3uPlay- list() function) for every character in a filename. It replaces any directory sep- arator with the platform-specific directory separator. And any other character is returned unchanged. ptg7913109 3.7. Example: M3u2pls 135 Like most cross-platform programming languages and libraries, Go uses Unix-style directory separators internally on all platforms, even on Windows. However, for user-visible output and for human-readable data files, we prefer to use the platform-specific directory separator. To achieve this we can use the filepath.Separator constant which holds the / character on Unix-like systems and the \ character on Windows. In this example we don’t know whether the paths we are reading use forward slashes or backslashes, so we have had to cater for both. However, if we know for sure that a path uses forward slashes we can use the filepath.FromSlash() function on it: This will return the path unchanged on Unix-like systems, but will replace forward slashes with backslashes on Windows. func writePlsPlaylist(songs []Song) { fmt.Println("[playlist]") for i, song := range songs { i++ fmt.Printf("File%d=%s\n", i, song.Filename) fmt.Printf("Title%d=%s\n", i, song.Title) fmt.Printf("Length%d=%d\n", i, song.Seconds) } fmt.Printf("NumberOfEntries=%d\nVersion=2\n", len(songs)) } This function writes out the songs data in .pls format. It writes the data to os.Stdout (i.e., to the console), so file redirection must be used to get the output into a file. The function begins by writing the section header ("[playlist]"), and then for every song it writes the song’s filename, title, and duration in seconds, each on their own lines. Since each key must be unique a number is appended to each one, starting from 1. And at the end the two items of metadata are written. 3.8. Exercises There are two exercises for this chapter, the first involving the modification of an existing command-line program, and the second requiring the creation of a web application (optionally) from scratch. 1. The previous section’s m3u2pls program does a decent job of converting .m3u playlist files into .pls format. But what would make the program much more useful is if it could also perform the reverse conversion, from .pls format to .m3u format. For this exercise copy the m3u2pls directo- ry to, say, my_playlist and create a new program called playlist that has the required functionality. Its usage message should be usage: playlist . ptg7913109 136 Chapter 3. Strings If the program is called with an .m3u file it should do exactly what the m3u2pls program does: Write the file’s data in .pls format to the console. But if the program is called with a .pls file it should write the file’s data in .m3u format, again to the console. The new functionality will require about 50 new lines of code. A straightforward solution is provided in the file play- list/playlist.go. 2. Data cleaning, matching, and mining applications that involve people’s names can often produce better results by matching names by the way they sound rather than by how they are spelled. Many algorithms for name matching English language names are available, but the oldest and sim- plest is the Soundex algorithm. The classic Soundex algorithm produces a soundex value of a capital letter followed by three digits. For example, the names “Robert” and “Rupert” both have the same soundex value of “R163” according to most Soundex algorithms. However, the names “Ashcroft” and “Ashcraft” have a soundex value of “A226” according to some Soundex algorithms (including the one in the exercise solution), but “A261” according to others. The exercise is to write a web application that supports two web pages. The first page (with path /) should present a simple form through which the user can enter one or more names to see their soundex values—this is illustrated in Figure 3.3’s left-hand screenshot. The second page (with path /test) should execute the application’s soundex() function on a list of strings and compare each result to what we would expect—this is illustrated in Figure 3.3’s right-hand screenshot. Figure 3.3 The Soundex application on Linux Readers who would like a jump-start could copy one of the other web applications (statistics, statistics_ans, quadratic_ans1, quadratic_ans2) to ptg7913109 3.8. Exercises 137 get the skeleton of the application up and running, and then just focus on the soundex and test page functionality. A solution is in the file soundex/soundex.go and is about 150 lines; the soundex() function itself is 20 lines although it does rely on an []int that maps capital letters to digits in a slightly subtle way. The solution’s algo- rithm is based on the Python implementation shown on the Rosetta Code web site ( which produces slightly different resultsto the Go implementation shown on that site and from the one shown on Wikipedia ( The test data is in the file soundex/soundex-test-data.txt. Naturally,readersare free to implement whichever version of the algorithm they prefer—or even implement a more advanced algorithm such as one of the Metaphone algorithms—and simply adjust the tests to match. ptg7913109 This page intentionally left blank ptg7913109 4 Collection Types §4.1. Values, Pointers, and Reference Types ➤ 140 §4.2. Arrays and Slices ➤ 148 §4.2.1. Indexing and Slicing Slices ➤ 153 §4.2.2. Iterating Slices ➤ 154 §4.2.3. Modifying Slices ➤ 156 §4.2.4. Sorting and Searching Slices ➤ 160 §4.3. Maps ➤ 164 §4.3.1. Creating and Populating Maps ➤ 166 §4.3.2. Map Lookups ➤ 168 §4.3.3. Modifying Maps ➤ 169 §4.3.4. Key-Ordered Map Iteration ➤ 170 §4.3.5. Map Inversion ➤ 170 §4.4. Examples ➤ 171 §4.4.1. Example: Guess Separator ➤ 171 §4.4.2. Example: Word Frequencies ➤ 174 This chapter’s first section explains Go’s values, pointers, and reference types since an understanding of these is necessary for the rest of the chapter and for subsequent chapters. Go’s pointers work just like those in C and C++, both syntactically and semantically—except that Go does not support pointer arith- metic, thus eliminating a whole category of potential bugs that can affect C and C++ programs. Nor does Go need free()or delete since Go has a garbage collector and manages memory automatically.★ Values of Go’s reference types are cre- ated in a unique and simple way and once created are used rather like Java or Python object references. Go’s values work like those in most other mainstream languages. This chapter’s other sections are devoted to Go’s built-in collection types. All the built-in collection types are covered—arrays, slices, and maps. These types are ★ Go’s delete() function is used to delete keys from maps as we will see later in this chapter. 139 ptg7913109 140 Chapter 4. Collection Types so versatile and efficient that between them they comfortably meet almost every need. The standardlibrary providessomeadditional,morespecializedcollection types—container/heap, container/list, and container/ring—that might be more efficient for particular use cases. A couple of tiny examples showing a heap and a list are presented in a later chapter (§9.4.3, ➤ 421). And Chapter 6 has an ex- ample that shows how to create a balanced binary tree (§6.5.3, ➤ 302). 4.1. Values, Pointers, and Reference Types In this section we discuss what variables hold (values, pointers, and references —including array values, and slice and map references), whereas in the follow- ing sections we explain how to actually use arrays, slices, and maps. In general, Go variables hold values. That is, we can think of a variable as “being” the value it stores. The exceptions are variables that refer to channels, functions, methods, maps, and slices—these hold references—and variables that hold pointers. Values that are passed to functions or methods are copied. This is cheap for Booleans and numbers because they only occupy from one to eight bytes each. Passing strings by value is also cheap because Go compilers can safely optimize passing them so that only a small amount of data is actually passed per string, no matter how large the string is, since Go strings are immutable. (The amount per string is 16 bytes on 64-bit machines and 8 bytes on 32-bit machines.★) Of course, if a passed-in string is modified (e.g., using the += operator), behind the scenes Go must do a copy on write which is potentially expensive for large strings—but this is a price that would have to be paid no matter what language was being used. Unlike C or C++, Go arrays are passed by value—so passing large arrays is expensive. Fortunately, arrays are rarely needed in Go programming since slices are used instead, as we will see in the next section. Passing a slice costs much the same as passing a string (i.e., 16 bytes on 64-bit machines and 12 bytes on 32-bit machines), no matter what the slice’s length or capacity.★ Nor is there any copy on write overhead if the slice is modified, because unlike strings, slices are mutable (i.e., if a slice is modified the modification is visible to all the variables—references—that refer to it). Figure 4.1 illustrates the relationship between variables and the memory they occupy. In the figure, memory addresses are shown in gray since they will vary, and bold is used to indicate changes. Conceptually, a variable is the name given to a piece of memory that holds a value of a particular type. So if we have the short variable declaration y := 1.5, ★ The sizes in bytes were measured on a 64-bit machine and on a 32-bit machine at the time of this writing. The amounts are implementation details that may vary but will never be large. ptg7913109 4.1. Values, Pointers, and Reference Types 141 Statement Variable Value Type Memory Address y := 1.5 y 1.5 float64 0xf8400000f8 y++ y 2.5 float64 0xf8400000f8 y 2.5 float64 0xf8400000f8 x 2.5 float64 Modifiable copy of y in Ceil()z := math.Ceil(y) z 3.0 float64 0xf84000000c0 Figure 4.1 Simple values in memory Go will set aside enough memory to store a float64 (i.e., 8 bytes) and will put the 1.5 value into this memory. From this point onward—while y remains in scope—Go will treat the variable y as synonymous with the memory that stores the float64 that y is associated with. So if we follow the declaration with the statement y++, Go will increment the value that y is associated with. However, if we pass y to a function or method, Go will pass a copy of y; in other words Go will create a new variable that is associated with the called function or method’s corresponding parameter name and will copy y’s value into the memory set aside for the new variable. Sometimes we want a function to modify a variable that we pass it. This can be done without formality for reference types as we will see, but value types are copied, so any modifications are applied to the copy and the original value is left unchanged. Also, it can be expensive to pass some values, because they are large (e.g., an array, or a struct with lots of fields). Furthermore, local variables are garbage-collected if they are no longer being used (e.g., when they are not being referred to and they go out of scope), yet in many situations we want to create variables whose lifetime is determined by us rather than by their enclosing scope. Parameters that are cheap to pass, parameters that are modifiable, and vari- ables whose lifetimes are independent of scope, can all be achieved by using pointers. A pointer is a variable that holds another variable’s memory address. Pointers are created to point to variables of a particular type—this ensures that Go knows how large (i.e., how many bytes) the pointed-to value occupies. A variable pointed to by a pointer can be modified through the pointer, as we will see shortly. Pointers are cheap to pass (8 bytes on 64-bit machines, 4 bytes on 32-bit machines), regardless of the size of the value they point to. And pointed- to variablespersist in memory for as long as there is at least one pointer pointing to them, so their lifetime is independent of the scope in which they were creat- ed.★ ★ C and C++ programmers should be aware that although a particular Go compiler may make internal distinctions between stack and heap memory, Go programmers never have to worry about this since Go handles all the memory management itself internally. ptg7913109 142 Chapter 4. Collection Types In Go the & operator is overloaded. When used as a binary operator it performs a bitwise AND. When used as a unary operator it returns the memory address of its operand—and it is a memory address that a pointer stores. In Figure 4.2’s third statement we assign the address of variable x of type int to variable pi which has type *int (pointer to int).The unary & is sometimes called the address of operator. The term pointer refers to the fact that a variable that holds the memory address of another variable is considered to be “pointing to” the other variable, as illustrated by the arrows in Figure 4.2. The * operator is also overloaded. It multiplies its operands when used as a binary operator. And when used as a unary operator it provides access to the value pointed to by the variable it is applied to. So, in Figure 4.2, *pi and x can be used interchangeably after the statement pi := &x (but not after pi is assigned to point to a different variable). And since they are both associated with the same int in memory, any changes to one affect the other. The unary * is sometimes called the contents of operator or the indirection operator or the dereference operator. Figure 4.2 also illustrates that if we change the pointed-to value (say, using x++), the value changes as we would expect,and when we dereference the pointer Statement Variable Value Type Memory Address x:=3 x 3 int 0xf840000148 y:=22 y 22 int 0xf840000150 x==3&&y==22 x 3 int 0xf840000148pi := &x pi 0xf840000148 *int 0xf840000158 *pi == 3 && x == 3 && y == 22 x 4 int 0xf840000148x++ pi 0xf840000148 *int 0xf840000158 *pi == 4 && x == 4 && y == 22 x 5 int 0xf840000148*pi++ pi 0xf840000148 *int 0xf840000158 *pi == 5 && x == 5 && y == 22 y y int 0xf840000150pi := &y pi 0xf840000150 *int 0xf840000158 *pi == 22 && x == 5 && y == 22 y 23 int 0xf840000150*pi++ pi 0xf840000150 *int 0xf840000158 *pi == 23 && x == 5 && y == 23 Figure 4.2 Pointers and values ptg7913109 4.1. Values, Pointers, and Reference Types 143 (*pi), it returns the new value. We can also change the value through the pointer. For example, *pi++ means increment the pointed-to value; of course, this will only compile if the value’s type supports the ++ operator,as Go’s built-in numbers do. A pointer doesn’t have to stay pointing to the same value all the time. For ex- ample, toward the bottom of the figure we set the pointer to point to a different value (pi := &y), and then change y through the pointer. We could easily have gone on to change y directly (say, using y++), and then *pi would return y’s new value. It is also possible to have pointers to pointers (and pointers to pointers to pointers, etc.). Using a pointer to refer to a value is called indirection. And if we use pointers to pointers we are said to be using multiple levels of indirection. This is quite common in C and C++, but not needed so often in Go because of Go’s use of reference types. Here is a very simple example. z:=37 // z is of type int pi := &z // pi is of type *int (pointer to int) ppi := &pi // ppi is of type **int (pointer to pointer to int) fmt.Println(z, *pi, **ppi) **ppi++ // Semantically the same as: (*(*ppi))++ and *(*ppi)++ fmt.Println(z, *pi, **ppi) 37 37 37 38 38 38 In this snippet, pi is a pointer of type *int (pointer to int) that is pointing to z of type int, and ppi is a pointer of type **int (pointer to pointer to int) that is pointing to pi. When dereferencing we use one * for each level of indirection, so *ppi dereferences ppi to produce an *int, that is, a memory address, and by applying the * operator a second time (**ppi), we get the pointed-to int. In addition to being the multiplication and dereferencing operator, the * oper- ator is also overloaded for a third purpose—as a type modifier. When an * is placed on the left of a type name it changes the meaning of the name from speci- fying a value of the given type to specifying a pointer to a value of the given type. This is shown in Figure 4.2’s “Type” column. Let’s look at a tiny example to illustrate some of what we’ve discussed so far. i:=9 j:=5 product := 0 swapAndProduct1(&i, &j, &product) fmt.Println(i, j, product) 5945 ptg7913109 144 Chapter 4. Collection Types Here we have created three variables of type int and given them initial values. Then we have called a custom swapAndProduct1() function that takes three int pointers and makes sure that the first two (pointed to) integers are in ascending order and sets the third one’s (pointed to) value to the product of the first two. Since the function takespointersrather than values,we must passthe addresses of the ints, not the ints themselves. Whenever we see the & address of operator being used in a function call,we should assumethat the corresponding variable’s value might be modified inside the function. Here is the swapAndProduct1() func- tion. func swapAndProduct1(x, y, product *int) { if *x>*y{ *x, *y=*y, *x } *product = *x **y // The compiler would be happy with: *product=*x**y } The function’s parameter declaration’s *int uses the * type modifier to specify that the parameters are all pointers to integers. This means, of course, that we can only pass the addresses of integer variables (using the & address of operator), not integer variables themselves or literal integer values. Within the function we are concerned with the values that the pointers point to, so we must use the * dereference operator throughout. In the last executable line we multiply two pointed-to values together and assign the result to another pointer’s pointed-to value. Go can distinguish when two consecutive *s mean multiplication and dereference rather than two dereferences, based on the context. Inside the function the pointers are called x, y, and product, but the values they point to are the ints i, j, and product, at the function’s call site. Writing functions in this way is common in C and older C++ code, but is less often necessary in Go. If we have just one or a few values it is more idiomatic in Go to return them, and if we have lots of values it is common to pass them as a slice or map (which can be cheaply passed without using pointers, as we will see shortly), or in a struct passed by pointer if they are all of different types. Here is a simpler alternative function that doesn’t use pointers: i:=9 j:=5 i, j, product := swapAndProduct2(i, j) fmt.Println(i, j, product) 5945 And here is how we would write the corresponding swapAndProduct2() function. ptg7913109 4.1. Values, Pointers, and Reference Types 145 func swapAndProduct2(x, y int) (int, int, int) { if x>y{ x, y = y, x } return x, y, x * y } This version of the function is perhaps clearer than the first one; but without us- ing pointers it has the disadvantage that it cannot perform the swap in-place. In C and C++ it is common to have functions which accept a pointer to a Boolean that is used to indicate success or failure. This can easily be done in Go by including a *bool in a function’s signature; but it is much more convenient to return a Boolean success flag (or best of all, an error value), as the last (or only) return value, which is standard practice in Go. In the code snippets shown so far, we have used the & address of operator to take the address of function parameters or local variables. Thanks to Go’s automatic memory management this is always safe, since so long as a pointer refers to a variable, that variable will be kept in memory. This is why it is safe to return pointers to local variables created inside functions in Go (something that is a disastrous error in C and C++ for nonstatic variables). In situations where we want to pass around modifiable values of nonreference types or to pass values of large types efficiently, we need to use pointers. Go provides two syntaxes for creating variables and at the same time acquiring pointers to them, one using the built-in new() function and the other using the address of operator. We will look at both syntaxes, and at how to create a plain custom struct value, for comparison. type composer struct { name string birthYear int } Given this struct definition we can create composer values or we can create pointers to composer values, that is, variables of type *composer. And in either case we can take advantage of Go’s support for struct initialization when we use braces. antónio := composer{"António Teixeira", 1707}// composer value agnes := new(composer) // pointer to composer, agnes.birthYear = "Agnes Zimmermann", 1845 julia := &composer{} // pointer to composer, julia.birthYear = "Julia Ward Howe", 1819 augusta := &composer{"Augusta Holmès", 1847}// pointer to composer ptg7913109 146 Chapter 4. Collection Types fmt.Println(antónio) fmt.Println(agnes, augusta, julia) {António Teixeira 1707} &{Agnes Zimmermann 1845} &{Augusta Holmès 1847} &{Julia Ward Howe 1819} When Go prints pointers to structs it prints the dereferenced struct but prefixed with the & addressof operator to indicate that it is a pointer. The part of the code snippet where the agnes and julia pointers are created illustrates the following equivalence when the type is one that can be initialized using braces: new(Type) ≡ &Type{} Both these syntaxes allocate a new zeroed value of the given Type and return a pointer to the value. If the Type isn’t a type that can be initialized using braces then we can use only the built-in new() function. And, of course, we don’t have to worry about the value’s lifetime or ever delete it, since Go’s memory management system takes care of all that for us. One advantage of using the &Type{} syntax for structs is that we can specify initial field values as we did here when creating the augusta pointer. (We can even specify only selected fields and leave the others at their zero values as we will see later; §6.4, ➤ 275.) In addition to values and pointers, Go has reference types. (Go also has inter- faces, but for almost all practical purposes we can consider an interface to be a kind of reference; interfaces are covered later; §6.3, ➤ 265.) A variable of a reference type refers to a hidden value in memory that stores the actual data. Variables holding reference types are cheap to pass (e.g., 16 bytes for a slice and 8 bytes for a map on 64-bit machines), and are used with the same syntax as a value (i.e., we don’t need to take a reference type’s address or dereference it to access the value it refers to). Once we reach the stage where we need to return more than four or five values from a function or method,it is best to passa slice if the values are homogeneous, or to use a pointer to a struct if they are heterogeneous. Passing a slice, or a pointer to a struct, is cheap, and allows us to modify the data in-place. We will look at a couple of small examples to illustrate these points. grades := []int{87, 55, 43, 71, 60, 43, 32, 19, 63} inflate(grades, 3) fmt.Println(grades) [261 165 129 213 180 129 96 57 189] Here we perform an operation on all the numbers in a slice of ints. Maps and slices are reference types, and any changes made to a map or to a slice’s items— ptg7913109 4.1. Values, Pointers, and Reference Types 147 whether directly or inside a function they have been passed to—are visible to all the variables that refer to them. func inflate(numbers []int, factor int) { for i:=range numbers { numbers[i] *= factor } } The grades slice is passed in as the parameter numbers—but unlike when we pass values, any changes applied to numbers are reflected in grades since they both refer to the same underlying slice. Since we want to modify the slice’s values in-place we have used a loop counter to access each item in turn. We didn’t use a for index, item … range loop since that gets a copy of each item from the slice it operates on—this would result in the copy being multiplied by the factor each time and then discarded, leaving the original slice unchanged. We could have used a for loop familiar in other languages (e.g., for i:=0; i < len(numbers); i++), but instead we have used the more convenient for index := range syntax. (All the for loop syntaxes are covered in the next chapter; §5.3, ➤ 203.) Let’s now imagine that we have a rectangle type that stores a rectangle’s position as its top-left and bottom-right x, y coordinates, and its fill color. We could represent the rectangle’s data using a struct. type rectangle struct { x0, y0, x1, y1 int fill color.RGBA } Now we can create a value of the rectangle type,print it, resize it, and then print it again. rect := rectangle{4, 8, 20, 10, color.RGBA{0xFF, 0, 0, 0xFF}} fmt.Println(rect) resizeRect(&rect, 5, 5) fmt.Println(rect) {4 8 20 10 {255 0 0 255}} {4 8 25 15 {255 0 0 255}} As we noted in the previous chapter, even though Go knows nothing of our cus- tom rectangle type it is still able to print it in a sensible way. The output shown below the code snippet clearly shows that the custom resizeRect() function cor- rectly did its job. And rather than passing the whole rectangle (at least 16 bytes ptg7913109 148 Chapter 4. Collection Types for the ints alone), we just passed its address (8 bytes on a 64-bit system,no mat- ter how large the struct is). func resizeRect(rect *rectangle, Δwidth, Δheight int) { (*rect).x1 += Δwidth // Ugly explicit dereference rect.y1 += Δheight // . automatically dereferences structs } The function’s first statement uses an explicit dereference just to show what is happening under the hood. The (*rect) refers to the actual rectangle value that the pointer points to, and the .x1 refers to the rectangle’s x1 field. The second statement shows the idiomatic way to work with struct values—or with pointers to structs—in the latter case relying on Go to do the dereferencing for us. This works because Go’s .(dot) selector operator automatically dereferences pointers to structs.★ Certain types in Go are reference types: maps, slices, channels, functions, and methods. Unlike with pointers, there is no special syntax for reference types since they are used just like values. It is also possible to have pointers to the ref- erence types, although this is really only useful—and sometimes essential—for slices. (We will see the use case for using a pointer to a slice in the next chapter; §5.7, ➤ 244.) If we declare a variable to hold a function, the variable actually gets a reference to the function. Function references know the signature of the function they refer to, so it is not possible to pass a reference to a function that doesn’t have the right signature—thus eliminating some really nasty errors and crashes which can occur in languages that allow functions to be passed by pointer but that don’t guarantee that such functions have the correct signature. We have already seen a few examples of passing function references—for example, when we passed a mapping function to the strings.Map() function (112 ➤ ; 132 ➤ ). We will see many more examples of pointers and reference types throughout the rest of the book. 4.2. Arrays and Slices A Go array is a fixed-length sequence of itemsof the same type.Multidimension- al arrays can be created simply by using items that are themselves arrays. Array items are indexed using the [] index operator by their 0-based position,so an array’s first item is array[0] and its last item is array[len(array)-1]. Arrays are mutable, so we can use the syntax array[index] on the left of an assignment ★ Go doesn’t have or need the -> dereferencing operator used in C and C++. Go’s .(dot) operator is sufficient for most situations (e.g., to access the fields in a struct or a pointer to a struct), and where it isn’t, we can explicitly dereference using as many * operators as there are levels of indirection. ptg7913109 4.2. Arrays and Slices 149 to set the array’s item at the given index position. We can also use this syntax in an expression on the right of an assignment or in a function call, to access the item. Arrays are created using the syntaxes: [length]Type [N]Type{value1, value2,…, valueN} […]Type{value1, value2,…, valueN} If the ... (ellipsis) operator is used in this context, Go will calculate the array’s length for us. (The ellipsis operator is overloaded for other purposes, as we will see later in this chapter and in Chapter 5.) In all cases an array’s length is fixed and unchangeable. Here are some examples that show how to create and index arrays. var buffer [20]byte var grid1 [3][3]int grid1[1][0], grid1[1][1], grid1[1][2]=8, 6, 2 grid2 := [3][3]int{{4, 3}, {8, 6, 2}} cities := [...]string{"Shanghai","Mumbai","Istanbul","Beijing"} cities[len(cities)-1]="Karachi" fmt.Println("Type Len Contents") fmt.Printf("%-8T %2d %v\n", buffer, len(buffer), buffer) fmt.Printf("%-8T %2d %q\n", cities, len(cities), cities) fmt.Printf("%-8T %2d %v\n", grid1, len(grid1), grid1) fmt.Printf("%-8T %2d %v\n", grid2, len(grid2), grid2) Type Len Contents [20]uint8 20 [0 0000000000000000000] [4]string 4 ["Shanghai" "Mumbai" "Istanbul" "Karachi"] [3][3]int 3 [[0 0 0] [8 6 2] [0 0 0]] [3][3]int 3 [[4 3 0] [8 6 2] [0 0 0]] Go guarantees that all array items are initialized to their zero value if they are not explicitly initialized—or are only partly initialized—when they are created, as the buffer, grid1, and grid2 variables illustrate. The length of an array is given by the len() function. Since arrays are of fixed size their capacity is alwaysequal to their length,so for arraysthe cap() function returns the same number as the len() function. Arrays can be sliced using the same slicing syntax as strings or slices,only the result is a slice and not an array. And just like strings and slices, arrays can be iterated using a for … range loop (§5.3, ➤ 203). In general, Go’s slices are more flexible, powerful, and convenient than arrays. Arrays are passed by value (i.e., copied)—although the cost of this can be avoid- ptg7913109 150 Chapter 4. Collection Types ed by passing pointers—whereas slices are cheap to pass, regardless of their length or capacity, since they are references. (A slice is passed as a 16-byte val- ue on 64-bit machines and as a 12-byte value on 32-bit machines, no matter how many items it contains.) Arrays are of fixed size whereas slices can be resized. The functions in Go’s standard library all use slices rather than arrays in their public APIs.★ We recommend always using slices unless there is a very specific need to use an array in a particular case. Both arrays and slices can be sliced using the syntaxes shown in Table 4.1 (➤ 151). A Go slice is a variable-length fixed-capacity sequence of items of the same type. Despite their fixed capacity, slices can be shrunk by slicing them and can be grown using the efficient built-in append() function, as we will see later in this section. Multidimensional slices can be created quite naturally by using items that are themselves slices—and the lengths of the inner slices in multidimen- sional slices may vary. Although arrays and slices store items of the same type there is no limitation in practice. This is because the type used could be an interface. So we could store items of any types provided that they all met the specified interface (i.e., had the method or methods that the interface requires). We can even make an array or slice’s type the empty interface, interface{}, which means that we could store any items of any types—although when we accessed an item we would need to use a type assertion or a type switch or introspection to make use of the item. (Interfaces are covered in Chapter 6; reflection is covered in §9.4.9, ➤ 427.) Slices are created using the syntaxes: make([]Type, length, capacity) make([]Type, length) []Type{} []Type{value1, value2,…, valueN} The built-in make() function is used to create slices, maps, and channels. When used to create a slice it creates a hidden zero-value initialized array,and returns a slice reference that refers to the hidden array. The hidden array,like all arrays in Go, is of fixed length, with the length being the slice’s capacity if the first syntax is used, or the slice’s length if the second syntax is used, or the number of items in braces if the composite literal (third and fourth) syntax is used. A slice’s capacity is the length of its hidden array, and its length is any amount up to its capacity. In the first syntax the slice’s length must be less than or equal to the capacity, although normally this syntax is used when we want the initial length to be less than the capacity. The second, third, and fourth syntaxes are used when we want the length and capacity to be the same. The composite ★ At the time of this writing, the Go documentation often uses the term array when describing parameters that are actually slices. ptg7913109 4.2. Arrays and Slices 151 Table 4.1 Slice Operations Syntax Description/result s[n] The item at index position n in slice s s[n:m]A slice taken from slice s from index positions n to m -1 s[n:] A slice taken from slice s from index positions n to len(s)- 1 s[:m]A slice taken from slice s from index positions 0 to m -1 s[:] A slice taken from slice s from index positions 0 to len(s)- 1 cap(s) The capacity of slice s; always ≥ len(s) len(s) The number of items in slice s; always ≤ cap(s) s = s[:cap(s)] Increase slice s’s length to its capacity if they are different literal (fourth) syntax is very convenient, since it allows us to create a slice with some initial values. The syntax []Type{} is equivalent to make([]Type,0); both create an empty slice. This isn’t useless since we can use the built-in append() function to effectively increase a slice’s capacity. However, for practical purposes, when we need an initially empty slice it is almost always better to create one using make(), giving it a length of zero and a nonzero capacity that is or approximates the number of items we expect the slice to end up with. Valid index positions for a slice range from 0 to len(slice)-1.A slice can be resliced to reduce its length,and if a slice’s capacity is greater than its length the slice can be resliced to increase its length up to its capacity. We can also increase a slice’s capacity using the built-in append() function; we will see examples later in this section. Figure 4.3 (➤ 152) provides a conceptual view of the relationship between slices and their hidden arrays. Here are the slices it shows. s := []string{"A","B","C","D","E","F","G"} t := s[:5]// [A BCDE] u:=s[3 : len(s)-1]// [D E F] fmt.Println(s, t, u) u[1]="x" fmt.Println(s, t, u) [A BCDEFG][ABCDE][DEF] [A BCDxFG][ABCDx][DxF] Since the slices s, t, and u all refer to the same underlying data, a change to one will affect any of the others that refer to the same data. ptg7913109 152 Chapter 4. Collection Types 0 1 2 3 4 5 6 Indexes "A" "B" "C" "D" "E" "F" "G" Hidden array s := []string{"A", … t := s[:5] u := s[3:len(s) - 1] len(s) == 7 len(t) == 5 len(u) == 3 cap(s) == 7 cap(t) == 7 cap(u) == 4 Figure 4.3 A conceptual view of some slices and their hidden array s:=new([7]string)[:] s[0], s[1], s[2], s[3], s[4], s[5], s[6]="A","B","C","D","E","F","G" Using the built-in make() function or the composite literal syntax are the best ways to create slices, but here we show an approach that is not used in practice but that makes the array–slice relationship obvious. The first statement creates a pointer to an array using the built-in new() function, and then immediately takes a slice of the entire array. This will produce a slice with a length and capacity equal to the array’s length, but with every item set to its zero value, in this case an empty string. The second statement completes the setup of the slice by setting the individual items to the initial values we want, after which this slice s is exactly the same as the one created in the previous snippet using the composite literal syntax. Here are the slice-based equivalents to the array examples we saw earlier, except that we have set the buffer’s capacity to be greater than its length just to show how it is done. buffer := make([]byte, 20, 60) grid1 := make([][]int, 3) for i:=range grid1 { grid1[i] = make([]int, 3) } grid1[1][0], grid1[1][1], grid1[1][2]=8, 6, 2 grid2 := [][]int{{4, 3, 0}, {8, 6, 2}, {0, 0, 0}} cities := []string{"Shanghai","Mumbai","Istanbul","Beijing"} cities[len(cities)-1]="Karachi" fmt.Println("Type Len Cap Contents") fmt.Printf("%-8T %2d %3d %v\n", buffer, len(buffer), cap(buffer), buffer) fmt.Printf("%-8T %2d %3d %q\n", cities, len(cities), cap(cities), cities) fmt.Printf("%-8T %2d %3d %v\n", grid1, len(grid1), cap(grid1), grid1) fmt.Printf("%-8T %2d %3d %v\n", grid2, len(grid2), cap(grid2), grid2) ptg7913109 4.2. Arrays and Slices 153 Type Len Cap Contents []uint8 20 60 [0 0000000000000000000] []string 4 4 ["Shanghai" "Mumbai" "Istanbul" "Karachi"] [][]int 3 3 [[0 0 0] [8 6 2] [0 0 0]] [][]int 3 3 [[4 3 0] [8 6 2] [0 0 0]] The buffer’s contents are only the first len(buffer) items; the other items are inaccessible unless we reslice the buffer—something we will see how to do later on in this section. We created grid1 as a slice of slices with an initial length of 3 (i.e., it can contain three slices), and a capacity of 3 (since the capacity defaults to the length if it isn’t specified). Then we set each of the grid’s outermost slices to contain their own 3-item slices. Naturally, we could have made the innermost slices have different lengths if we wanted. For grid2 we had to specify every value since we created it using the composite literal syntax and Go would have no other way of knowing how many items we wanted. After all, we could have created a slice of different length slices—for example, grid2 := [][]int{{9, 7}, {8}, {4, 2, 6}},which would make grid2 a slice of length 3 whose slices’ lengths are 2, 1, and 3. 4.2.1. Indexing and Slicing Slices A slice is a reference to a hidden array and slices of slices are also references to the same hidden array. Here is an example to illustrate what this means. s := []string{"A","B","C","D","E","F","G"} t:=s[2:6] fmt.Println(t, s, "=", s[:4], "+",s[4:]) s[3]="x" t[len(t)-1]="y" fmt.Println(t, s, "=", s[:4], "+",s[4:]) [C DEF][ABCDEFG]=[ABCD]+[EFG] [C xEy][ABCxEyG]=[ABCx]+[EyG] When we change the data—whether via the original s slice or from the t slice of the s slice—the same underlying data is changed, so both slices are affected. The code snippet also illustrates that given a slice s and an index position i (0 ≤ i ≤ len(s)), s is equal to the concatenation of s[:i] and s[i:]. We saw a similar equality in the previous chapter in reference to strings: s == s[:i] + s[i:] // s is a string; i is an int; 0 <= i <= len(s) ptg7913109 154 Chapter 4. Collection Types Figure 4.4 shows slice s, including all its valid index positions and the slices used in the code snippet. The first index position in any slice is 0 and the last is always len(s)-1. s[2:6] s[:4] s[4:] Slices "A" "B" "C" "D" "E" "F" "G" Slice s 0 1 2 3 4 5 6 len(s)-7 len(s)-6 len(s)-5 len(s)-4 len(s)-3 len(s)-2 len(s)-1 Indexes Figure 4.4 Anatomy of a slice Unlike strings, slices don’t support the + or += operators. Nonetheless, it is easy to append to slices—and also to insert and remove items, as we will see shortly (§4.2.3, ➤ 156). 4.2.2. Iterating Slices One frequent requirement is to iterate over all the items in a slice. If we want to access the items without modifying them we can use a for … range loop; and if we need to modify items we can use a for loop with a loop counter. Here is an example of the former. amounts := []float64{237.81, 261.87, 273.93, 279.99, 281.07, 303.17, 231.47, 227.33, 209.23, 197.09} sum := 0.0 for _, amount := range amounts { sum += amount } fmt.Printf("Σ%.1f → %.1f\n", amounts, sum) Σ[237.8 261.9 273.9 280.0 281.1 303.2 231.5 227.3 209.2 197.1] → 2503.0 The for … range loop assigns a 0-based loop counter, which in this case we have discarded using the blank identifier (_), and a copy of the corresponding item from the slice. The copy is cheap even for strings (since they are passed by reference). This means that any changes that are applied to the item affect only the copy, not the item in the slice. Naturally, we can use slicing to iterate over just a portion of the slice. For example, if we just wanted to iterate over the first five items we would write for _, amount := range amounts[:5]. If we want to modify the items in the slice we must use a for loop that just provides valid slice indexes and not copies of the slice’s items. ptg7913109 4.2. Arrays and Slices 155 for i:=range amounts { amounts[i] *= 1.05 sum += amounts[i] } fmt.Printf("Σ%.1f → %.1f\n", amounts, sum) Σ[249.7 275.0 287.6 294.0 295.1 318.3 243.0 238.7 219.7 206.9] → 2628.1 Here we have increased each item in the slice by 5% and accumulated their sum. Slices can, of course, contain custom items. Here is a custom type with a single custom method. type Product struct { name string price float64 } func (product Product) String() string { return fmt.Sprintf("%s (%.2f)",, product.price) } This defines the Product type as a struct with string and float64 fields. We have also defined a String() method to control how Go prints Product items using the %v verb. (We discussed print verbs earlier; §3.5, 93 ➤ . We briefly introduced custom types and methods in §1.5, 21 ➤ ; much more coverage is provided in Chapter 6.) products := []*Product{{"Spanner", 3.99}, {"Wrench", 2.49}, {"Screwdriver", 1.99}} fmt.Println(products) for _, product := range products { product.price += 0.50 } fmt.Println(products) [Spanner (3.99) Wrench (2.49) Screwdriver (1.99)] [Spanner (4.49) Wrench (2.99) Screwdriver (2.49)] Here we have created a slice of pointers to Products ([]*Product), and immedi- ately initialized the slice with three *Products. This works because Go is smart enough to realize that a []*Product requires pointers to Products. What we have written is really a shorthand for products := []*Product{&Product{"Spanner", 3.99}, &Product{"Wrench", 2.49}, &Product{"Screwdriver", 1.99}}.(Recall from §4.1, 140 ➤ , that we can use the &Type{} syntax to create a new value of the type and immediately get a pointer to it.) ptg7913109 156 Chapter 4. Collection Types If we had not defined a Product.String() method the %v verb (which is used im- plicitly by fmt.Println() and similar functions) would simply print the memory addresses of the Products rather than the Products themselves. Notice also that the Product.String() method takes a Product value, not a *Product—this isn’t a problem, though, since Go is smart enough to dereference *Products to make them work with custom methods that take Product values.★ We noted earlier that the for … range loop cannot be used to modify the items it iterates over. Yet here we have successfully incremented all the prices in the products slice. At each iteration the product variable is assigned a copy of a *Product; this is a pointer that points to the same underlying Product as the corresponding one in the products slice. Thus, the modification we are applying is to the pointed-to Product value, not to the copy of the *Product pointer. 4.2.3. Modifying Slices If we need to append to a slice we can use the built-in append() function. This function takes the slice to be appended to and one or more individual items to append. If we want to append a slice to a slice we must use the ... (ellipsis) operator to tell Go to pass the slice to be added as individual values. The values to append must be of the same type as the slice’s value type. In the case of a string we can append its individual bytes to a byte slice by using the ellipsis syntax. s := []string{"A","B","C","D","E","F","G"} t := []string{"K","L","M","N"} u := []string{"m","n","o","p","q","r"} s=append(s, "h","i","j")// Append individual values s=append(s, t...) // Append all of a slice's values s=append(s, u[2:5]...) // Append a subslice b := []byte{'U','V'} letters := "wxy" b=append(b, letters...) // Append a string's bytes to a byte slice fmt.Printf("%v\n%s\n",s,b) [A BCDEFGhijKLMNopq] UVwxy The built-in append() function takes a slice and one or more values and returns a (possibly new) slice which has the original slice’s contents,plus the given value or values as its last item or items. If the original slice’s capacity is sufficient for the new items (i.e., its length plus the number of new items is within its capac- ★ One compiler does this as follows. Whenever a method is created that operates on a value, say it is called Method(), a wrapper method of the same name and signature is created that has a pointer receiver—in effect, func (value *Type) Method() { return (*value).Method() }. ptg7913109 4.2. Arrays and Slices 157 ity), append() puts the new value or values in the empty position or positions at the end and returns the original slice with its length increased by the number of items added. If the original slice doesn’t have sufficent capacity, the append() function creates a new slice under the hood and copies the original slice’s items into it, plus the new value or values at the end, and returns the new slice—hence the need to assign append()’s return value to the original slice variable. It sometimes occurs that we want to insert items at the front or in the middle of a slice, not just at the end. Here are some examples that use a custom InsertStringSliceCopy() function that takes a slice to insert into, a slice to insert, and the index position where the insertion should be made. s := []string{"M","N","O","P","Q","R"} x := InsertStringSliceCopy(s, []string{"a","b","c"}, 0)// At the front y := InsertStringSliceCopy(s, []string{"x","y"}, 3)// In the middle z := InsertStringSliceCopy(s, []string{"z"}, len(s)) // At the end fmt.Printf("%v\n%v\n%v\n%v\n",s,x,y,z) [M NOPQR] [a bcMNOPQR] [M NOxyPQR] [M NOPQRz] The custom InsertStringSliceCopy() function creates a new slice (which is why slice s is unchanged at the end of the snippet), making use of the built-in copy() function to copy the first slice it is given and to insert the second slice. func InsertStringSliceCopy(slice, insertion []string, index int) []string { result := make([]string, len(slice)+len(insertion)) at := copy(result, slice[:index]) at += copy(result[at:], insertion) copy(result[at:], slice[index:]) return result } The built-in copy() function takes two slices(which could be portionsof the same slice—even overlapping ones) that contain items of the same type. The function copies the items into the first (destination) slice from the second (source) slice and returns the number of items copied. If the source slice is empty, the copy() function will safely do nothing. If the destination slice’s length is insufficient to accommodate the source slice’s items, the items that don’t fit are silently ignored. If the destination slice’s capacity is greater than its length, we can increase its length to its capacity with the statement slice = slice[:cap(slice)], before doing the copy. ptg7913109 158 Chapter 4. Collection Types The slices passed to the built-in copy() function must be of the same type—ex- cept that if the first (destination) slice is a []byte the second (source) argument may be a []byte or a string. If the source is a string, its bytes are copied into the first argument. (An example of this use is shown in Chapter 6, ➤ 268.) In the custom InsertStringSliceCopy() function, we begin by creating a new slice (result) that is large enough to hold the items from the two slices passed in. Then we copy a subslice of the first slice (slice[:index]) into the result slice. Next we copy the insertion slice into the result slice starting at the position in the result slice we have reached (at). Then we copy the rest of the first slice (slice[index:]) into the result slice at the next position we have reached (at).For this last copy we ignore the copy() function’s return value since we don’t need it. And finally, we return the result slice. If the index position is 0, the slice[:index] in the first copy statement will be slice[:0] (i.e., an empty slice), so no copying is done. Similarly, if the index is greater than or equal to the length of the slice the slice[index:] in the last copy statement will effectively be slice[len(slice):] (i.e., an empty slice), so again, no copying is done. Here is a function that has almost the same behavior as the InsertStringSlice- Copy() function, but with much shorter and simpler code. The difference is that the InsertStringSlice() function changes the original slice (and possibly the in- serted slice), whereas the InsertStringSliceCopy() function does not. func InsertStringSlice(slice, insertion []string, index int) []string { return append(slice[:index], append(insertion, slice[index:]...)...) } The InsertStringSlice() function appends the end of the original slice from the index position onto the end of the insertion slice,and then appends the resultant slice onto the end of the original slice at the index position. The returned slice is the original slice with the insertion applied. (Recall that append() takes a slice and one or more values, so we must use the ellipsis syntax to transform a slice into its individual values—and in this example, we must do so twice.) Items can be removed from the beginning and end of slices using Go’s standard slice syntax, but removing from the middle can require a little care. We will start by seeing how to remove from the start, end, and middle of a slice, working on the slice in-place. Then we will see how to take a copy of a slice with items removed that leaves the original slice unchanged. s := []string{"A","B","C","D","E","F","G"} s=s[2:] // Remove s[:2] from the front fmt.Println(s) [C DEFG] ptg7913109 4.2. Arrays and Slices 159 Removing items from the start of a slice is easily achieved by reslicing. s := []string{"A","B","C","D","E","F","G"} s = s[:4]// Remove s[4:] from the end fmt.Println(s) [A BCD] Removing items from the end of a slice is achieved by reslicing, just the same as for removing at the start. s := []string{"A","B","C","D","E","F","G"} s=append(s[:1], s[5:]...) // Remove s[1:5] from the middle fmt.Println(s) [A F G] Retrieving items from the middle of a slice is easy—for example, to get the three middle items of slice s, we would use the expression s[2:5]. But to remove items from the middle of a slice is slightly tricky. Here we have done the removal using the append() function to append the subslice of slice s that follows what we want to delete, to the subslice of slice s that precedes what we want to delete, and assigning the resultant slice back to s. Clearly, using append() and assigning back to the original slice to remove items, changes the original slice. Here are some examples that use a custom RemoveStringSliceCopy() function that returns a copy of the slice it is given, but with the items from the start and end index positions removed. s := []string{"A","B","C","D","E","F","G"} x := RemoveStringSliceCopy(s, 0, 2)// Remove s[:2] from the front y := RemoveStringSliceCopy(s, 1, 5)// Remove s[1:5] from the middle z := RemoveStringSliceCopy(s, 4, len(s)) // Remove s[4:] from the end fmt.Printf("%v\n%v\n%v\n%v\n",s,x,y,z) [A BCDEFG] [C DEFG] [A F G] [A BCD] Since the RemoveStringSliceCopy() function copies the items, the original slice is left intact. func RemoveStringSliceCopy(slice []string, start, end int) []string { result := make([]string, len(slice)-(end-start)) at := copy(result, slice[:start]) ptg7913109 160 Chapter 4. Collection Types copy(result[at:], slice[end:]) return result } In the custom RemoveStringSliceCopy() function,we begin by creating a new slice (result) that is large enough to hold the items it will contain. Then we copy a subslice of the slice up to the start position (slice[:start]) into the result slice. Next we copy the slice from the end position (slice[end:]) into the result slice at the position we have reached (at). And finally, we return the result slice. It is also possible to create a simpler RemoveStringSlice() function that works on the slice it is given rather than making a copy. func RemoveStringSlice(slice []string, start, end int) []string { return append(slice[:start], slice[end:]...) } This is a generalization of the remove from the middle example that used the built-in append() function shown earlier. The returned slice is the original slice with the items from the start position up to (but excluding) the end position removed. 4.2.4. Sorting and Searching Slices The standard library’s sort package provides functions for sorting slices of ints, float64s, and strings, for checking if such a slice is sorted, and for searching for an item in a sorted slice using the fast binary search algorithm. There are also generic sort.Sort() and sort.Search() functions that can easily be used with custom data. These functions are listed in Table 4.2. The way that Go sorts numbers holds no surprises,as we saw in an earlier chap- ter (73 ➤ ). However, strings are sorted purely in terms of the bytes that repre- sent them, as we discussed in the previous chapter (§3.2, 86 ➤ ). This means, for example, that string sorting is case-sensitive. Here are a couple of string sorting examples and the results they produce. files := []string{"Test.conf","util.go","Makefile","misc.go","main.go"} fmt.Printf("Unsorted: %q\n", files) sort.Strings(files) // Standard library sort function fmt.Printf("Underlying bytes: %q\n", files) SortFoldedStrings(files) // Custom sort function fmt.Printf("Case insensitive: %q\n", files) Unsorted: ["Test.conf" "util.go" "Makefile" "misc.go" "main.go"] Underlying bytes: ["Makefile" "Test.conf" "main.go" "misc.go" "util.go"] Case insensitive: ["main.go" "Makefile" "misc.go" "Test.conf" "util.go"] ptg7913109 4.2. Arrays and Slices 161 Table 4.2 The Sort Package’s Functions Syntax Description/result sort.Float64s(fs) Sorts fs of type []float64 into ascending order sort.Float64sAreSorted(fs) Returns true if fs of type []float64 is sorted sort.Ints(is) Sorts is of type []int into ascending order sort.IntsAreSorted(is) Returns true if is of type []int is sorted sort.IsSorted(d) Returns true if d of type sort.Interface is sorted sort.Search(size, fn) Returns the index position in a sorted slice in scope of length size where function fn with the signature func(int) bool returns true (see text) sort.SearchFloat64s(fs, f) Returns the index position of f of type float64 in sorted fs of type []float64 sort.SearchInts(is, i) Returns the index position of i of type int in sorted is of type []int sort.SearchStrings(ss, s) Returns the index position of s of type string in sorted ss of type []string sort.Sort(d) Sorts d of type sort.Interface (see text) sort.Strings(ss) Sorts ss of type []string into ascending order sort.StringsAreSorted(ss) Returns true if ss of type []string is sorted The standard library’s sort.Strings() function takes a []string and sorts the strings in-place in ascending order in terms of their underlying bytes. If the strings have all been encoded using the same character to bytes mappings (e.g., they were all created in the current program or by other Go programs), this results in code-point ordering. The custom SortFoldedStrings() function works in the same way, except that it sorts case-insensitively using the sort package’s generic sort.Sort() function. The sort.Sort() function can sort items of any type that provide the methods in the sort.Interface, that is, items of a type that provide the Len(), Less(), and Swap() methods, each with the required signatures. We have created a custom type, FoldedStrings, that provides these methods. Here is the complete implementation of the SortFoldedStrings() function, the FoldedStrings type, and the supporting methods. func SortFoldedStrings(slice []string) { sort.Sort(FoldedStrings(slice)) } type FoldedStrings []string func (slice FoldedStrings) Len() int { return len(slice) } ptg7913109 162 Chapter 4. Collection Types func (slice FoldedStrings) Less(i, j int) bool { return strings.ToLower(slice[i]) < strings.ToLower(slice[j]) } func (slice FoldedStrings) Swap(i, j int) { slice[i], slice[j] = slice[j], slice[i] } The SortFoldedStrings() function simply calls the standard library’s sort.Sort() function to do the work—having (very cheaply) converted the given []string into a FoldedStrings value using Go’s standard conversion syntax. In general, whenever we create a custom type that is based on a built-in type we can promote a value of that built-in type to the custom type by doing a conversion in this way. (Custom types are covered in Chapter 6.) The FoldedStrings type provides the three methods needed to satisfy the sort. Interface interface. All the methods are trivial;case-insensitivity is achieved by using the strings.ToLower() function in the Less() method. (And if we wanted to sort in descending order we could simply change the Less() method’s < less than operator to a > greater than operator.) The SortFoldedStrings() function is perfectly adequate for 7-bit ASCII (i.e., En- glish) strings, but is unlikely to produce a satisfactory ordering for non-English languages as we discussed in the previous chapter (§3.2, 86 ➤ ). Sorting Unicode strings with correct accounting for non-English languages is not a trivial under- taking. It is explained in detail in the Unicode Collation Algorithm document ( If we want to search a slice to find the index position of a particular item (if it contains the item), we can easily do so using a for … range loop. files := []string{"Test.conf","util.go","Makefile","misc.go","main.go"} target := "Makefile" for i, file := range files { if file == target { fmt.Printf("found \"%s\" at files[%d]\n", file, i) break } } found "Makefile" at files[2] Using a simple linear search like this is the only option for unsorted data and is fine for small slices (up to hundreds of items). But for larger slices—especially if we are performing searches repeatedly—the linear search is very inefficient, on average requiring half the items to be compared each time. ptg7913109 4.2. Arrays and Slices 163 Go provides a sort.Search() method which uses the binary search algorithm: This requires the comparison of only log2(n) items (where n is the number of items) each time. To put this in perspective, a linear search of 1000000 items requires 500000 comparisons on average, with a worst case of 1000000 compar- isons; a binary search needs at most 20 comparisons, even in the worst case. sort.Strings(files) fmt.Printf("%q\n", files) i := sort.Search(len(files), func(i int) bool { return files[i] >= target }) if i= operator for slices that are sorted in ascending order or the <= operator for slices sorted in descending order. The function must be a closure, that is, it must be created in the scope of the slice it is to work on since it must capture the slice as part of its state. (Closures are covered in §5.6.3, ➤ 225.) The sort.Search() function returns an int; only if this is less than the length of the slice and the item at that index position matches the target, can we be sure that we have found the item we are looking for. Here is a variation that searches a []string that has been sorted case-insensi- tively and that assumes a lowercase target string. target := "makefile" SortFoldedStrings(files) fmt.Printf("%q\n", files) caseInsensitiveCompare := func(i int) bool { return strings.ToLower(files[i]) >= target } i := sort.Search(len(files), caseInsensitiveCompare) if i 0 && same { return separators[sepIndex] } } return "" } This function’s purpose is to find the first []int in the counts slices whose counts are all the same—and nonzero. The function iterates over each “row” in counts (one per separator), and initially assumes that all the row’s counts are the same. It sets the initial count to the first count, that is, to the number of times the separator occurs in the first line that was read. Then it iterates over the rest, that is, over the separator counts for each of the other lines that was read. If a different count is encountered the inner for loop is broken out of and the next separator tried. If the inner for loop ptg7913109 174 Chapter 4. Collection Types completes without setting same to false, and the count is greater than zero, we have found what we want and immediately return it. If no separator matches we return an empty string—this is our convention to mean that the fields are whitespace-separated, or not separated at all. func report(separator string) { switch separator { case "": fmt.Println("whitespace-separated or not separated at all") case "\t": fmt.Println("tab-separated") default: fmt.Printf("%s-separated\n", separator) } } The report() function is trivial, writing a simple description of the separator used by the file that was read. This example has shown both one- and two-dimensional slices (separators, lines, and counts) in typical use. In the next example we will look at maps, slices, and sorting. 4.4.2. Example: Word Frequencies Textual analysis has a variety of uses, from data mining to the study of language itself. In this subsection we will review an example that performs one of the most basic forms of textual analysis: It counts the frequencies of words in the files it is given. Frequency data can be presented in two different but equally sensible ways—as an alphabetical list of words with their frequencies, and as an ordered list of frequency values and the words that have the corresponding frequencies. The wordfrequency program (in file wordfrequency/wordfrequency.go) produces both kinds of output, as illustrated below. $ ./wordfrequency small-file.txt Word Frequency ability 1 about 1 above 3 ... years 1 you 128 Frequency → Words ptg7913109 4.4. Examples 175 1 ability, about, absence, absolute, absolutely, abuse, accessible, ... 2 accept, acquired, after, against, applies, arrange, assumptions, ... ... 128 you 151 or 192 to 221 of 345 the Even for a small file the number of words and the number of different frequen- cies can be quite large, so here we have elided most of the output. Producing the first part of the output is straightforward. We can use a map of type map[string]int with word keys and frequency values. But to get the second part of the output we will need to invert the map—and this isn’t quite so easy because it is likely that more than one word will have the same frequency. The solution is to invert to a multivalued map of type map[int][]string, that is,a map whose keys are frequencies and whose values are all the words that have the corresponding frequency. We will begin with the program’s main() function and work top-down, and as usual, will omit the imports. func main() { if len(os.Args) == 1 || os.Args[1]=="-h" || os.Args[1]=="--help" { fmt.Printf("usage: %s [ [... ]]\n", filepath.Base(os.Args[0])) os.Exit(1) } frequencyForWord := map[string]int{} // Same as: make(map[string]int) for _, filename := range commandLineFiles(os.Args[1:]) { updateFrequencies(filename, frequencyForWord) } reportByWords(frequencyForWord) wordsForFrequency := invertStringIntMap(frequencyForWord) reportByFrequency(wordsForFrequency) } The main() function starts by dealing with the command line and then gets down to work. We begin by creating the simple map that will keep track of the frequency of each unique word in the files read in. We have used the composite literal syntax to create the initially empty map, just to show how it is done. Once we have the map we iterate over each filename given on the command line and for each one attempt to update the frequencyForWord map. ptg7913109 176 Chapter 4. Collection Types Once the first map is complete we output the first report: an alphabetical list of all the unique words encountered and their corresponding frequencies. Then we create an inverted version of the map and output the second report: a numerically ordered list of frequencies and their associated words. func commandLineFiles(files []string) []string { if runtime.GOOS == "windows" { args := make([]string, 0, len(files)) for _, name := range files { if matches, err := filepath.Glob(name); err != nil { args = append(args, name) // Invalid pattern } else if matches != nil {// At least one match args = append(args, matches...) } } return args } return files } The commandLineFiles() function simply returns the []string it is given on Unix- like platforms such as Linux and Mac OS X, since on these platforms the shell automatically does file globbing (i.e., replaces, say, *.txt with any matching text files, e.g., README.txt, INSTALL.txt, etc.). The Windows shell (cmd.exe) does not do file globbing, so if the user enters, say, *.txt on the command line, that is what the program will receive. To provide reasonable cross-platform uniformity, we do the globbing ourselves when the program is run on Windows. (Another way to handle cross-platform differences is to have platform-specific .go files—this is covered in a later chapter, §, ➤ 410.) func updateFrequencies(filename string, frequencyForWord map[string]int) { var file *os.File var err error if file, err = os.Open(filename); err != nil { log.Println("failed to open the file: ", err) return } defer file.Close() readAndUpdateFrequencies(bufio.NewReader(file), frequencyForWord) } This function is used purely for the file handling. It opens the given file for reading, defers the closing of the file to when the function returns, and passes on the actual work to the readAndUpdateFrequencies() function. By passing the file reader as a *bufio.Reader (produced by the bufio.NewReader() call),we ensure ptg7913109 4.4. Examples 177 that the called function can read the file as strings line by line rather than having to read raw bytes. func readAndUpdateFrequencies(reader *bufio.Reader, frequencyForWord map[string]int) { for { line, err := reader.ReadString('\n') for _, word := range SplitOnNonLetters(strings.TrimSpace(line)) { if len(word) > utf8.UTFMax || utf8.RuneCountInString(word) > 1 { frequencyForWord[strings.ToLower(word)] += 1 } } if err != nil { if err != io.EOF { log.Println("failed to finish reading the file: ", err) } break } } } The first part of this function should be very familiar by now. We create an infinite loop and read the file line by line, breaking out of the loop when we reach the end of the file or if an error occurs (in which case we report the error to the user). We don’t terminate the program when we hit an error because there might be many files to read and for this program we prefer to do as much work as possible and report any problems that were encountered rather than stopping at the first error. The inner for loop is where the interesting processing is done. Any given line might have punctuation, numbers, symbols, and other nonword characters, so we iterate word by word having split the line into words and discarding any non-word characters using a custom SplitOnNonLetters() function. And the string we feed that function in the first place has any whitespace trimmed off both ends. We only want to include words that contain at least two letters. The easiest way to do this is to use a one-clause if statement, that is, if utf8.RuneCountIn- String(word) > 1, which works fine. The simple if statement just described is potentially a bit expensive because it will parse the entire word. So in the program we use a two-clause if statement where the first clause takes a much cheaper approach. The first clause checks to see if the number of bytes in the word is greater than utf8.UTFMax (which is a constant of value 4, the maximum number of bytes required to represent a single UTF-8 character). This is a really fast test because Go strings know ptg7913109 178 Chapter 4. Collection Types how many bytes they contain and Go’s binary Boolean operators (&& and ||) are short-circuiting (§2.2, 56 ➤ ). Of course, words consisting of four or fewer bytes (e.g., four 7-bit ASCII characters or a couple of 2-byte UTF-8 characters) will fail this first check, but that isn’t a problem because the second check (the rune count) will be fast because it will always have four or fewer characters to count. Is it worth using the two-clause if statement in this situation? It really depends on the input—the more words that need processing and the longer they are, the more potential for savings. The only way to know for certain is to benchmark using real or at least typical data. func SplitOnNonLetters(s string) []string { notALetter := func(char rune) bool { return !unicode.IsLetter(char) } return strings.FieldsFunc(s, notALetter) } This function is used to split a string on nonword characters. First we create an anonymous function that has the signature required by the strings.Fields- Func() function and which returns true for nonletters and false for letters. Then we return the result of calling the strings.FieldsFunc() function with the given string and with the notALetter() function. (We discussed the strings.Fields- Func() function in the previous chapter; 107 ➤ .) func reportByWords(frequencyForWord map[string]int) { words := make([]string, 0, len(frequencyForWord)) wordWidth, frequencyWidth := 0, 0 for word, frequency := range frequencyForWord { words = append(words, word) if width := utf8.RuneCountInString(word); width > wordWidth { wordWidth = width } if width := len(fmt.Sprint(frequency)); width > frequencyWidth { frequencyWidth = width } } sort.Strings(words) gap := wordWidth + frequencyWidth - len("Word")-len("Frequency") fmt.Printf("Word %*s%s\n", gap, "","Frequency") for _, word := range words { fmt.Printf("%-*s%*d\n", wordWidth, word, frequencyWidth, frequencyForWord[word]) } } Once the frequencyForWord map has been populated, the reportByWords() func- tion is called to output its data. We want the output to be in alphabetical (ac- ptg7913109 4.4. Examples 179 tually, Unicode code point) order, so we begin by creating an empty []string to hold the words that is large enough to hold all the words in the map. We also want to know the width in characters of the longest word and of the highest frequency (i.e., how many digits it has) so that we can produce our output in neat columns: The wordWidth and frequencyWidth variables are used to record these widths. The first for loop iterates over the items in the map. Each word is appended to the words []string, a very cheap operation because words’s capacity is already large enough so all that the append() function has to do is put the given word at the len(words) index position and increment the words slice’s length by one. For each word we count the number of characters it contains and set wordWidth to this amount if it is larger than the existing value. Similarly, we count the number of characters needed to represent the frequency—we can safely use len() for this to count bytes since the fmt.Sprint() function takes a number and returns a string with decimal digits all of which are 7-bit ASCII characters. So at the end of the first for loop we have the widths of the two columns we want to output. Once the words slice has been populated we sort it. We don’t have to worry about case-sensitivity because all the words are lowercase (this was done in the readAndUpdateFrequencies() function; 177 ➤ ). After sorting the words we print the two column titles. First we print “Word”, then we print spaces so that the y of “Frequency” will be right-aligned with the last digit of the frequencies. This is achieved by printing a single space ("") with a field width of gap characters using the %*s format specifier. An alternative would be to use a format specifier of plain %s and to pass a string of spaces produced by strings.Repeat(" ", gap).(String formatting was covered in the previous chapter; §3.5, 93 ➤ .) And finally, we print the words and their frequencies in two columns with appropriate widths in ascending alphabetical word order. func invertStringIntMap(intForString map[string]int) map[int][]string { stringsForInt := make(map[int][]string, len(intForString)) for key, value := range intForString { stringsForInt[value] = append(stringsForInt[value], key) } return stringsForInt } The function begins by creating an empty inverted map. Although we don’t know how many items there will be, we have assumed that there will be about the same number as in the original map—after all, there can’t be more. The processing is straightforward: We simply iterate over the original map and use each value as a key in the inverted map, and add each key to the inverted map’s ptg7913109 180 Chapter 4. Collection Types corresponding slice value. Since the new map’s values are slices, no data is lost, even if the original map has multiple keys with the same value. func reportByFrequency(wordsForFrequency map[int][]string) { frequencies := make([]int, 0, len(wordsForFrequency)) for frequency := range wordsForFrequency { frequencies = append(frequencies, frequency) } sort.Ints(frequencies) width := len(fmt.Sprint(frequencies[len(frequencies)-1])) fmt.Println("Frequency → Words") for _, frequency := range frequencies { words := wordsForFrequency[frequency] sort.Strings(words) fmt.Printf("%*d %s\n", width, frequency, strings.Join(words, ", ")) } } This function is structurally very similar to the reportByWords() function. It begins by creating a slice of frequencies which it then sorts into ascending order. Then it computes the width needed to accommodate the largest frequency and uses that for the first column’s width. Next, it outputs the report’s title. And finally, it iterates over the frequencies and outputs each one with the words that have that frequency in ascending alphabetical order, comma-separating the words if there is more than one. We have now reviewed this chapter’s two complete examples and gained some insight into using pointers in Go and into the power and convenience of Go’s slice and map types. In the next chapter, we will look at how to create custom functions; this will complete the foundations in Go procedural programming. Once functions have been covered we will be ready to tackle object-oriented programming, and after that, concurrent programming. 4.5. Exercises There are five exercises, each one requiring the creation of a small function, and drawing on the coverage of slices and maps presented in this chapter. We have put all five functions in the same .go file (chap4_ans/chap4_ans.go), and added a main() function that makes use of them all to make testing easier. (Proper unit testing is covered later, in Chapter 9, §, ➤ 414.) 1. Create a function that accepts an []int and returns an []int which is a copy of the given []int but with duplicates removed. For example, given an argument of []int{9, 1, 9, 5, 4, 4, 2, 1, 5, 4, 8, 8, 4, 3, 6, 9, 5, 7, 5}, the function should return []int{9, 1, 5, 4, 2, 8, 3, 6, 7}. In the chap4_ans.go solution file the function is called UniqueInts(). The function uses composite ptg7913109 4.5. Exercises 181 literal syntax rather than the built-in make() function and is 11 lines long. It should be quite easy to do. 2. Create a function that accepts an [][]int (i.e., a two-dimensional slice of ints), and returns a single []int that contains all the ints from the two-di- mensional slice’s first slice, then from its second slice, and so on. For exam- ple, if the function is called Flatten(): irregularMatrix := [][]int{{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11}, {12, 13, 14, 15}, {16, 17, 18, 19, 20}} slice := Flatten(irregularMatrix) fmt.Printf("1x%d: %v\n", len(slice), slice) 1x20: [1 234567891011121314151617181920] The Flatten() function in chap4_ans.go is a mere nine lines. The function is slightly subtle to ensure that it works correctly even when the lengths of the inner slicesvary (asthey do in the irregularMatrix),but is quite straight- forward. 3. Create a function that accepts an []int and a column count (as an int), and that returns an [][]int where each inner slice’s length is equal to the given number of columns. For example, if the argument is []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20}, here are some sample results, each preceded by the number of columns that was passed: 3 [[1 2 3] [4 5 6] [7 8 9] [10 11 12] [13 14 15] [16 17 18] [19 20 0]] 4 [[1 2 3 4] [5 6 7 8] [9 10 11 12] [13 14 15 16] [17 18 19 20]] 5[[12345][678910][11 12 13 14 15] [16 17 18 19 20]] 6[[123456][789101112][13 14 15 16 17 18] [19 20 0000]] Notice that since there are 20 ints, neither 3 nor 6 columns are exact multiples, so we have padded the last inner slice with zeros when necessary to keep all column (i.e., inner slice) lengths the same. The Make2D() function in chap4_ans.go is 12 lines long and makes use of a helper function that’s7 lines long. The Make2D() function and its helper need a little bit of thought to get right, but aren’t difficult. 4. Create a function that accepts a []string containing the lines of an .ini-style file and that returns a map[string]map[string]string whose keys are group names and whose values are key–value maps of each group’s keys and values. Blank lines and lines beginning with ; should be ignored. Each group is indicated by a name in square brackets on its own line, and ptg7913109 182 Chapter 4. Collection Types each group’s keys and values are indicated by one or more lines of the form key=value. Here is an example []string that the function could process. iniData := []string{ "; Cut down copy of Mozilla application.ini file", "", "[App]", "Vendor=Mozilla", "Name=Iceweasel", "Profile=mozilla/firefox", "Version=3.5.16", "[Gecko]", "MinVersion=1.9.1", "MaxVersion=1.9.1.*", "[XRE]", "EnableProfileMigrator=0", "EnableExtensionManager=1", } Given this data, the function should return the following map which we have “pretty-printed” to make it easier to see its structure. map[Gecko: map[MinVersion: 1.9.1 MaxVersion: 1.9.1.*] XRE: map[EnableProfileMigrator: 0 EnableExtensionManager: 1] App: map[Vendor: Mozilla Profile: mozilla/firefox Name: Iceweasel Version: 3.5.16]] The ParseIni() solution function assumes a group of “General” for any key–values that are not within the scope of a group. It is 24 lines long and might take a bit of care to get right. 5. Create a function that accepts a map[string]map[string]string that repre- sents an .ini file’s data. The function should print out the data as an .ini file with groupsin alphabeticalorder and keyswithin groups in alphabetical order, and with a blank line between each group. For example, given the data from the previous exercise the output should be: [App] Name=Iceweasel Profile=mozilla/firefox Vendor=Mozilla ptg7913109 4.5. Exercises 183 Version=3.5.16 [Gecko] MaxVersion=1.9.1.* MinVersion=1.9.1 [XRE] EnableExtensionManager=1 EnableProfileMigrator=0 The PrintIni() solution function is 21 lines long and should be easier to do than the previous exercise’s ParseIni() function. ptg7913109 This page intentionally left blank ptg7913109 5 Procedural Programming §5.1. Statement Basics ➤ 186 §5.1.1. Type Conversions ➤ 190 §5.1.2. Type Assertions ➤ 191 §5.2. Branching ➤ 192 §5.2.1. If Statements ➤ 192 §5.2.2. Switch Statements ➤ 195 §5.3. Looping with For Statements ➤ 203 §5.4. Communication and Concurrency Statements ➤ 205 §5.4.1. Select Statements ➤ 209 §5.5. Defer, Panic, and Recover ➤ 212 §5.5.1. Panic and Recover ➤ 213 §5.6. Custom Functions ➤ 219 §5.6.1. Function Arguments ➤ 220 §5.6.2. The init() and main() Functions ➤ 224 §5.6.3. Closures ➤ 225 §5.6.4. Recursive Functions ➤ 227 §5.6.5. Choosing Functions at Runtime ➤ 230 §5.6.6. Generic Functions ➤ 232 §5.6.7. Higher Order Functions ➤ 238 §5.7. Example: Indent Sort ➤ 244 The purpose of this chapter is to complete the coverage of Go procedural pro- gramming that the earlier chapters began. Go can be used to write purely pro- cedural programs, to write object-oriented programs, or to write programs that combine both paradigms. It is important to learn procedural Go since object- oriented Go builds on the procedural foundations, as does concurrent Go pro- gramming. 185 ptg7913109 186 Chapter 5. Procedural Programming The previous chapters described and illustrated Go’s built-in data types, in the course of which many of Go’s statements and control structures were used, and many small custom functions were created. In this chapter we will review Go’s statements and control structures in more detail, and also look much more closely at creating and using custom functions. Table 5.1 provides a list of Go’s built-in functions, most of which have already been covered.★ Some of this chapter’s material has been seen less formally in earlier chapters, and some of the material refers to aspects of Go programming that are covered in subsequent chapters. Forward and backward cross-references are provided where appropriate. 5.1. Statement Basics Formally, Go’s syntax requires the use of semicolons (;) as statement termina- tors in many contexts. However, as we have seen, very few semicolons are need- ed in real Go programs. This is because the compiler will conveniently insert semicolons automatically at the end of nonblank lines that end with an identifi- er, a number literal, a character literal, a string literal, certain keywords (break, continue, fallthrough, return), an increment or decrement operator (++ or --), or a closing parenthesis, bracket, or brace (),],}). Two common cases where semicolons must be manually inserted are when we want to have two or more statements on the same line and in plain for loops (§5.3, ➤ 203). An important consequence of the automatic semicolon insertion is that an opening brace cannot occur on its own line. // Correct ✓ for i:=0;i<5; i++ { fmt.Println(i) } // WRONG! (This won't compile.) ✗ for i:=0;i<5; i++ { fmt.Println(i) } The right-hand code snippet won’t compile because the compiler will insert a semicolon after the ++. Similarly, if we had an infinite loop (for) with the brace starting on the next line, the compiler would insert a semicolon after the for, and again the code wouldn’t compile. The æsthetics of brace placement usually generate endless arguments—but not in Go. This is partly because the automatic semicolons constrain brace placement and partly because many Go users use the gofmt program which ★ Table 5.1 does not list the built-in print() and println() functions since they should not be used. They exist for the convenience of Go compiler implementers and may be removed from the language. Use functions like fmt.Print() instead (§3.5, 93 ➤ ). ptg7913109 5.1. Statement Basics 187 Table 5.1 Built-In Functions Syntax Description/result append(s,...) The slice it was given plus the new items at the end if the slice’s capacity is sufficient; otherwise a new slice with the original items plus the new items at the end (see §4.2.3, 156 ➤ ) cap(x) The capacity of slice x or the channel buffer capacity of chan- nel x or the length of array (or the array pointed to by) x; see also len() (see §4.2, 148 ➤ ) close(ch) Closes channel ch (but not legal for receive-only channels). No more data can be sent to the channel. Data can continue to be received from the channel (e.g., any sent but not yet received values), and when there are no more values in the channel, receivers will get the channel data type’s zero value. complex(r, i)A complex128 with the given r (real) and i (imaginary) parts, both of type float64 (see §, 70 ➤ ) copy(dst, src) copy(b, s) Copies (possibly overlapping) items from the src slice into the dst slice, truncating if there isn’t enough room; or copies s of type string’s bytes to b of type []byte (see §4.2.3, 156 ➤ and ➤ 268) delete(m, k) Deletes the item with key k from map m or safely does nothing if there’s no such key (see §4.3, 164 ➤ ) imag(cx) The imaginary part of cx of type complex128 as a float64 (see §, 70 ➤ ) len(x) The length of slice x or the number of items queued in channel x’s buffer or the length of array (or the array pointed to by) x or the number of items in map x or the number of bytes in string x; see also cap() (see §4.2.3, 156 ➤ ) make(T) make(T, n) make(T, n, m) A reference to a slice, map, or channel of type T.If n is given this is a slice’s length and capacity, or a hint to a map of how many itemsto expect,or a channel’s buffer size. For slicesonly, n and m may be given to specify the length and capacity (see 150 ➤ for slices, 165 ➤ for maps, and Chapter 7 for channels). new(T)A pointer to a value of type T(see §4.1, 140 ➤ ) panic(x) Raises a catchable runtime exception with value x (see §5.5.1, ➤ 213) real(cx) The real part of cx of type complex128 as a float64 (see §, 70 ➤ ) recover() Catches a runtime exception (see §5.5.1, ➤ 213) ptg7913109 188 Chapter 5. Procedural Programming formatsGo programs in a standardized way. In fact,all the Go standard library’s source code uses gofmt which is why the code has such a consistent layout, even though it is the product of many different programmers’ work.★ Go supports the ++ (increment) and -- (decrement) operators listed in Table 2.4 (59 ➤ ). They are both postfix operators, that is, they must follow the operand they apply to, and they do not return a value. These constraints prevent the operators from being used as expressions, and mean that they cannot be used in semantically ambiguous contexts—for example, we cannot apply one of these operators to an argument in a function call or write i = i++ in Go (although we could in C and C++ where the results are undefined). Assignments are made using the = assignment operator. Variables can be both created and assigned by using = in conjunction with var—for example, var x =5 creates a new variable x of type int and with value 5.(Exactly the same could be achieved using var x int = 5 or x := 5.) The type of the variable assigned to must be compatible with the value being assigned. If = is used without var the variable on its left-hand side must already exist. Multiple comma-separated variables can be assigned to, and we can use the blank identifier (_), which is compatible with any type, to ignore any of the values being assigned. Multiple assignments make it easy to swap two values without the need for an explicit temporary variable—for example, a, b = b, a. The short variable declaration operator (:=) is used to both declare a new vari- able and assign to it in a single statement. Multiple comma-separated variables can be used in much the same way as when using the = operator, except that at least one nonblank variable must be new. If there is a variable that already exists it will be assigned to without creating a new variable—unless the := is at the start of a new scope such as in an if or for statement’sinitializing statement (see §5.2.1, ➤ 192; §5.3, ➤ 203). a, b, c := 2, 3, 5 for a:=7;a<8; a++ {// a is unintentionally shadowing the outer a b := 11 // b is unintentionally shadowing the outer b c = 13 // c is the intended outer c ✓ fmt.Printf("inner: a→%d b→%d c→%d\n",a,b,c) } fmt.Printf("outer: a→%d b→%d c→%d\n",a,b,c) inner: a→7b→11 c→13 outer: a→2b→3 c→13 ★ At the time of this writing, gofmt did not support line wrapping to a maximum line width, and in some cases gofmt will join two or more wrapped lines to make one long line. The book’s source code was automatically extracted from live examples and test programs and inserted into the book’s camera-ready PDF file—but this is subject to a hard 75-character-per-line limit. So, for the book’s code, gofmt was used, and then long lines were manually wrapped. ptg7913109 5.1. Statement Basics 189 This code snippet shows how the := operator can create “shadow” variables. In this snippet, inside the for loop the a and b variables shadow variables from the outer scope, and while legal, this is almost certainly a programming error. On the other hand, there is only one c variable (from the outer scope), so its usage is correct and as intended. Variables that shadow other variables can be very convenient, as we will see shortly, but careless use can cause problems. As we will discuss later in the chapter, we can write return statements in func- tions that have one or more named return values, without specifying any re- turn values. In such cases, the returned values will be the named return values, which are initialized with their zero values on entry to the function, and which we can change by assigning to them in the body of the function. func shadow() (err error){// THIS FUNCTION WILL NOT COMPILE! x, err := check1() // x is created; err is assigned to if err != nil { return // err correctly returned } if y, err := check2(x); err != nil {// y and inner err are created return // inner err shadows outer err so nil is wrongly returned! } else { fmt.Println(y) } return // nil returned } In the shadow() function’s first statement the x variable is created and assigned to, but the err variable is simply assigned to since it is already declared as the shadow() function’s return value. This works because the := operator must create at least one nonblank variable and that condition is met here. So, if err is not nil, it is correctly returned. An if statement’s simple statement, that is, the optional statement that follows the if and precedes the condition, starts a new scope (§5.2.1, ➤ 192).So, both the y and the err variables are created, the latter being a shadow variable. If the err is not nil the err in the outer scope is returned (i.e., the err declared as the shadow() function’s return value), which is nil since that was the value assigned to it by the call to check1(), whereas the call to check2() was assigned to the shadowing inner err. Fortunately, this function’s shadow problem is merely a phantom, since the Go compiler will stop with an error message if we use a bare return when any of the return variables has been shadowed. So, this function will not compile as it stands. One easy solution is to add a line at the start of the function that declares the variables (e.g., var x, y int or x, y:=0,0), and change := to = for the check1() and ptg7913109 190 Chapter 5. Procedural Programming check2() calls. (For an example of this approach see the custom americanise() function; 35 ➤ .) Another solution is to use an unnamed return value. This forces us to return an explicit value, so in this case the first two return statements would both become return err (each returning a different but correct err value), and the last one would become return nil. 5.1.1. Type Conversions Go provides a means of converting between different—compatible—types, and such conversions are useful and safe. For conversions between non- numeric types no loss of accuracy occurs. But for conversions between nu- meric types, loss of accuracy or other effects may occur. For example, if we have x := uint16(65000) and then use the conversion y := int16(x), since x is outside the int16 range, y’s value is set to the unsurprising—but probably undesirable—value of -536. Here is the conversion syntax: resultOfType := Type(expression) For numbers, essentially we can convert any integer or floating-point number to another integer or floating-point type (with possible loss of accuracy if the target type is smaller than the source type). The same applies to converting between complex128 and complex64 types. We discussed numeric conversions in §2.3 (58 ➤ and 69 ➤ ). A string can be converted to a []byte (its underlying UTF-8 bytes) or to a []rune (its Unicode code points), and both a []byte and a []rune can be converted to a string.A single character is a rune (i.e., an int32), and can be converted to a one- character string. String and character conversions were covered in Chapter 3 (87 ➤ and 88 ➤ ; also Table 3.2, 85 ➤ , and Tables 3.8 and 3.9, 114–115 ➤ ). Let’s look at a tiny illustrative example, starting with a simple custom type. type StringSlice []string This type also has a custom StringSlice.String() function (not shown) that returns a string representation of the string slice in the form used to create a custom StringSlice using composite literal syntax. fancy := StringSlice{"Lithium","Sodium","Potassium","Rubidium"} fmt.Println(fancy) plain := []string(fancy) fmt.Println(plain) ptg7913109 5.1. Statement Basics 191 StringSlice{"Lithium", "Sodium", "Potassium", "Rubidium"} [Lithium Sodium Potassium Rubidium] The fancy StringSlice is printed using its own StringSlice.String() function. But once we convert it to a plain []string, it is printed like any other []string. (Creating custom types with their own methods is covered in Chapter 6.) Conversions for other types will work if the expression and Type’s underlying types are the same, or if the expression is an untyped constant that can be represented by type Type, or if Type is an interface type and the expression implements Type’s interface.★ 5.1.2. Type Assertions A type’s method set is the set of all the methods that can be called on a value of the type—this set is empty for types that have no methods. The Go interface{} type is used to represent the empty interface, that is, a value of a type whose method set includes the empty set. Since every type has a method set that includes the empty set (no matter how many methods it has), an interface{} can be used to represent a value of any Go type. Furthermore, we can convert an interface{} to a value of the actual type it holds using a type switch (see §, ➤ 197), or a type assertion, or by doing introspection with Go’s reflect package (§9.4.9, ➤ 427).✪ The use of variables of type interface{} (or of custom interface types) can arise when we are handling data received from external sources, when we want to create generic functions, and when doing object-oriented programming. To access the underlying value, one approach is to use a type assertion using one of these syntaxes: resultOfType, boolean := expression.(Type)//Checked resultOfType := expression.(Type) //Unchecked; panic() on failure A successful checked type assertion returns the expression as a value of the specified Type and true to indicate success. If the checked type assertion fails (i.e., the expression’s type is not compatible with the specified Type), a zero value of the specified Type and false are returned. Unchecked type assertions either return the expression as a value of the specified Type or call the built-in panic() ★ Other more obscure conversions are also possible; these are covered in the Go specification (golang. org/doc/go_spec.html). ✪ Python programmers may find it helpful to think of interface{} as being like an instance of object, and Java programmersas being like an instance of Object,although unlike Java’s Object, interface{} can be used to represent both custom and built-in types. For C and C++ programmers, interface{} is rather like a void* that knows what type it is. ptg7913109 192 Chapter 5. Procedural Programming function which will result in program termination if the panic isn’t recovered. (Panicking and recovery is covered later; §5.5, ➤ 212.) Here is a tiny example to illustrate the syntaxes in use. var i interface{} = 99 var s interface{} = []string{"left","right"} j := i.(int) // j is of type int (or a panic() has occurred) fmt.Printf("%T→%d\n",j,j) if i, ok := i.(int); ok { fmt.Printf("%T→%d\n",i,i)// i is a shadow variable of type int } if s, ok := s.([]string); ok { fmt.Printf("%T→%q\n",s,s)// s is a shadow variable of type []string } int→99 int→99 []string→["left" "right"] It is quite common when doing type assertions to use the same name for the result variable as for the original variable,that is,to use shadow variables. And, generally, we use checked type assertions only when we expect the expression to be of the specified type. (If the expression could be any one of a number of types, we can use a type switch; §, ➤ 197.) Note that if we printed the original i and s variables (both of type interface{}) they would be printed as an int and a []string. This is because when the fmt package’s print functions are faced with interface{} types, they are sensible enough to print the actual underlying values. 5.2. Branching Go provides three branching statements: if, switch, and select—the latter is discussed further on (§5.4.1, ➤ 209). A branching effect can also be achieved using a map whose keys are used to select the “branch” and whose values are corresponding functions to call—something we will see later in the chapter (§5.6.5, ➤ 230). 5.2.1. If Statements Go’s if statement has the following syntax: if optionalStatement1; booleanExpression1 { block1 } else if optionalStatement2; booleanExpression2 { ptg7913109 5.2. Branching 193 block2 } else { block3 } There may be zero or more else if clauses and zero or one final else clause. Each block consists of zero or more statements. The braces are mandatory, but a semicolon is needed only if an optional state- ment ispresent. The optional statement isa simplestatementin Go terminology: This means that it may be only an expression,a channel send (using the <- oper- ator), an increment or decrement statement, an assignment, or a short variable declaration. If variables are created in an optional statement (e.g., using the := operator),their scope extendsfrom the point of declaration to the end of the com- plete if statement—so they exist in the if or else if they are declared in, and in every following branch, and cease to exist at the end of the if statement. The Boolean expressions must be of type bool. Go will not automatically convert non-bools, so we must always use a comparison operator—for example, if i == 0. (The Boolean and comparison operators are listed in Table 2.3, 57 ➤ .) We have already seen numerous examples of if statements in use, and will see many more in the rest of the book. Nonetheless, we will look at two tiny examples, the first to show the value of the optional simple statement, and the second to illustrate a Go if statement idiom. // Canonical ✓ if α := compute(); α < 0 { fmt.Printf("(%d)\n",-α) } else { fmt.Println(α) } // Long-winded! { α := compute() if α < 0 { fmt.Printf("(%d)\n",-α) } else { fmt.Println(α) } } These two code snippets print exactly the same thing. The right-hand snippet must use extra braces to limit the scope of the α variable, whereas the left-hand snippet automatically limits the variable’s scope to the if statement. The second if statement example is the ArchiveFileList() function which is tak- en from the archive_file_list example (in file archive_file_list/archive_file_ list.go).Later on we will use this function’sbody to compare if and switch state- ments. ptg7913109 194 Chapter 5. Procedural Programming func ArchiveFileList(file string) ([]string, error){ if suffix := Suffix(file); suffix == ".gz" { return GzipFileList(file) } else if suffix == ".tar" || suffix == ".tar.gz" || suffix == ".tgz" { return TarFileList(file) } else if suffix == ".zip" { return ZipFileList(file) } return nil, errors.New("unrecognized archive") } The example program reads the files given on the command line, and for those archive files that it can handle (.gz,.tar,.tar.gz,.zip), it prints the name of the archive file and an indented list of the files the archive contains. Notice that the scope of the suffix variable created in the first if clause extends throughout the entire if … else if … statement, so it is visible in every branch, just like the α variable in the previous example. The function could have been written using a final else statement, but it is very common in Go to use the structure shown here: an if statement and zero or more else if statements each of which ends with a return statement, with this followed by a return statement rather than a final else statement that ends with a return. func Suffix(file string) string { file = strings.ToLower(filepath.Base(file)) if i := strings.LastIndex(file, "."); i > -1 { if file[i:] == ".bz2" || file[i:] == ".gz" || file[i:] == ".xz" { if j := strings.LastIndex(file[:i], "."); j > -1 && strings.HasPrefix(file[j:], ".tar"){ return file[j:] } } return file[i:] } return file } The Suffix() function is included for completeness: It takes a filename (which may include a path), and returns the lowercased suffix—also called the exten- sion—that is,the last part of the name that begins with a period. If the filename has no period, it is returned as is (but without any path); if the filename ends in .tar.bz2,.tar.gz,or.tar.xz then this is the suffix that is returned. ptg7913109 5.2. Branching 195 5.2.2. Switch Statements There are two kinds of switch statement: expression switches and type switches. Expression switches will be familiar to C, C++, and Java programmers, whereas type switches are specific to Go. Both kinds are syntactically very similar, but unlike C, C++, and Java, Go’s switch statements do not fall through (so there is no need to put a break at the end of every case); instead we can request fallthrough explicitly by using the fallthrough statement when it is needed. Expression Switches Go’s expression switch statement has the following syntax: switch optionalStatement; optionalExpression { case expressionList1: block1 ... case expressionListN: blockN default: blockD } The semicolon is required if the optional statement is present, regardless of whether the optional expression is present. Each block consists of zero or more statements. If the switch has no optional expression the compiler assumes an expression of true. The optional statement is the same kind of simple statement that can be used with if statements (193 ➤ ). If variables are created in the optional state- ment (e.g., using the := operator), their scope extends from the point of declara- tion to the end of the complete switch statement—so they exist in every case and in the default case, and cease to exist at the end of the switch statement. The most efficient way to order cases is from most likely to least likely, although this only really matters when there are lots of cases and the switch is executed repeatedly. Since cases do not automatically fall through, there is no need to put a break at the end of each case’s block. If fallthrough is wanted we simply use a fallthrough statement. The default case is optional and if present may appear anywhere. If no case’s expression matches, the default case is executed if it is present; otherwise processing continues from the statement following the switch statement. Each case must have an expression list of one or more comma-separated expressions whose type matches the switch statement’s optional expression’s type. If no optional expression is present the compiler sets it to true, that is, of type bool, in which case every expression in each case clause’s expression list must evaluate to a bool. If a case or default clause has a break statement, the switch statement will im- mediately be broken out of, with control passing to the statement following the ptg7913109 196 Chapter 5. Procedural Programming switch statement, or—if the break statement specifies a label—to the innermost enclosing for, switch,orselect statement that has the specified label. Here is a very simple example of a switch statement that has no optional statement and no optional expression. func BoundedInt(minimum, value, maximum int) int { switch { case value < minimum: return minimum case value > maximum: return maximum } return value } Since there is no optional expression the compiler sets the expression to true; this means that each case clause expression must evaluate to a bool. Here both expressions use Boolean comparison operators. switch { case value < minimum: return minimum case value > maximum: return maximum default: return value } panic("unreachable") Here is an alternative body for the BoundedInt() function. The switch statement now covers every possible case, so control can never reach the end of the function. Nonetheless, Go expects a return at the end—or a panic(),sowehave used the latter to better express the function’s semantics. The ArchiveFileList() function shown in the previous subsection (194 ➤ ) used an if statement to determine which function to call. Here is a naïve switch statement-based version. switch suffix := Suffix(file); suffix {// Naïve and noncanonical! case ".gz": return GzipFileList(file) case ".tar": fallthrough case ".tar.gz": fallthrough ptg7913109 5.2. Branching 197 case ".tgz": return TarFileList(file) case ".zip": return ZipFileList(file) } This switch statement has both a statement and an expression. In this case the expression is of type string so each case’s expression list must contain one or more comma-separated strings to match. We have used the fallthrough statement to ensure that all tar files are processed by the same function. The suffix variable’s scope extends throughout the switch statement to every case (and would extend to the default case if one was present), and ends at the end of the switch statement since at that point the suffix ceases to exist. switch Suffix(file) {// Canonical ✓ case ".gz": return GzipFileList(file) case ".tar",".tar.gz",".tgz": return TarFileList(file) case ".zip": return ZipFileList(file) } Here is a more compact and canonical version of the preceding switch state- ment. Instead of a statement and an expression we have simply used an expres- sion: The Suffix() function (that we saw earlier; 194 ➤ ) returns a string. And instead of using fallthrough statements for tar files, we have used a comma- separated list of all the matching suffixes as that case clause’s expression list. Go’s expression switch statements are much more versatile than those provided by C, C++, and Java, and in many cases can be used instead of—and are more compact than—if statements. Type Switches As we noted when we covered type assertions (§5.1.2, 191 ➤ ), when we use variables of type interface{} we often want to access the underlying value. If we know the type we can use a type assertion, but if the type may be any one of a number of possible types we can use a type switch statement. Go’s type switch statement has the following syntax: switch optionalStatement; typeSwitchGuard { case typeList1: block1 ... case typeListN: blockN ptg7913109 198 Chapter 5. Procedural Programming default: blockD } The optional statement is the same as in expression switches and if statements. And the case clauses work the same way as for expression switches except that they list one or more comma-separated types. The optional default clause and fallthrough statements are just the same as for expression switches, and as usual, each block consists of zero or more statements. The type switch guard is an expression whose result is a type. If the expression is assigned using the := operator, the variable created has the value of the value in the type switch guard expression, but its type depends on the case clauses: In a case clause with one type in its type list, the variable has that type in that case, and in case clauses that have two or more types, the variable’s type is that of the type switch guard expression. The kind of type testing supported by the type switch statement is generally frowned upon by object-oriented programmers who instead prefer to rely on polymorphism. Go supports a kind of polymorphism through duck typing (as we will see in Chapter 6), but nonetheless there are times where explicit type testing makes sense. Here is an example of how we might call a simple type classifier function and the output it produces. classifier(5,-17.9,"ZIP", nil, true, complex(1, 1)) param #0 is an int param #1 is a float64 param #2 is a string param #3 is nil param #4 is a bool param #5's type is unknown The classifier() function uses a simple type switch. It is a variadic function, that is, it can accept a variable number of arguments. And since the argument type is interface{}, the arguments can be of any types. (Functions, including variadic functions and the ellipsis, are covered later in this chapter; §5.6, ➤ 219.) func classifier(items ...interface{}) { for i, x := range items { switch x.(type){ case bool: fmt.Printf("param #%d is a bool\n",i) case float64: fmt.Printf("param #%d is a float64\n",i) ptg7913109 5.2. Branching 199 case int, int8, int16, int32, int64: fmt.Printf("param #%d is an int\n",i) case uint, uint8, uint16, uint32, uint64: fmt.Printf("param #%d is an unsigned int\n",i) case nil: fmt.Printf("param #%d is nil\n",i) case string: fmt.Printf("param #%d is a string\n",i) default: fmt.Printf("param #%d's type is unknown\n",i) } } } The type switch guard used here has the same format as a type assertion, that is, variable.(Type), but using the keyword type instead of an actual type to stand for any type. Sometimes we might want to access an interface{}’s underlying value as well as its type. This can be done by making the type switch guard an assignment (using the := operator), as we will see in a moment. One common use case for type testing is when we are dealing with data from external sources. For example, if we are parsing data encoded using JSON (JavaScript Object Notation), we must somehow convert the data to the corre- sponding Go data types. This can be done using Go’s json.Unmarshal() function. If we give the function a pointer to a struct with fields that match the JSON data,this function will populate the struct’s fields converting each item of JSON data into the data type of its corresponding struct field. But if we do not know the JSON data’s structure in advance we cannot give the json.Unmarshal() func- tion a struct. In such cases we can give the function a pointer to an interface{} which the json.Unmarshal() function will set to refer to a map[string]interface{} whose keys are JSON field names and whose values are the corresponding val- ues stored as interface{}s. Here is an example that shows how we can unmarshal a raw JSON object of unknown structure and how we can create and print a corresponding string representation of the JSON object. MA := []byte(`{"name": "Massachusetts", "area": 27336, "water": 25.7, "senators": ["John Kerry", "Scott Brown"]}`) var object interface{} if err := json.Unmarshal(MA, &object); err != nil { fmt.Println(err) } else { jsonObject := object.(map[string]interface{}) ➊ ptg7913109 200 Chapter 5. Procedural Programming fmt.Println(jsonObjectAsString(jsonObject)) } {"senators": ["John Kerry", "Scott Brown"], "name": "Massachusetts", "water": 25.700000, "area": 27336.000000} If no error occurred when unmarshaling, the object variable of type interface{} will refer to a variable of type map[string]interface{} whose keys are the JSON object’s field names. The jsonObjectAsString() function accepts a map of this type and returns a corresponding JSON string. We use an unchecked type assertion (199 ➤ , ➊) to convert the object of type interface{} to the jsonObject variable of type map[string]interface{}.(Note that the output shown here is split over two lines to fit the book’s page width.) func jsonObjectAsString(jsonObject map[string]interface{}) string { var buffer bytes.Buffer buffer.WriteString("{") comma := "" for key, value := range jsonObject { buffer.WriteString(comma) switch value := value.(type){// shadow variable ➊ case nil: ➋ fmt.Fprintf(&buffer, "%q: null", key) case bool: fmt.Fprintf(&buffer, "%q: %t", key, value) case float64: fmt.Fprintf(&buffer, "%q: %f", key, value) case string: fmt.Fprintf(&buffer, "%q: %q", key, value) case []interface{}: fmt.Fprintf(&buffer, "%q: [", key) innerComma := "" for _, s := range value { if s, ok := s.(string); ok {// shadow variable ➌ fmt.Fprintf(&buffer, "%s%q", innerComma, s) innerComma = ", " } } buffer.WriteString("]") } comma = ", " } buffer.WriteString("}") return buffer.String() } ptg7913109 5.2. Branching 201 This function converts a map representing a JSON object and returns a corre- sponding string of the object’s data in JSON format. JSON arrays inside maps representing JSON objects are themselves represented by the []interface{} type. The function makes one simplifying assumption regarding JSON arrays: It assumes that they have only string items. To access the data we use a for … range loop (§5.3, ➤ 203) over the map’s keys and values and use a type switch to access and handle each different value type. The switch’s type switch guard (200 ➤ , ➊) assigns the value (of type interface{}) to a new variable called value which has the type of the matching case. This is a situation where it makes sense to shadow a variable (although we are free to create a new variable if we wish). So, if the interface{} value’s type is bool, the inner value will be a bool and will match the second case, and similarly for the other cases. To write the values to the buffer we have used the fmt.Fprintf() function since this is more convenient than writing, say, buffer.WriteString(fmt.Sprintf(…)) (200 ➤ , ➋). The fmt.Fprintf() function writes to the io.Writer passed as its first argument. A bytes.Buffer is not an io.Writer—but a *bytes.Buffer is, which is why we pass the buffer’s address. This matter is covered more fully in Chap- ter 6, but in brief, io.Writer is an interface that can be fulfilled by any value that provides a suitable Write() method. The bytes.Buffer.Write() method takes a pointer receiver (i.e., a *bytes.Buffer, not a bytes.Buffer value), so only a *bytes.Buffer fulfills the interface, which means that we must pass the buffer’s address to the fmt.Fprintf() function, not the buffer value itself. If the JSON object contains JSON arrays, we use an inner for … range loop to iterate over each of the []interface{}’s items and use a checked type assertion (200 ➤ , ➌) which means that we add items to our output only if they really are strings. Again, we use a shadow variable (this time s of type string), since we don’t want the interface{}, but rather the value it refers to. (Type assertions were covered earlier; §5.1.2, 191 ➤ .) Of course, if we knew the original JSON object’s structure in advance we could simplify the code a great deal. We would need a struct to hold the data and a method for outputting it in string form. Here is the code to unmarshal and print in such cases. var state State if err := json.Unmarshal(MA, &state); err != nil { fmt.Println(err) } fmt.Println(state) {"name": "Massachusetts", "area": 27336, "water": 25.700000, "senators": ["John Kerry", "Scott Brown"]} ptg7913109 202 Chapter 5. Procedural Programming This code looks very similar to the code we had before. However,there is no need for a jsonObjectAsString() function; instead we need to define a State type and a corresponding State.String() method. (Once again, we had to split the output over two lines to fit the book’s page width.) type State struct { Name string Senators []string Water float64 Area int } The struct is similar to ones we have seen before. Note, though, that each field must begin with an uppercase letter to make it exported (public) since the json.Unmarshal() function can only populate exported fields. Also, although Go’s encoding/json package does not distinguish between different numeric types—it treats all JSON numbers as float64s—the json.Unmarshal() function is smart enough to populate fields of other numeric types as necessary. func (state State) String() string { var senators []string for _, senator := range state.Senators { senators = append(senators, fmt.Sprintf("%q", senator)) } return fmt.Sprintf( `{"name": %q, "area": %d, "water": %f, "senators": [%s]}`, state.Name, state.Area, state.Water, strings.Join(senators, ", ")) } This method returns a State value as a JSON data string. Most Go programs should not need type assertions and type switches; and even when they are needed, their use should be fairly rare. One use case is where we are passing values that satisfy one interface and want to check if they also satisfy another. (This topic is covered in Chapter 6; e.g., §6.5.2, ➤ 289.) Another use case is when data from external sources must be converted into Go data types. For ease of maintenance, it is almost always best that such code is isolated from the rest of the program. This allows the program to work wholly in terms of Go data types and means that any maintenance needed due to changes to the format or types received from external sources can be localized. ptg7913109 5.3. Looping with For Statements 203 5.3. Looping with For Statements Go uses two kinds of for statements for looping, plain for statements and for … range statements. Here are their syntaxes: for {//Infinite loop block } for booleanExpression {//While loop block } for optionalPreStatement; booleanExpression; optionalPostStatement { ➊ block } for index, char := range aString {//String per character iteration ➋ block } for index := range aString {//String per character iteration ➌ block // char, size := utf8.DecodeRuneInString(aString[index:]) } for index, item := range anArrayOrSlice {//Array or slice iteration ➍ block } for index := range anArrayOrSlice {//Array or slice iteration ➎ block // item := anArrayOrSlice[index] } for key, value := range aMap {//Map iteration ➏ block } for key := range aMap {//Map iteration ➐ block // value := aMap[key] } for item := range aChannel {//Channel iteration block } The braces are mandatory, but a semicolon is only needed if an optional pre- or post-statement is used (➊); both statements must be simple statements. If variables are created in an optional statement or to capture the values produced by a range clause (e.g., using the := operator), their scope extends from the point of declaration to the end of the complete for statement. ptg7913109 204 Chapter 5. Procedural Programming The Boolean expression in the plain for loop syntax (203 ➤ , ➊) must be of type bool since Go will not automatically convert non-bools. (The Boolean and comparison operators are listed in Table 2.3, 57 ➤ .) The second for … range over a string syntax (203 ➤ , ➌) gives byte offset indexes. For a 7-bit ASCII string s, of value "XabYcZ", this produces indexes 0, 1, 2, 3, 4, 5. But for a UTF-8 string s, of value "XαβYγZ", the indexes produced are 0, 1, 3, 5, 6, 8. The first for … range over a string syntax (203 ➤ , ➋) is almost always more convenient than the second (203 ➤ , ➌). The second for … range over an array or slice syntax (203 ➤ , ➎) produces item indexes from 0 to len(slice)-1for nonempty slices or arrays. This syntax and the first for … range over an array or slice syntax (203 ➤ , ➍) are often useful. These two syntaxes in particular account for why fewer plain for loops (203 ➤ , ➊) are needed in Go programs. The for … range loops over map key–value items (203 ➤ , ➏) and over map keys (203 ➤ , ➐) produce the items or keys in an arbitrary order. If sorted order is required one solution is to use the second syntax (203 ➤ , ➐) and populate a slice with the keys and then sort the slice—we saw an example of this in the previous chapter (§4.3.4, 170 ➤ ). Another solution is to use an ordered data structure in the first place—for example, an ordered map. We will see an example of this in the next chapter (§6.5.3, ➤ 302). If any of the syntaxes (203 ➤ , ➋–➐) are used on an empty string, array, slice, or map, the for loop harmlessly does nothing and the flow of control continues at the following statement. A for loop can be broken out of at any time with a break statement, with control passing to the statement following the for loop, or—if the break statement specifies a label—to the innermost enclosing for, switch, or select statement that has the specified label. It is also possible to make the flow of control return to the for loop’s condition or range clause to force the next iteration (or the end of the loop), by using a continue statement. We have already seen numerous examples of for statements in use; these in- clude for … range loops (89 ➤ , 172 ➤ , and 180 ➤ ), infinite loops (23 ➤ and 45 ➤ ), and the plain for loop (100 ➤ ) that is needed less frequently in Go since the other loops are often more convenient. And we will, of course, see many more for loop examples in the rest of the book, including some later in this chapter; so here we will confine ourselves to one small example. Suppose that we have two-dimensional slices (e.g., of type [][]int), and want to search them to see if they contain a particular value. Here are two ways we can perform the search. Both use the second for … range over an array or slice syntax (203 ➤ , ➎). ptg7913109 5.3. Looping with For Statements 205 found := false for row := range table { for column := range table[row] { if table[row][column] == x { found = true break } } if found { break } } found := false FOUND: for row := range table { for column := range table[row] { if table[row][column] == x { found = true break FOUND } } } A label is an identifier followed by a colon. Both code snippets achieve the same thing but the right-hand snippet is shorter and clearer because as soon as the searched-for value (x) is found, it breaks to the outer loop by using a break statement that specifies a label. The advantages of breaking to a label are even greater if we are in a deeply nested series of loops (e.g., iterating over three-dimensional data). Labels can be applied to for loops, switch statements, and select statements. Both break and continue statements can specify labels and can be used inside for loops. It is also possible to use break statements—either bare or specifying a label—inside switch and select statements. Labels can also appear as statements in their own right in which case they may be the targets of goto statements (using the syntax goto label). If a goto state- ment jumps past any statement that creates a variable, the Go program’s be- havior is undefined—if we are lucky it will crash, but probably it will continue to run and produce spurious results. One use case for goto statements is when automatically generating code,since in thiscircumstance goto can be convenient and the concerns about spaghetti code don’t necessarily apply. Although, at the time of this writing, more than 30 of Go’s source files use goto statements, none of the book’s examples use the goto statement, and we advocate avoiding it.★ 5.4. Communication and Concurrency Statements Go’s communication and concurrency features are covered in Chapter 7, but for completeness of our coverage of procedural programming we will describe their basic syntax here. ★ goto statements have been generally despised since Edsger Dijkstra’s famous 1968 letter titled “Go-to statement considered harmful” ( ptg7913109 206 Chapter 5. Procedural Programming A goroutine is a function or method invocation that executes independently and concurrently in relation to any other goroutines in a program. Every Go pro- gram has at least one goroutine, the main goroutine in which the main pack- age’s main() function executes. Goroutines are rather like lightweight threads or coroutines, in that they can be created in large numbers (whereas even small numbers of threads can consume a huge amount of machine resources). Go- routines all share the same address space, and Go provides locking primitives to allow data to be safely shared across goroutines. However, the recommended approach to concurrent Go programming is to communicate data, rather than to share it. A Go channel is a bidirectional or unidirectional communication pipe that can be used to communicate (i.e., send and receive) data between two or more goroutines. Between them, goroutines and channels provide a means of lightweight (i.e., scalable) concurrency that does not use shared memory and so does not require locking. Nonetheless, as with all other approaches to concurrency, care must be exercised when creating concurrent programs and maintenance is usually more challenging than for nonconcurrent programs. Most operating systems are ex- cellent at running multiple programs at the same time, so exploiting this can reduce maintenance—for instance, by running multiple programs (or multiple copies of the same program) each operating on different data. Good program- mers write concurrent programs only when the approach has clear advantages that outweigh the increased maintenance burden. A goroutine is created using the go statement with the following syntaxes: go function(arguments) go func(parameters){block }(arguments) We must either call an existing function or call an anonymous function created on the spot. The function may have zero or more parameters just like any other function,and if it has parameters,corresponding arguments must be passed the same as with any other function call. Execution of the called function begins immediately—but in a separate goroutine—and execution of the current goroutine (i.e., the one that has the go statement) resumes immediately from the next statement. So, after a go state- ment, there are at least two goroutines running, the original one (initially the main goroutine), and the newly created one. In rare cases it is sufficient to start off a bunch of goroutines and wait for them all to finish, with no communication necessary. In most situations, though, goroutines need to work cooperatively together, and this can best be achieved by giving them the ability to communicate. Here are the syntaxes used for sending and receiving data: ptg7913109 5.4. Communication and Concurrency Statements 207 channel <- value // Blocking send <-channel // Receive and discard x := <-channel // Receive and store x, ok := <-channel // As above & check for channel closed & empty Nonblocking sends are possible using the select statement, and to some extent using buffered channels. Channels are created with the built-in make() function with these syntaxes: make(chan Type) make(chan Type, capacity) If no buffer capacity is specified the channel is synchronous,so it will block until the sender is ready to send and the receiver is ready to receive. If a capacity is given the channel is asynchronous and communication will progress without blocking so long as there is unused capacity for sends and there is data in the channel to be received. Channels are bidirectional by default, but we can make them unidirectional if we want to—for example, to better express our semantics in a way that the compiler can enforce. In Chapter 7 we show how to create unidirectional channels, and from then on use unidirectional channels whenever appropriate. Let’s put all the syntax just discussed in context with a tiny example.★ We will write a createCounter() function which will return a channel that will send an int whenever we ask to receive from it. The first value received will be the start value that we pass to the createCounter() function and each subsequent value will be one more than the one before. Here is how we might create two independent counter channels (each operating in its own goroutine), and the results they produce. counterA := createCounter(2)// counterA is of type chan int counterB := createCounter(102)// counterB is of type chan int for i:=0;i<5; i++ { a := <-counterA fmt.Printf("(A→%d, B→%d) ", a, <-counterB) } fmt.Println() (A→2, B→102) (A→3, B→103) (A→4, B→104) (A→5, B→105) (A→6, B→106) We have shown the receives in two different ways just to show how it is done. The first receive assigns the received value to a variable, and the second passes the received value as an argument to a function. ★ This example was inspired by Andrew Gerrand’s blog, -unique-numbe.(There really isn’t an “r” on the end.) ptg7913109 208 Chapter 5. Procedural Programming The two calls to the createCounter() function are made in the main goroutine, and the two other goroutines, each one created by createCounter(), are both initially blocked. In the main goroutine, as soon as we attempt to receive from one of the channels a send takes place and we receive the value. Then the sending goroutine is blocked again, waiting for a new receive request. The two channels are “infinite”, in that they can always send a value. (Of course, if we reach the int limit the next value will wrap.) Once the five values we want have been received from each channel the channels are again blocked and ready for use later on. How can we get rid of the goroutines that we are using for the counter channels if they are no longer needed? This requires us to get them to break out of their infinite loops,so that they stop sending more data, and then to close the channels they are using. We will see one way to do this in the following subsection—and, of course, Chapter 7 which is devoted to concurrency has much more coverage. func createCounter(start int) chan int { next := make(chan int) go func(i int) { for { next <- i i++ } }(start) return next } This function accepts a starting value and creates a channel for sending and receiving ints. It then begins executing an anonymous function in a new go- routine, passing it the start value. The function has an infinite loop that simply sends an int and then increments the int at each iteration. Because the chan- nel was created with zero capacity the send blocks until a receive is requested from the channel. The blocking only affectsthe anonymous function’sgoroutine, so the rest of the program’s goroutines can continue to run unconcerned. Once the goroutine has been set running (and, of course, at this point it immediately blocks), the function’s following statement is immediately executed, and this re- turns the channel to its caller. In some situations we may have multiple goroutines executing, each with its own communication channel. We can monitor their communications using a select statement. ptg7913109 5.4. Communication and Concurrency Statements 209 5.4.1. Select Statements Go’s select statement has the following syntax:★ select { case sendOrReceive1: block1 ... case sendOrReceiveN: blockN default: blockD } In a select statement Go evaluates each send or receive statement in order from first to last. If any of these statements can proceed (i.e., is not blocked), then of those that can proceed, an arbitrary choice is made as to which one to use. If none can proceed (i.e., if they are all blocked), there are two possible scenarios. If a default case is present, the default case is executed and execution resumes from the statement following the select; but if there is no default case the select will block until at least one communication can proceed. A consequence of the select statement’s logic is as follows. A select with no default case is blocking and will only complete when one communication case (receive or send) has occurred. A select with a default case is nonblocking and executes immediately, either because a communication case occurred, or if no communication channel is ready, by executing the default case. To get to grips with the syntax we will review two short examples. The first ex- ample is rather contrived but does give a good idea of how the select statement works. The second example shows a more realistic approach to use. channels := make([]chan bool, 6) for i:=range channels { channels[i] = make(chan bool) } go func() { for { channels[rand.Intn(6)] <- true } }() In this snippet we have created six channels which can send and receive Booleans. We have then created a single goroutine that hasan infinite loop with- in which one of the channels is chosen at random and sent a true value on every iteration. The goroutine immediately blocks, of course, since the channels are unbuffered and we have not yet tried to receive from any of them. ★ Go’s select statement has nothing to do with the POSIX select() function used to monitor file descriptors—for that, use the syscall package’s Select() function. ptg7913109 210 Chapter 5. Procedural Programming for i:=0;i<36; i++ { var x int select { case <-channels[0]: x = 1 case <-channels[1]: x = 2 case <-channels[2]: x = 3 case <-channels[3]: x = 4 case <-channels[4]: x = 5 case <-channels[5]: x = 6 } fmt.Printf("%d ",x) } fmt.Println() 646541212155462365154432333536522362 In this snippet we use the six channels to simulate rolls of a fair die (strictly speaking, a pseudo-random die).★ The select statement waits for one of the channels to have something to send—the select blocks since we have not pro- vided a default case—and as soon as one or more channels are ready to send one case is chosen pseudo-randomly. Since the select is inside a plain for loop it is executed a fixed number of times. Now we will look at a more realistic example. Suppose that we want to perform the same expensive computation on two separate data sets and that the compu- tation produces a sequence of results. Here is a skeleton of a function that per- forms such a computation. func expensiveComputation(data Data, answer chan int, done chan bool) { // setup ... finished := false for !finished { // computation ... answer <- result } done <- true } ★ For proper pseudo-random numbers see the math/rand and crypto/rand packages. ptg7913109 5.4. Communication and Concurrency Statements 211 The function is given some data to work on, and two channels. The answer channel is used to send each result to the monitoring code and the done channel is used to notify the monitoring code that the computation has finished. // setup ... const allDone = 2 doneCount := 0 answerα := make(chan int) answerβ := make(chan int) defer func() { close(answerα) close(answerβ) }() done := make(chan bool) defer func() { close(done) }() go expensiveComputation(data1, answerα, done) go expensiveComputation(data2, answerβ, done) for doneCount != allDone { var which, result int select { case result = <-answerα: which = 'α' case result = <-answerβ: which = 'β' case <-done: doneCount++ } if which != 0 { fmt.Printf("%c→%d ", which, result) } } fmt.Println() α→3 β→3 α→0 β→9 α→0 β→2 α→9 β→3 α→6 β→1 α→0 β→8 α→8 β→5 α→0 β→0 α→3 Here is the code that sets up the channels, starts the expensive computations, monitors progress, and cleans up at the end—and there isn’t a lock in sight. We begin by creating two channels to accept results, answerα and answerβ, and a channel to keep track of when the computations are finished, done. We create anonymous functions in which the channels are closed and call these in defer statements so that they will be closed when they are no longer needed, that is, when the enclosing function returns. Next, we start off the expensive computa- tions (in their own goroutines), giving each one its own unique data to work on, and for communications, its own unique answer channel, and the done channel that is shared. ptg7913109 212 Chapter 5. Procedural Programming We could have given both expensive computations the same answer channels, but if we did that we would not know which one had given which result (which might not matter, of course). If we wanted to use the same channel and wanted to identify the origin of any particular result we could make a single answer channel that operated on a struct—for example, type Answer struct{ id, answer int }. With the expensive computations started in their goroutines (but blocked, since their channels are unbuffered), we are ready to receive their results. The for loop starts with fresh which and result values on every iteration, and the blocking select statement executes an arbitrary case from those that are ready to proceed. If an answer is ready we set which to indicate its origin and print the origin and the result. If the done channel is ready we increment the doneCount counter—and when this reaches the number of expensive computations we started we know that they are all finished and the for loop ends. Once outside the for loop we know that both expensive computations’ goroutines will no longer send any data on the channels (since they broke out of their own infinite for loops when they were finished; 210 ➤ ). When the function returns the channels are closed by the defer statements and any resources they use are released. After this the garbage collector is free to get rid of the goroutines themselves since they are no longer executing and the channels they were using are closed. Go’s communication and concurrency features are very flexible and versatile; Chapter 7 is devoted to the subject. 5.5. Defer, Panic, and Recover The defer statement is used to defer the execution of a function or method (or of an anonymous function created on the spot) until just before the enclosing function or method returns, but after the return values (if any) have been evaluated. This makes it possible to modify a function’s named return values inside a deferred function (e.g., by assigning to them using the = assignment operator).If more than one defer statement is used in a function or method,they are executed in LIFO (Last In First Out) order. The most common uses of a defer statement are to ensure that a successfully opened file is closed when we are finished with it, to close channels that are no longer needed, or to catch panics. var file *os.File var err error if file, err = os.Open(filename); err != nil { log.Println("failed to open the file: ", err) return ptg7913109 5.5. Defer,Panic, and Recover 213 } defer file.Close() This is an extract from the wordfrequency program’s updateFrequencies() function that was discussed in the previous chapter (176 ➤ ). It shows a typical pattern for opening a file and deferring closing the file if the open succeeded. This pattern of creating a value and deferring some kind of close function that cleans up the value (e.g., freeing up any resources the value uses), prior to the value being garbage-collected, is standard in Go.★ We can, of course, apply this pattern to our own types by providing them with a Close() or Cleanup() function that can be the subject of a defer statement, although this is rarely needed in practice. 5.5.1. Panic and Recover Go provides an exception handling mechanism through the use of its built-in panic() and recover() functions. These functions could be used to provide a general-purpose exception handling mechanism, similar to those available in some other languages (e.g., C++, Java, and Python): But to do so is considered to be poor Go style. Go distinguishes between errors—things that might go wrong and that a program should handle gracefully (e.g., a file that could not be opened)—and exceptions—something that “cannot” happen (e.g., a precondition which should always be true that’s actually false). The idiomatic way to handle errorsin Go is to return an error as the last (or only) return value from functions and methods and to always check any returned errors. (The one case where it is common to ignore returned error values is when printing to the console.) For “cannot happen” situations we can call the built-in panic() function with any value we like (e.g., a string that explains the invariant that has been broken).In other languages we might use an assertion for these situations,but in Go we call panic(). During early development and prior to any releases the simplest and probably the best approach is to call panic() to terminate the program to force problems to be impossible to ignore so that they get fixed. Once we start deploy- ing our application it is best to avoid termination when problems occur if at all possible, and this can be done while still leaving any remaining panic() calls in place by adding deferred recover() calls in our packages. During recovery we can catch and log any panics (so that they remain as visible problems), and re- turn non-nil errors to callers who can then try to restore the program to a sane state from which it can safely continue to run. ★ In C++ destructors are used to clean up values. In Java and Python cleanup is problematic since they cannot guarantee when or even if their finalizer()/__del__() method will be called. ptg7913109 214 Chapter 5. Procedural Programming When the built-in panic() function is called normal execution of the enclosing function or method stops immediately. Then, any deferred functions or methods are called—just as they would have been had the function returned normally. And, finally, control is returned to the caller—as if the called function or method had called panic(), so the process is then repeated in the caller: Execution stops, deferreds are called, and so on. When main() is reached there is no caller to return to, so at this point the program is terminated with a stack trace dumped to os.Stderr including the value that was given to the original panic() call. If a panic occursthe processjust describedisnormally what unfolds. However,if one of the deferred functions or methods contains a call to the built-in recover() function (which may be called only inside a deferred function or method), the panic is stopped in its tracks. At this point we can respond to the panic any way we like. One solution is to ignore the panic, in which case control will pass to the caller of the function with the deferred recover() call which will then continue to execute normally. This approach is not recommended, but if used, at the very least the panic should be logged so that the problem isn’t completely hidden. An- other solution is to do whatever cleanup we like and then call panic() ourselves to continue the propagation of the problem. A more common solution is to create an error value and set that as the (or one of the) return values of the function with the deferred recover() call, thus turning the exception (i.e., a panic()) into an error (i.e., an error). In almost every case, the Go standard library uses error values rather than panics. For our own custom packages, it is best not to use panic(); or, rather, not to allow panic()s to leave the custom package by using recover() to capture panics and to return errors instead, just like the standard library does. An illustrative example is Go’s basic regular expression package, regexp. This has a few functions for creating regular expressions, including regexp.Compile() and regexp.MustCompile(). The first of these returns a compiled regular expres- sion and nil, or, if the string passed to it isn’t a valid regular expression, nil and an error. The second of these returns a compiled regular expression or it panics. The first function is ideal for when the regular expression comes from an exter- nal source (e.g., is entered by the user or read from a file). The second function is best when the regular expression is hard-coded into the program since it will ensure that when we run the program, if we made a mistake with a regular ex- pression the program will immediately terminate due to the panic. When should we allow panics to terminate our programs and when should we stop them with recover()? There are two competing interests that we must consider. As programmers we want our programs to crash as soon as possible if there is a logical error so that we can identify and fix the problem. But we don’t want our programs to crash at all once they have been deployed. For problems that can be caught just by running the program (e.g., invalid regular expressions), we should use panic() (or functions that panic such as ptg7913109 5.5. Defer,Panic, and Recover 215 regexp.MustCompile()) since we would never deploy an application that crashes as soon as it is run. We must be careful that we do this only in functions we are certain will be called simply by running the program—for example, the main package’s init() function (if it has one), the main package’s main() function, and any init() functions in our custom packages that our program imports—plus, of course, any functions or methods that these functions call. If we use a test suite we can, of course, extend our use of panics to any function or method that the test suite causes to be invoked. Naturally, we must also be sure that such potential panic cases are always exercised no matter what the program’s flow of control. For functions and methods that may or may not be called during any particular run we should use recover() if we call panic() ourselves or if we call functions or methods that panic, and turn panics into errors. Ideally recover()s should be used as close to the panic()s they handle as possible, and where possible and appropriate they should restore the program to a sane state before setting their enclosing function or method’s error return value. For the main package’s main() function we could put in a top-level “catchall” recover() that logs any caught panics—but unfortunately, the program would then terminate after the deferred recover() had been handled. This can be worked around, as we will see shortly. We will look at two examples, the first demonstrating how to convert panics into errors, and the second showing how to make programs more robust. Imagine we have the following function buried deep within a package we are using, but which we cannot change because it is from a third party over whom we have no control. func ConvertInt64ToInt(x int64) int { if math.MinInt32 <= x && x <= math.MaxInt32 { return int(x) } panic(fmt.Sprintf("%d is out of the int32 range", x)) } This function safely converts an int64 to an int or panics if the conversion would produce an invalid result. Why would a function like this use panic() in the first place? We might want to force a crash as soon as something goes wrong so as to flush out programming errors as early as possible. Another use case is where we have a function that calls one or more other functions and so on, but where if anything goes wrong we want to immediately return control to the original function—so we make the called functions panic if they hit a problem, and catch the panic (wherever it came from) using recover(). Normally, we want packages to report problems as errors rather than to panic, so it is fairly common to use panic()s inside a ptg7913109 216 Chapter 5. Procedural Programming package, and to use recover()s to ensure that the panics don’t leak out and are reported as errors. And another use case is to put calls like panic("unreachable") in places that our logic says cannot be reached (e.g., at the end of a function which always returns by using return statements before reaching the end), or calling panic() if a pre- or post-condition is broken. Doing this ensures that if we ever break the logic of such functions we will soon know about it. If none of the above reasons apply then we ought to avoid panicking and return a non-nil error when problems occur. So, in this example, we want to return an int and nil if a conversion succeeds and int and an error if a conversion fails. Here is a wrapper function that achieves what we want: func IntFromInt64(x int64) (i int, err error){ defer func() { if e:=recover(); e != nil { err = fmt.Errorf("%v",e) } }() i = ConvertInt64ToInt(x) return i, nil } When this function is called, as usual, Go automatically sets the return values to the zero values for their types, in this case 0 and nil. If the call to the custom ConvertInt64ToInt() function returns normally, we assign its result to the i return value, and return i along with nil to signify that no error occurred. But if the ConvertInt64ToInt() function panics, we catch the panic in the deferred anonymous function and set err to be an error with its text set to the textual representation of the panic it caught. As the IntFromInt64() function shows, it is straightforward to convert panics into errors. For our second example we will consider how to make a web server robust in the face of panics. Back in Chapter 2 we reviewed the statistics example (§2.4, 72 ➤ ). If we made a programming error in that server—for example, if we accidentally passed nil as an image.Image value and called a method on it—we would get a panic that without a call to recover() would terminate the program. This is, of course, a very unsatisfactory situation if the web site is important to us, especially if we want it to run unattended some of the time. What we want is for the server to continue running even if a panic occurs, and to log any panics so that we can track them down and fix them at our leisure. We have created a modified version of the statistics example (in fact, of the statistics_ans solution), in file statistics_nonstop/statistics.go. One modifica- tion that we have made is to add an extra button to the web page, Panic!, that can ptg7913109 5.5. Defer,Panic, and Recover 217 be clicked to make a panic occur for testing purposes. The most important mod- ification is that we have made the server able to survive panics. And to help us see what is going on, we also log whenever a client is successfully served, when we get a bad request, and if the server was restarted. Here is a tiny sample of a typical log. [] served OK [] served OK [] bad request: '6y' is invalid [] served OK [] caught panic: user clicked panic button! [] served OK We have told the log package not to use timestamps simply to make the log output more attractive for the book. Before looking at the changes we have made, let us briefly remind ourselves of the original code. func main() { http.HandleFunc("/", homePage) if err := http.ListenAndServe(":9001", nil); err != nil { log.Fatal("failed to start server", err) } } func homePage(writer http.ResponseWriter, request *http.Request) { // ... } This web site has only one page, although the technique we will present can just as easily be applied to sites with multiple pages. If a panic occurs that is not caught by a recover(), that is, if a panic reaches the main() function, the server will terminate, so this is what we must protect against. func homePage(writer http.ResponseWriter, request *http.Request) { defer func() {// Needed for every page if x:=recover(); x != nil { log.Printf("[%v] caught panic: %v", request.RemoteAddr, x) } }() // ... } For a web server to be robust in the face of panics we must make sure that every page handler function has a deferred anonymous function that calls recover(). ptg7913109 218 Chapter 5. Procedural Programming This will stop any panic from being propagated. However, it cannot stop the page handler from returning (since deferred statements are executed just be- fore a function returns), but that doesn’t matter since the http.ListenAndServe() function will call the page handler afresh whenever the page it handles is re- quested. Of course, for a large web site with lots of page handlers, adding a deferred function to catch and log panics involves a lot of code duplication and is easy to forget. This can be solved by creating a wrapper function that has the code needed by each page handler. Using the wrapper we can omit the recover code from the page handlers, so long as we change the http.HandleFunc() calls. http.HandleFunc("/", logPanics(homePage)) Here we have the original homePage() function (i.e., one that does not have a de- ferred function that calls recover()), relying instead on the logPanics() wrapper function to take care of panics. func logPanics(function func(http.ResponseWriter, *http.Request)) func(http.ResponseWriter, *http.Request) { return func(writer http.ResponseWriter, request *http.Request) { defer func() { if x:=recover(); x != nil { log.Printf("[%v] caught panic: %v", request.RemoteAddr, x) } }() function(writer, request) } } This function takes an HTTP handler function as its sole argument and creates and returns an anonymous function that includes a deferred (also) anonymous function that catches and logs panics, and that calls the passed-in handler func- tion. This has the same effect as adding the deferred panic catcher and logger that we saw in the modified homePage() function, but is much more convenient since we don’t have to add the deferred function to any page handler; instead we pass each page handler function to the http.HandleFunc() using the logPanics() wrapper. A version of the statistics program that uses this technique is in file statis- tics_nonstop2/statistics.go. Anonymous functions are covered in the next sec- tion’s subsection on closures (§5.6.3, ➤ 225). ptg7913109 5.6. Custom Functions 219 5.6. Custom Functions Functions are the bedrock of procedural programming and Go provides first- class support for them. Go methods (covered in Chapter 6) are very similar to Go functions, so this section is relevant for both procedural and object-oriented programming. Here are the fundamental syntaxes for function definitions: func functionName(optionalParameters) optionalReturnType { body } func functionName(optionalParameters)(optionalReturnValues){ body } A function can take zero or more parameters. If there are no parameters the parentheses are empty. If there is one or more, they are written params1 type1, …, paramsN typeN, where params1 is either a single parameter name or a comma- separated list of two or more parameter names of the given type. Parameters must be passed in the order given: There is no equivalent to Python’s named pa- rameters, although a similar effect can be achieved as we will see later (§, ➤ 222). The very last parameter’s type may be preceded by an ellipsis (...). Such functions are called variadic; this means that the function will accept zero or more values of that type as that parameter’s value and inside the function that parameter will be of type []type. A function may return zero or more values. If there are none the open brace follows the parameter’s closing parenthesis. If there is one unnamed return value it can be written as type. If there are two or more unnamed return values, parentheses must be used and they are written as (type1,…, typeN). If there are one or more named return values, parentheses must be used and they are written as (values1 type1,…, valuesN typeN), where values1 is either a single return value name or a comma-separated list of two or more return value names of the given type. Function return values may all be unnamed or all be named, but not a mixture of both. Functions that have one or more return values must have at least one return statement—or have a call to panic() as their final statement. If return values are unnamed, the return statement must specify as many values as there are return values, each with a type matching the corresponding return value. If the return values are named the return statement can either specify values just like in the unnamed case or be bare (i.e., giving no explicit values to return). Note that although bare returns are legal, they are considered poor style—none of the book’s examples uses them. ptg7913109 220 Chapter 5. Procedural Programming If a function has one or more return values its last executable statement must be a return or a panic(). Go compilers are smart enough to realize that a func- tion that ends with a panic won’t return normally and so doesn’t need a return statement at that point. Unfortunately, current Go compilers don’t understand that if a function ends with an if statement that has an unconditional else statement that ends with a return statement, or a switch statement that has a default case that ends with a return statement, no additional return is needed afterward. A common practice in such cases is to either not end with an else or default case and put the return statement after the if or switch, or simply put a panic("unreachable") statement at the end—we saw examples of both approach- es earlier (196 ➤ ). 5.6.1. Function Arguments We have already seen many examples of custom Go functions that accept a fixed number of arguments of specified types. By using a parameter type of interface{} we can create functions that take arguments of any type. And by using a parameter type that is an interface type—either our own custom interface or one from the standard library—we can create functions that take arguments of any type that has a specific set of methods: We will look at these issues in Chapter 6 (§6.3, ➤ 265). In this subsection we will look at other possibilities regarding function argu- ments. In the first subsubsection we will see how to use functions’ return values directly asargumentsto other functions. In the second subsubsectionwe will see how to create functions that accept a variable number of arguments. And in the final subsubsection we will discuss a technique that makes it possible to create functions that can accept optional arguments. Function Calls as Function Arguments If we have a function or method that accepts one or more parameters,we can, of course, call it with corresponding arguments. And in addition, we can call the function with another function or method—providing that the other function returns exactly the number of arguments required (and of the right types). Here is an example of a function that takes the lengths of the sides of a triangle (as three ints) and outputs the triangle’s area using Heron’s formula. for i:=1;i<=4; i++ { a, b, c := PythagoreanTriple(i, i+1) Δ1 := Heron(a, b, c) Δ2 := Heron(PythagoreanTriple(i, i+1)) fmt.Printf("Δ1 == %10f == Δ2 == %10f\n",Δ1,Δ2) } ptg7913109 5.6. Custom Functions 221 Δ1 == 6.000000 == Δ2 == 6.000000 Δ1 == 30.000000 == Δ2 == 30.000000 Δ1 == 84.000000 == Δ2 == 84.000000 Δ1 == 180.000000 == Δ2 == 180.000000 First we obtain the lengths using Euclid’s formula for Pythagorean triples, then we apply Heron’s formula using the Heron() function which takes exactly three int arguments. Then we repeat the computation, only this time we use the PythagoreanTriple() function directly as the Heron() function’s argument, leaving Go to convert the PythagoreanTriple()’s three return values into the Heron() function’s three arguments. func Heron(a, b, c int) float64 { α, β, γ := float64(a), float64(b), float64(c) s := (α + β + γ)/2 return math.Sqrt(s *(s - α)*(s - β)*(s - γ)) } func PythagoreanTriple(m, n int) (a, b, c int) { if m 1 && (os.Args[1]=="-a" || os.Args[1]=="--ascii"){ os.Args = append(os.Args[:1], os.Args[2:]...) // Strip out arg. IsPalindrome = func(s string) bool { // Simple ASCII-only version if len(s) <= 1 { return true } if s[0]!=s[len(s)-1]{ return false } return IsPalindrome(s[1 : len(s)-1]) } } else { IsPalindrome = func(s string) bool {// UTF-8 version // ... same as earlier ... } } } We have made the choice of IsPalindrome() implementation dependent on a command-line argument. If the argument is given, we strip it out of the os.Args slice (so the rest of the program doesn’t have to know or care about it),and create a 7-bit ASCII version of the IsPalindrome() function. The stripping out is slight- ly subtle since we want os.Args to have its first string and its third and subse- quent strings but not its second string (which is "-a" or "--ascii"). We can’t use os.Args[0] in the append() call because the first argument must be a slice, so we use os.Args[:1] which is a one-item slice containing os.Args[0] (§4.2.1,153 ➤ ).If the ASCII argument isn’t present, we create the same version as we saw earlier that works correctly on both 7-bit ASCII and UTF-8 Unicode strings. In the rest of the program the IsPalindrome() function can be called normally,but the actual code that gets executed will vary depending on which version was created. (The source code for this example is in palindrome/palindrome.go.) 5.6.6. Generic Functions Earlier in the chapter we created a function for finding the smallest of the int arguments it was passed (221 ➤ ). The algorithm used in that function could also be applied to other numeric types, or even to strings, since it works for any type that supports the < less than operator. In C++, for cases like this, we would create a generic function that is parameterized by type which would result in ptg7913109 5.6. Custom Functions 233 the compiler creating as many versions of the function as we need (i.e., one per type used). In Go, at the time of this writing, there is no support for type parameterization, so to get the same effect as C++ achieves we must manually create the functions we need (e.g., MinimumInt(), MinimumFloat(), MinimumString()). This way we end up with one function per type used (just like in C++, except that in Go each function must have a unique name). Go offers various alternative approaches which avoid the need to create func- tions that are the same except for the types they operate on, at the cost of some runtime efficiency. For small functions that are not used often or which are more than fast enough already, the alternative approaches can be convenient. Here are some examples that use a generic Minimum() function. i := Minimum(4, 3, 8, 2, 9).(int) fmt.Printf("%T %v\n",i,i) f := Minimum(9.4,-5.4, 3.8, 17.0,-3.1, 0.0).(float64) fmt.Printf("%T %v\n",f,f) s := Minimum("K","X","B","C","CC","CA","D","M").(string) fmt.Printf("%T %q\n",s,s) int 2 float64 -5.4 string "B" The function returns a value of type interface{} which we convert to the built-in type we expect using an unchecked type assertion (§5.1.2, 191 ➤ ). func Minimum(first interface{}, rest ...interface{}) interface{} { minimum := first for _, x := range rest { switch x := x.(type){ case int: if x < minimum.(int) { minimum = x } case float64: if x < minimum.(float64) { minimum = x } case string: if x < minimum.(string) { minimum = x } } } ptg7913109 234 Chapter 5. Procedural Programming return minimum } This function takes at least one value (first) and zero or more other values (rest). We use the interface{} type since that can be used for any type in Go. We initially assume that the first value is the smallest and then iterate over the rest of the values. Whenever we find a value that is smaller than the current minimum we set the minimum to this value. And at the end we return the minimum—as an interface{}, hence the need to convert it to a built-in type at the Minimum() function’s call site using an unchecked type assertion. We still have duplicate code—in each case’s if statement’s block—but if there were a lot of duplicate code we could simply set a Boolean in each case (e.g., change = true), and then follow the switch with an if change statement that contained all the common code. Clearly, using this Minimum() function is less efficient than having type-specific minimum functions. However, it is worth knowing the technique because it is useful for cases where the type testing overheads and conversion inconvenience are outweighed by the advantage of having to define the function only once. The problem of duplicate code within a generic function isn’t so easy to work around if one or more of the interface{} arguments are actually slices. For example, here is a function that, given a slice and an item of the same type as the slice’s items, returns the index position of the first occurrence of the item in the slice—or −1 if the item isn’t in the slice. func Index(xs interface{}, x interface{}) int { switch slice := xs.(type){ case []int: for i, y := range slice { if y == x.(int) { return i } } case []string: for i, y := range slice { if y == x.(string) { return i } } } return -1 } ptg7913109 5.6. Custom Functions 235 We have only bothered to implement the int and string cases—both of which contain essentially the same code. Here is an example of the Index() function in use and the output it produces. (The code is taken from the contains/contains.go test program.) xs := []int{2, 4, 6, 8} fmt.Println("5 @", Index(xs, 5), " 6 @", Index(xs, 6)) ys := []string{"C","B","K","A"} fmt.Println("Z @", Index(ys, "Z"), " A @", Index(ys, "A")) 5 @-1 6 @ 2 Z@-1 A @ 3 What we really need to be able to do is treat the slice generically—that way we could have just one loop and do the type-specific testing inside it. Here is a function that achieves this—and it produces the same output as the above code snippet if we replace calls to Index() with calls to IndexReflectX(). func IndexReflectX(xs interface{}, x interface{}) int {// Long-winded way if slice := reflect.ValueOf(xs); slice.Kind() == reflect.Slice { for i:=0; i < slice.Len(); i++ { switch y := slice.Index(i).Interface().(type){ case int: if y == x.(int) { return i } case string: if y == x.(string) { return i } } } } return -1 } The function begins by using Go’s reflection support (provided by the reflect package; §9.4.9, ➤ 427), to convert the xs interface{} into a slice-typed re- flect.Value. Such values provide the methods we need to traverse the slice’s items and to extract any items we are interested in. Here, we access each item in turn and use the reflect.Value.Interface() function to pull out the value as an interface{} which we immediately assign to y inside a type switch. This en- sures that y has the item’s actual type (e.g., int or string) which can be directly compared with the unchecked type-asserted x value. ptg7913109 236 Chapter 5. Procedural Programming In fact, the reflect package can take on far more of the work, so we can simplify this function considerably. func IndexReflect(xs interface{}, x interface{}) int { if slice := reflect.ValueOf(xs); slice.Kind() == reflect.Slice { for i:=0; i < slice.Len(); i++ { if reflect.DeepEqual(x, slice.Index(i)) { return i } } } return -1 } Here we rely on the reflect.DeepEqual() function to do the comparison for us. This versatile reflection function can also be used to compare arrays, slices, and structs. Here is a type-specific function for finding the index of an item in a slice. func IntSliceIndex(xs []int, x int) int { for i, y := range xs { if x==y{ return i } } return -1 } This is much nicer and simpler than the generic versions but requires us to create one function like this per type we want to work on—with the only changes being their names and the types specified in the functions’ signatures. We can combine the benefits of a generic approach—implementing one algo- rithm—with the simplicity and efficiency of type-specific functionsby using cus- tom types, a topic that is covered more thoroughly in the next chapter. Here is a type-specific function for finding the index position of an item in an []int and the generic function it uses to do the actual work. func IntIndexSlicer(ints []int, x int) int { return IndexSlicer(IntSlice(ints), x) } func IndexSlicer(slice Slicer, x interface{}) int { for i:=0; i < slice.Len(); i++ { if slice.EqualTo(i, x) { ptg7913109 5.6. Custom Functions 237 return i } } return -1 } The IntIndexSlicer() function takes an []int to search and an int to find and passes these on to the generic IndexSlicer() function. The IndexSlicer() func- tion operates in terms of a Slicer value—the Slicer type is a custom interface that is met by any value that provides the Slicer methods (Slicer.EqualTo() and Slicer.Len()). type Slicer interface { EqualTo(i int, x interface{}) bool Len() int } type IntSlice []int func (slice IntSlice) EqualTo(i int, x interface{}) bool { return slice[i] == x.(int) } func (slice IntSlice) Len() int { return len(slice) } The Slicer interface specifies the two methods we need to implement the generic IndexSlicer() function. The IntSlice type is based on an []int (which is why the IntIndexSlicer() function can convert the []int it is passed into an IntSlice without formali- ty), and implements the two methods required to fulfill the Slicer interface. The IntSlice.EqualTo() method takes a slice index position and a value and re- turns true if the item at the given index is equal to the value. The Slicer inter- face specifies the value as a generic interface{} rather than an int so that the Slicer interface can be implemented by other slice types (e.g., FloatSlice and StringSlice), so we must convert the value to the actual type. In this case we can safely use an unchecked type assertion because we know that the value ul- timately comes from a call to the IntIndexSlicer() function which has a corre- sponding int argument. We can implement other custom slice types that satisfy the Slicer interface and can then be used with the generic IndexSlicer() function. type StringSlice []string func (slice StringSlice) EqualTo(i int, x interface{}) bool { return slice[i] == x.(string) } ptg7913109 238 Chapter 5. Procedural Programming func (slice StringSlice) Len() int { return len(slice) } The only difference between the StringSlice and the IntSlice is the underlying slice’s type ([]string rather than []int) and the unchecked type assertion’s type (string vs. int). The same applies to the FloatSlice (which uses []float64 and float64). This last example uses techniques that we saw earlier when we discussed custom sorting (§4.2.4, 160 ➤ ), and are used to implement the standard library’s sort package’s sort functions. Full coverage of custom interfaces and custom types is provided in Chapter 6. When working with slices (or maps) it is often possible to create generic func- tions that don’t need to do type testing or type assertions, and that don’t need to use custom interfaces. Instead, we can make our generic functions higher order functions that abstract away all the type-specific aspects, as we will see in the next subsection. 5.6.7. Higher Order Functions A higher order function is a function that takes one or more other functions as arguments and uses them in its own body. Let’s look at a very short and simple higher order function—but one whose functionality may not be immediately apparent. func SliceIndex(limit int, predicate func(i int) bool) int { for i:=0; i < limit; i++ { if predicate(i) { return i } } return -1 } This is a generic function that returns the index position of an item in a slice for which the predicate() function returns true. So, this function can do exactly the same job as the Index(), IndexReflect(), IntSliceIndex(), and IntIndexSlicer() functions discussed in the previous subsection—but with no code duplication and no type switching or type assertions. The SliceIndex() function doesn’t know or care about the slice’s or the item’s types—indeed, the function knows nothing of the slice or the item it (indirect- ly) operates on. The function expects its first argument to be the length of the slice and the second argument to be a function that returns a Boolean for any given index position in the slice indicating whether the desired item is at that position. ptg7913109 5.6. Custom Functions 239 Here are four example calls and their results. xs := []int{2, 4, 6, 8} ys := []string{"C","B","K","A"} fmt.Println( SliceIndex(len(xs), func(i int) bool { return xs[i] == 5 }), SliceIndex(len(xs), func(i int) bool { return xs[i] == 6 }), SliceIndex(len(ys), func(i int) bool { return ys[i] == "Z" }), SliceIndex(len(ys), func(i int) bool { return ys[i] == "A" })) -1 2 -1 3 The anonymous functions passed as second arguments to the SliceIndex() function are, of course, closures—so the slices they refer to (xs and ys) must be in scope when these functions are created. (The technique used here is the same as Go’s standard library uses for the sort.Search() function.) In fact, the SliceIndex() function is a general-purpose function that need not have anything to do with slices. i := SliceIndex(math.MaxInt32, func(i int) bool { return i>0 && i%27 == 0 && i%51 == 0 }) fmt.Println(i) 459 In this snippet we have used the SliceIndex() function to find the least nat- ural number that is divisible by both 27 and 51. The way this works is slight- ly subtle. The SliceIndex() function iterates from 0 to the given limit (in this case math.MaxInt32), and at each iteration it calls the anonymous function (the predicate). As soon as the predicate returns true, the SliceIndex() function re- turns, passing back the index position that was reached. In this case the “index position” is actually the natural number we were looking for. In addition to searching unsorted slices it is often useful to filter them to discard items that are no longer of interest. Here is a simple higher order filtering function in action. The function filters an []int using the function it is passed to determine which ints are kept and which are dropped. readings := []int{4,-3, 2,-7, 8, 19,-11, 7, 18,-6} even := IntFilter(readings, func(i int) bool { return i%2 == 0 }) fmt.Println(even) [4 2 8 18 -6] Here, we have filtered out odd numbers from the readings slice. ptg7913109 240 Chapter 5. Procedural Programming func IntFilter(slice []int, predicate func(int) bool) []int { filtered := make([]int, 0, len(slice)) for i:=0;i 3999 { panic("RomanForDecimal() only handles integers [0, 3999]") } var buffer bytes.Buffer for i, decimal := range decimals { remainder := x / decimal x %= decimal if remainder > 0 { ptg7913109 244 Chapter 5. Procedural Programming buffer.WriteString(strings.Repeat(romans[i], remainder)) } } return buffer.String() }) } The RomanForDecimal() function is declared globally (well, inside its package; see Chapter 9) as a memoizeFunction and created in the init() function. The decimals and romans slices are local to the init() function, but are kept alive so long as the RomanForDecimal() function is used since it is a closure that has captured them. Go functions (and methods) are incredibly flexible and powerful, and offer several different ways to achieve genericity when that is required. 5.7. Example: Indent Sort In this section we will review a custom function that sorts a slice of strings. What makesthis function special (and why the standard library’s sort.Strings() function is insufficient on its own) is that the strings are sorted hierarchically, that is, within their level of indentation. (The function is in the book’s source code in the file indent_sort/indent_sort.go.) Note that the algorithm used for the SortedIndentedStrings() function makes a critical simplifying assumption: A string’s indentation is measured by the number of leading spaces or tabs it has, so we are able to work in terms of single bytes rather than having to concern ourselves with multibyte whitespace characters. (If we really want to handle multibyte whitespace characters, one easy solution is to replace each such character with a single space or tab in the stringsbefore feeding them to the SortedIndentedStrings() function,for example, using the strings.Map() function.) Let’s begin with a program that calls the function and outputs some unsorted indented strings and the sorted strings, side by side for comparison (➤ 245). Between them, the SortedIndentedStrings() function and its helper functions and types use recursive functions, function references, and pointers to slices. So, although what the function does is easy to see, the implementation of the solution requires some thought. The solution uses some important Go function features that were introduced in this chapter, as well as some ideas and tech- niques that were covered in Chapter 4 and which are explained more fully in Chapter 6. The keys to our solution are the custom Entry and Entries types. For each string in the original slice we will create an Entry whose key field will be used for sorting, whose value field is the original string, and whose children field is a slice ptg7913109 5.7. Example: Indent Sort 245 func main() { fmt.Println("| Original | Sorted |") fmt.Println("|-------------------|-------------------|") sorted := SortedIndentedStrings(original) // original is a []string for i:=range original { // set in a global var fmt.Printf("|%-19s|%-19s|\n", original[i], sorted[i]) } } | Original | Sorted | |-------------------|-------------------| |Nonmetals |Alkali Metals | | Hydrogen | Lithium | | Carbon | Potassium | | Nitrogen | Sodium | | Oxygen |Inner Transitionals| |Inner Transitionals| Actinides | | Lanthanides | Curium | | Europium | Plutonium | | Cerium | Uranium | | Actinides | Lanthanides | | Uranium | Cerium | | Plutonium | Europium | | Curium |Nonmetals | |Alkali Metals | Carbon | | Lithium | Hydrogen | | Sodium | Nitrogen | | Potassium | Oxygen | of the string’s child Entrys (which could be empty, but if not, whose own Entrys could have children, and so on, recursively). type Entry struct { key string value string children Entries } type Entries []Entry func (entries Entries) Len() int { return len(entries) } func (entries Entries) Less(i, j int) bool { return entries[i].key < entries[j].key } ptg7913109 246 Chapter 5. Procedural Programming func (entries Entries) Swap(i, j int) { entries[i], entries[j] = entries[j], entries[i] } The sort.Interface interface defined in the sort package specifiesthree methods, Len(), Less(), and Swap(), with the same signatures as the Entries methods of the same names. This means that we can trivially sort an Entries value using the standard library’s sort.Sort() function. func SortedIndentedStrings(slice []string) []string { entries := populateEntries(slice) return sortedEntries(entries) } This is the exported (public) function that does the work, although we have refactored it to pass on everything to helper functions. The populateEntries() function takes a []string and returns a corresponding Entries (of underlying type []Entry). The sortedEntries() function takes the Entries and returns a corresponding []string hierarchically (by indentation) sorted. func populateEntries(slice []string) Entries { indent, indentSize := computeIndent(slice) entries := make(Entries, 0) for _, item := range slice { i, level := 0, 0 for strings.HasPrefix(item[i:], indent) { i += indentSize level++ } key := strings.ToLower(strings.TrimSpace(item)) addEntry(level, key, item, &entries) } return entries } The populateEntries() function begins by obtaining what signifies one level of indent in the given slice as a string (e.g., a string of four spaces) and the indent’s size (the number of bytes one level of indent occupies). It then creates an empty Entries value and iterates over every string in the slice. For each string it determines the string’s level of indentation,and then creates a sort key. Next, the function calls the custom addEntry() function, passing it the string’s level, the key, the string itself (item), and the address of the entries so that the addEntry() function can create a new Entry and add it to the entries in the right way. And at the end the entries are returned. ptg7913109 5.7. Example: Indent Sort 247 func computeIndent(slice []string) (string, int) { for _, item := range slice { if len(item) > 0 && (item[0]==''|| item[0]=='\t'){ whitespace := rune(item[0]) for i, char := range item[1:] { if char != whitespace { return strings.Repeat(string(whitespace), i), i } } } } return "", 0 } This function is used to determine what character (space or tab) is used for indentation, and how many of these characters are used to signify one level of indent. The function must potentially iterate over all the strings, because top-level strings have no indentation. But as soon as it finds one string with a leading tab or space it returns a string representing one level of indent and a count of how many characters are used for one level of indent. func addEntry(level int, key, value string, entries *Entries) { if level == 0 { *entries = append(*entries, Entry{key, value, make(Entries, 0)}) } else { addEntry(level-1, key, value, &((*entries)[entries.Len()-1].children)) } } This is a recursive function that createseach new Entry and adds it to the entries either directly or as the child of another Entry if the value string is indented. We must parameterize the function with an *Entries rather than pass the entries by reference (which is the default for slices), since we want to append to the entries and appending to a reference would produce a useless local copy leaving the original data untouched. If level is 0 then we have a top-level entry that must be added directly to the *entries. In fact, the situation is subtler—the level is relative to the given *entries, which is initially the top-level Entries, but if this function is called recursively, could be another Entry’s child Entries. The new Entry is added using the built-in append() function, and using the * contents of operator to access the underlying entries value that is being pointed to. This ensuresthat any changes ptg7913109 248 Chapter 5. Procedural Programming are visible to the caller. The new Entry that is added has the given key and value, and its own empty children Entries. This is the function’s recursive outcase. If the level is greater than 0 we have an indented entry that must be added as a child of the first preceding entry that has one less level of indentation. The code to do this is a single recursive call to the addEntry() function and the last argument is probably the most complicated expression we have seen so far. The subexpression entries.Len() - 1 produces an int that is the index position of the last item in the Entries value that *entries points to. (The Entries.Len() method takes an Entries value rather than an *Entries, but Go is smart enough to automatically dereference the entries pointer and call the method on the pointed-to Entries value.) The complete expression (apart from the outer &( …) part) accesses the last Entry in the Entries’ children field (which itself is of type Entries). So, if we consider the expression as a whole we are taking the address of the last Entry in the Entries children field—this is of type *Entries which is exactly what’s needed for the recursive call. To help clarify what is happening, here is some less compact code that does exactly the same thing as the addEntry() function’s recursive call to itself in the else block. theEntries := *entries lastEntry := &theEntries[theEntries.Len()-1] addEntry(level-1, key, value, &lastEntry.children) First, we create the theEntries variable to hold the value pointed to by the *en- tries pointer—this is cheap since no actual copying is done; in effect, theEntries is set as an alias to the pointed-to Entries value. Next we take the address of (i.e.,a pointer to) the last entry. If we didn’t take the addresswe would get a new entry that is a copy of the last entry—and a copy is of no use here. Finally, we do the recursive addEntry() call, passing the address of the last entry’s children field. func sortedEntries(entries Entries) []string { var indentedSlice []string sort.Sort(entries) for _, entry := range entries { populateIndentedStrings(entry, &indentedSlice) } return indentedSlice } When this sortedEntries() function is called,the structure of the Entries reflects that of the “Original” strings that the program outputs, with indented strings being children of their parent (as children of type Entries fields), and so on (245 ➤ ). ptg7913109 5.7. Example: Indent Sort 249 Once the Entries value has been populated,the SortedIndentedStrings() function calls this function to create a corresponding sorted []string. The function begins by creating an empty []string which it will use to hold the results. It then sorts the entries—thanks to the Entries’ support of the sort.Interface methods the sort.Sort() function can be used without formality and the sorting is by each Entry’s key field since that is how the Entries.Less() method is implemented (245 ➤ ). The sort applies only to the top-level Entrys, of course, and has no effect on their unsorted children. To sort the children, and their children, recursively, the function iterates over the top-level entries and for each one calls the populateIndentedStrings() function. The call is made with the Entry and with a pointer to the []string we want to populate. Slices can be passed to functions to be updated (e.g., to have items replaced), but here we need to append to the slice. The built-in append() function sometimes returns a reference to a new slice (if the original’s capacity is insufficient). The way we handle this here is to pass a pointer to a slice and set the contents that the pointer points to, to the slice returned by append().(If we didn’t use a pointer we would simply get a local slice which would not be visible to the caller.) An alternative approach is to pass in the slice value, and return the appended slice which must then be assigned to the original slice (e.g., slice = function(slice)); however, this can be tricky to get right with recursive functions. func populateIndentedStrings(entry Entry, indentedSlice *[]string) { *indentedSlice = append(*indentedSlice, entry.value) sort.Sort(entry.children) for _, child := range entry.children { populateIndentedStrings(child, indentedSlice) } } This function appends the given entry value to the slice that is being built up, sorts the entry’s children, and then calls itself recursively for every child. This has the effect of sorting every entry’s children, and their children, and so on, so that the entire indented slice of strings is sorted. We have now completed our coverage of Go’s built-in data types and procedural programming support. In the next chapter we will build on all this to present Go’s object-oriented programming facilities, and in the chapter after that we will study Go’s support for concurrent programming. ptg7913109 250 Chapter 5. Procedural Programming 5.8. Exercises There are four exercises for this chapter. The first involves changing one of the examples, some of whose code was shown in the chapter, and the others require the creation of new functions. All the exercises are very short—the first is easy, the second is straightforward, but the third and fourth are quite challenging! 1. Copy the archive_file_list directory to, say, my_archive_file_list. Then modify the archive_file_list/archive_file_list.go file: Remove all the code for supporting different ArchiveFileList() functions except for Archive- FileListMap() which should be renamed ArchiveFileList(). Then add the capability of handling .tar.bz2 (tarballs compressed with bzip2) files. This involves deleting about 11 lines from main(), deleting four functions,import- ing an additional package, adding one entry to the FunctionForSuffix map, and adding a few lines to the TarFileList() function. A solution is given in archive_file_list_ans/archive_file_list.go. 2. Create nonrecursive versions of the recursive IsPalindrome() functions shown earlier in the chapter (229 ➤ and 232 ➤ ). In the palindrome_ans/pal- indrome.go solution file the nonrecursiveASCII-only function is 10 lines long and is radically different in structure to the recursive version. On the other hand, the nonrecursive UTF-8 function is 14 lines long and is very similar to the recursive version—although it does require some care. 3. Create a CommonPrefix() function that takes a []string argument and re- turns a string with the common prefix (which might be an empty string) of all the strings passed in. A solution is given in common_prefix/common_pre- fix.go; the solution is 22 lines long and uses a [][]rune to hold the strings to ensure that when iterating we work in terms of whole characters even if the strings have non-ASCII characters. The solution builds up the result in a bytes.Buffer. Despite being very short, this function is by no means easy! (Some examples follow the next exercise.) 4. Create a CommonPathPrefix() function that takes a []string of paths and re- turns a string with the common prefix (which might be an empty string) of all the paths passed in—the prefix must consist of zero or more complete path components. A solution is given in common_prefix/common_prefix.go; the solution is 27 lines long and uses a [][]string to hold the paths and the filepath.Separator to identify the platform-specific path separator. The so- lution builds up the result in a []string and joins them together as a single path at the end using the filepath.Join() function. Despite being really short, this function is challenging! (Some examples follow.) Here is the output of the common_prefix program that exercises the functions referred to in Exercises 3 and 4. The first of each pair of lines is a slice of strings and the second line has the common prefix produced by the CommonPrefix() and CommonPathPrefix() functions with an indication of whether the common prefixes are the same. ptg7913109 5.8. Exercises 251 $ ./common_prefix ["/home/user/goeg" "/home/user/goeg/prefix" "/home/user/goeg/prefix/extra"] char × path prefix: "/home/user/goeg" == "/home/user/goeg" ["/home/user/goeg" "/home/user/goeg/prefix" "/home/user/prefix/extra"] char × path prefix: "/home/user/" != "/home/user" ["/pecan/π/goeg" "/pecan/π/goeg/prefix" "/pecan/π/prefix/extra"] char × path prefix: "/pecan/π/" != "/pecan/π" ["/pecan/π/circle" "/pecan/π/circle/prefix" "/pecan/π/circle/prefix/extra"] char × path prefix: "/pecan/π/circle" == "/pecan/π/circle" ["/home/user/goeg" "/home/users/goeg" "/home/userspace/goeg"] char × path prefix: "/home/user" != "/home" ["/home/user/goeg" "/tmp/user" "/var/log"] char × path prefix: "/" == "/" ["/home/mark/goeg" "/home/user/goeg"] char × path prefix: "/home/" != "/home" ["home/user/goeg" "/tmp/user" "/var/log"] char × path prefix: "" == "" ptg7913109 This page intentionally left blank ptg7913109 6 Object-Oriented Programming §6.1. Key Concepts ➤ 254 §6.2. Custom Types ➤ 256 §6.2.1. Adding Methods ➤ 258 §6.2.2. Validated Types ➤ 263 §6.3. Interfaces ➤ 265 §6.3.1. Interface Embedding ➤ 270 §6.4. Structs ➤ 275 §6.4.1. Struct Aggregation and Embedding ➤ 275 §6.5. Examples ➤ 282 §6.5.1. Example: FuzzyBool—A Single-Valued Custom Type ➤ 282 §6.5.2. Example: Shapes—A Family of Custom Types ➤ 289 §6.5.3. Example: Ordered Map—A Generic Collection Type ➤ 302 The aim of this chapter is to show how to do object-oriented programming the Go way. Programmerscoming from a proceduralbackground(e.g.,C)should find everything in this chapter builds nicely on what they already know and what the earlier chapters covered. But programmers coming from an inheritance- based object-oriented background (e.g., C++, Java, Python) will need to put aside many of the concepts and idioms they are used to—in particular, those relating to inheritance—since Go takes a radically different approach to object-oriented programming. Go’s standard library mostly provides packages of functions, although where appropriate it also provides custom types that have methods. In earlier chap- ters we created values of some custom types (e.g., regexp.Regexp and os.File) and called methods on them. Furthermore, we even created our own simple custom types and corresponding methods—for example, to support printing and sorting—so the basic use of Go types and calling their methods is already fa- miliar. The chapter’s very short first section introduces some key concepts in Go object- oriented programming. The second section covers the creation of methodless custom types, with subsections on adding methods to custom types and on cre- 253 ptg7913109 254 Chapter 6. Object-Oriented Programming ating construction functions and validating field data to provide all the funda- mentals needed to create fully fledged custom types. The third section covers interfaces—these are fundamental to Go’s approach to type-safe duck typing. The fourth section covers structs, introducing many new details that have not been seen earlier. The chapter’s long final section presents three complete examples of custom types that draw on most of the material covered in the chapter’s earlier sections and a fair amount of material from earlier chapters. The first of these examples is a simple custom single-valued data type, the second is a small family of data types, and the third is a generic collection type. 6.1. Key Concepts What makes Go object orientation so different from, say, C++, Java, and (to a lesser extent) Python, is that it does not support inheritance. When object-ori- ented programming first became popular inheritance was touted as one of its biggest advantages. But now, after a few decades of experience, it has turned out that this feature has some significant drawbacks,especially when it comesto maintaining large systems. Instead of using both aggregation and inheritance like most other object-oriented languages, Go supports aggregation (also called composition)and embedding exclusively. To see the difference between aggrega- tion and embedding let’s look at a tiny code snippet. type ColoredPoint struct { color.Color // Anonymous field (embedding) x, y int // Named fields (aggregation) } Here, color.Color is a type from the image/color package and x and y are ints. In Go terminology, color.Color, x, and y are all fields in the ColoredPoint struct. The color.Color field is anonymous (since it has no variable name) and is therefore an embedded field. The x and y fields are named aggregated fields. If we created a ColoredPoint value (e.g., point := ColoredPoint{}), the fields would be accessed as point.Color, point.x, and point.y. Notice also that when accessing a field of a type from another package we only use the last component of the name, that is, Color and not color.Color.(We will discuss these matters in more detail in §, ➤ 261, §6.3, ➤ 265, and §6.4, ➤ 275.) The terms “class”, “object”, and “instance” are so well established in convention- al inheritance-hierarchy-style object orientation, that in Go we avoid them al- together. Instead, we talk about “types” and “values”, where values of custom types may have methods. Without inheritance there are no virtual functions. Go’s answer to this is to support type-safe duck typing. In Go, parameters can be specified as concrete ptg7913109 6.1. Key Concepts 255 types (e.g., int, string,*os.File, MyType), or as interfaces—that is, values that provide the methods that fulfill the interface. For a parameter specified as an interface we can pass any value—so long as it has the methods that the interface requires. For example, if we have a value that providesa method with the signa- ture Write([]byte) (int, error) we can pass that value as an io.Writer (i.e., as a value that satisfiesthe io.Writer interface) to any function that has an io.Writer parameter, no matter what the value’s actual type is. This is incredibly flexible and powerful, especially when combined with Go’s clever support for accessing the methods of embedded values. One advantage of inheritance is that some methods need only be implemented once in a base class for all the subclasses to benefit. Go provides two solutions for this. One solution is to use embedding. If we embed a type, the method need be created only once for the embedded type and is available to all those types that include the embedded type.★ Another solution is to provide separate versions of the method for all the types we want to have it—but to make these thin wrappers (typically one-liners) that simply pass on the work to a single function that all the methods make use of. Another unusual aspect of Go object-oriented programming is that interfaces, values, and methods are kept separate. Interfaces are used to specify method signatures, structs are used to specify aggregated and embedded values, and methods are used to specify operations on custom types (which are often structs). There is no explicit connection between a custom type’s methods and any particular interface—but if the type’s methods fulfill one or more interfaces, values of the type can be used wherever values of those interfaces are expected. Of course, every type fulfills the empty interface (interface{}), so any value can be used where the empty interface is specified. One way of thinking about the Go approach is to consider that is-a relationships are defined by interfaces, that is, purely in terms of method signatures. So, a value that satisfies the io.Reader interface (i.e., that has a method with the signature Read([]byte) (int, error)) isn’t a reader because of what it is (a file, a buffer, or some custom type), but because of what methods it provides, in this case Read(). This is illustrated in Figure 6.1 (➤ 256). The has-a relationship is expressed by using structs in which we aggregate or embed values of the particular types we need as the constituents of a custom type. Although it isn’t possible to add methods to built-in types, it is very easy to create custom types based on built-in types and to add any methods we like to them. Values of such types can call the methods we have provided, and can also be used with any functions, methods, or operators that their underlying type provides. For example, if we have type Integer int,we can add values of thistype using the int + operator without formality. And once we have a custom type, we ★ In some other languages, what in Go terminology is called embedding,is known as delegation. ptg7913109 256 Chapter 6. Object-Oriented Programming Abstract Interfaces io.ReadWriter io.Reader Read([]byte) (int, error) io.Writer Write([]byte) (int, error) Concrete Types *os.File // is-a io.ReadWriter *bytes.Buffer // is-a io.ReadWriter *zlib.Writer // is-a io.Writer *bufio.Writer // is-a io.Writer *bufio.Reader // is-a io.Reader *strings.Reader // is-a io.Reader *tar.Reader // is-a io.Reader … Figure 6.1 Interfaces and types for reading and writing byte slices can also add custom methods—for example, func (i Integer) Double() Integer { return i * 2}, as we will see shortly (§6.2.1, ➤ 258). Not only are custom types based on built-in types easy to create, they are also very efficient to use. Converting to or from a built-in type has no runtime cost since the conversion is effectively done at compile time. In view of this it is en- tirely practical to “promote” values of built-in types to a custom type to use the custom type’s methods, and to “demote” such values to the underlying built-in type when we want to pass them to functions whose parameters are of built-in types. We saw an example of promotion earlier when we converted a []string to a FoldedStrings (162 ➤ ), and we will see an example of demotion when we first look at the Count type later in this chapter (➤ 259). 6.2. Custom Types Custom types are created using Go’s type statement with the following syntax: type typeName typeSpecification The typeName can be any valid Go identifier that is unique to the enclosing package or function. The typeSpecification can be any built-in type (such as a string, int, slice, map, or channel), an interface (see §6.3, ➤ 265), a struct (seen in earlier chapters, with more coverage later in this one, §6.4, ➤ 275), or a function signature. In some situations creating a custom type is sufficient, while in others we need to add methods to the custom type to make it really useful. Here are some examples of methodless custom types. type Count int type StringMap map[string]string type FloatChan chan float64 ptg7913109 6.2. Custom Types 257 None of these custom typeslook particularly useful on their own,although using types like this can improve a program’s readability and can also make it possi- ble to change the underlying type later on, so they serve as a basic abstraction mechanism. var i Count = 7 i++ fmt.Println(i) sm := make(StringMap) sm["key1"]="value1" sm["key2"]="value2" fmt.Println(sm) fc := make(FloatChan, 1) fc <- 2.29558714939 fmt.Println(<-fc) 8 map[key2:value2 key1:value1] 2.29558714939 Types like Count, StringMap, and FloatChan that are based directly on a built-in type can be used in the same way as the built-in type—for example, we can use the built-in append() function on a custom type StringSlice []string type—but we must convert them (at no cost because it is done at compile time) to the un- derlying built-in type if we need to pass them to a function that expects their un- derlying type. And sometimes we need to do the reverse and promote the value of a built-in type to a custom type to benefit from the custom type’s methods. We saw an example of this when we converted a []string to a FoldedStrings value in the SortFoldedStrings() function (162 ➤ ). type RuneForRuneFunc func(rune) rune When working with higher order functions (§5.6.7, 238 ➤ ) it is often convenient to define custom types that specify the signatures of the functions we want to pass. Here we have specified the signature of a function that takes and returns a single rune. var removePunctuation RuneForRuneFunc The removePunctuation variable is created to refer to a function of type RuneFor- RuneFunc (i.e., with the signature func(rune) rune). Like all Go variables it is au- tomatically initialized to its zero value, so in this case to nil. phrases := []string{"Day; dusk, and night.","All day long"} removePunctuation = func(char rune) rune { ptg7913109 258 Chapter 6. Object-Oriented Programming if unicode.Is(unicode.Terminal_Punctuation, char) { return -1 } return char } processPhrases(phrases, removePunctuation) Here we have created an anonymous function that matches the RuneForRuneFunc signature and passed it to the custom processPhrases() function. func processPhrases(phrases []string, function RuneForRuneFunc) { for _, phrase := range phrases { fmt.Println(strings.Map(function, phrase)) } } Day dusk and night All day long Using RuneForRuneFunc as a type rather than the underlying func(rune) rune is more meaningful for human readers; it also provides some abstraction. (The strings.Map() function was covered in Chapter 3, 111 ➤ .) Creating custom types based on built-in types or function signatures can be useful, but doesn’t get us very far. What we need are custom methods, the subject of the next subsection. 6.2.1. Adding Methods A method is a special kind of function that is called on a value of a custom type and that is (usually) passed the value it is called on. The value is passed as a pointer or value depending on how the method is defined. The syntax for defin- ing methods is almost identical to that for defining functions, except that be- tween the func keyword and the method’s name we must write the receiver—this is written in parentheses, either as the type of value that the method belongs to or as a variable name and the type. When the method is called, the receiver’s variable (if present) is automatically set to the value or pointer that the method was called on. We can add one or more methods to any custom type. A method’s receiver is always a value of the type or a pointer to a value of the type. However, ev- ery method name must be unique for any given type. One consequence of the unique name requirement is that we cannot have two methods of the same name where one takes a pointer and the other a value. Another consequence is that there is no support for overloaded methods, that is, methods with the same name but different signatures. One way to provide the equivalent of overloaded ptg7913109 6.2. Custom Types 259 methods is to use variadic methods (i.e., methods that take a variable number of arguments; see §5.6, 219 ➤ and §, 221 ➤ ); however, the Go way is to use uniquely named functions. For example, the strings.Reader type provides three different read methods: strings.Reader.Read(), strings.Reader.ReadByte(), and strings.Reader.ReadRune(). type Count int func (count *Count) Increment() { *count++ } func (count *Count) Decrement() { *count-- } func (count Count) IsZero() bool { return count == 0 } This simple int-based custom type supports three methods, of which the first two are declared to accept pointer receivers since both modify the value they are called on.★ var count Count i := int(count) count.Increment() j := int(count) count.Decrement() k := int(count) fmt.Println(count, i, j, k, count.IsZero()) 0010true This snippet shows the Count type in action. It doesn’t look like much, but we will use it in this chapter’s fourth section (➤ 278). Let’s look at a slightly more elaborate custom type, this time based on a struct. (We will return to this example in §6.3.1, ➤ 270.) type Part struct { Id int // Named field (aggregation) Name string // Named field (aggregation) } func (part *Part) LowerCase() { part.Name = strings.ToLower(part.Name) } func (part *Part) UpperCase() { part.Name = strings.ToUpper(part.Name) } ★ In C++ and Java receivers are always called this and in Python self; in Go the practice is to give them more meaningful type-specific names. ptg7913109 260 Chapter 6. Object-Oriented Programming func (part Part) String() string { return fmt.Sprintf("«%d %q»", part.Id, part.Name) } func (part Part) HasPrefix(prefix string) bool { return strings.HasPrefix(part.Name, prefix) } We have made the String() and HasPrefix() methods accept a value receiver purely to show how it is done. Of course,methodsthat accept values rather than pointers cannot modify the value they are called on. part := Part{5,"wrench"} part.UpperCase() part.Id += 11 fmt.Println(part, part.HasPrefix("w")) «16 "WRENCH"» false When custom types are based on structs we can create values of them using the type name and a brace-enclosed list of initializing values. (And as we will see in the next subsection, Go also has a syntax whereby we can provide just those values we want to, and rely on Go to zero-initialize the rest.) Once the part value is created we can call methods on it (e.g., Part.UpperCase()), access its exported (public) fields (e.g., Part.Id), and print it, safe in the knowl- edge that Go’s print functions are intelligent enough to use the custom type’s String() method if it has one. A type’s method set is the set of all the methods that can be called on a value of the type. If we have a pointer to a value of a custom type, its method set consists of all the methods defined for the type—whether they accept a value or a pointer. If we call a method that takes a value, on a pointer, Go is smart enough to dereference the pointer and pass the underlying value as the method’s receiver. If we have a value of a custom type, its method set consists of all those methods defined for the type that accept a value receiver—but not those methods that accept a pointer receiver. This isn’t as limiting as it sounds, since if we have a value we can still call a method that has a pointer receiver and rely on Go to pass the value’s address—providing the value is addressable (i.e., it is a variable, a dereferenced pointer, an array or slice item, or an addressable field in a struct). So, given the call value.Method() where Method() requires a pointer and value is an addressable value, Go will treat the code as if we had written (&value).Method(). ptg7913109 6.2. Custom Types 261 The *Count type’s method set consists of the three methods, Increment(), Decre- ment(), and IsZero(); whereas the Count type’s method set is the single method, IsZero(). All these methods can be called on a *Count—and also on a Count value, providing that it is addressable, as we saw in the snippet shown earlier (259 ➤ ). The *Part type’s method set consists of the four methods, LowerCase(), Upper- Case(), String(),and HasPrefix();while the Part type’smethod set consistsof just the String() and HasPrefix() methods. However,the LowerCase() and UpperCase() methods can also be used on addressable Part values, as the earlier code snippet illustrates (260 ➤ ). Defining methods that accept value receivers works well for types that are small, such as numbers. Such methods cannot modify the value they are called on since they get a copy of the value as their receiver. If we have values of large types or values we want to modify we need to use methods that accept pointer receivers. This makes method calls as cheap as possible (since the receiver is passed as a 32- or 64-bit pointer, no matter how large the value the method is called on is). Overriding Methods As we will see later in this chapter, it is possible to create struct types that include one or more types (including interface types) as embedded fields (§6.4.1, ➤ 275). A particular convenience of this approach is that any methods on the embedded type can be called on the custom struct as if they were the struct’s own methods, and they will be passed the embedded field as their receiver. type Item struct { id string // Named field (aggregation) price float64 // Named field (aggregation) quantity int // Named field (aggregation) } func (item *Item) Cost() float64 { return item.price * float64(item.quantity) } type SpecialItem struct { Item // Anonymous field (embedding) catalogId int // Named field (aggregation) } Here, the SpecialItem embeds an Item type. This means that we can call the Item’s Cost() method on a SpecialItem. special := SpecialItem{Item{"Green", 3, 5}, 207} fmt.Println(, special.price, special.quantity, special.catalogId) fmt.Println(special.Cost()) ptg7913109 262 Chapter 6. Object-Oriented Programming Green 3 5 207 15 When we call special.Cost(), since the SpecialItem type does not have its own Cost() method, Go uses the Item.Cost() method—and passes it the embedded Item value, not the entire SpecialItem that the method was originally called on. As we will see later, if any of the embedded Item’s fields has the same name as one of the SpecialItem’s fields, we can still access the embedded Item’s fields by using the type as part of the name—for example, special.Item.price. It is possible to override the methods of an embedded field simply by creating a new method for the embedding struct that has the same name as one of the embedded fields’ methods. For example, suppose we have a new item type: type LuxuryItem struct { Item // Anonymous field (embedding) markup float64 // Named field (aggregation) } As it stands, if we call the Cost() method on a LuxuryItem, the embedded Item.Cost() method will be used, just like it is for SpecialItems. Here are three different implementations that override the embedded method (only one of which may be used, of course!). /* func (item *LuxuryItem) Cost() float64 {// Needlessly verbose! return item.Item.price * float64(item.Item.quantity) * item.markup } func (item *LuxuryItem) Cost() float64 {// Needless duplication! return item.price * float64(item.quantity) * item.markup } */ func (item *LuxuryItem) Cost() float64 {// Ideal ✓ return item.Item.Cost() * item.markup } The last implementation takes advantage of the embedded Cost() method. Of course, there is no requirement that overriding methods make any use of the embedded methods they override if we don’t want them to. (Embedding fields in structs is covered later; §6.4.1, ➤ 275.) ptg7913109 6.2. Custom Types 263 Method Expressions Just as we can assign and pass functions, we can also assign and pass method expressions.A method expression is a function that must be passed a value of the method’stype as its first argument. (The term unbound method is often used in other languages for this concept.) asStringV := Part.String // Effective signature: func(Part) string sv := asStringV(part) hasPrefix := Part.HasPrefix // Effective signature: func(Part, string) bool asStringP := (*Part).String // Effective signature: func(*Part) string sp := asStringP(&part) lower := (*Part).LowerCase // Effective signature: func(*Part) lower(&part) fmt.Println(sv, sp, hasPrefix(part, "w"), part) «16 "WRENCH"» «16 "WRENCH"» true «16 "wrench"» Here we have created four method expressions: asStringV() which takes a Part value as its sole argument, hasPrefix() which takes a Part as its first argument and a string as its second argument, and asStringP() and lower() which both take a *Part as their sole argument. Method expressions are an advanced feature which can be very useful on the uncommon occasions when they are needed. All the custom types we have created so far potentially suffer from a critical flaw. None of them provide any means of ensuring that the data they are initialized with is valid (or is forced to be valid), nor any means of ensuring that the type’s data (or fields in the case of struct types) cannot be given invalid data. For ex- ample, the Part.Id and Part.Name fieldscan be set to any valueswe like. But what if we wanted to apply constraints—for example, only allow IDs to be positive integers greater than zero, or only allow names that had a specific format? We will address these issues in the next subsection where we will create a small but complete custom type with validated fields. 6.2.2. Validated Types For many simple custom types, no validation is necessary. For example, we might have type Point { X, Y int }, for which any values of X and Y are valid. Furthermore, since Go guarantees to initialize all variables (including struct fields) to their zero values, the need for explicit constructors is reduced. For those situations where the default zero-value initialization isn’t sufficient, we can create a construction function. Go doesn’t support constructors, so we must call construction functions explicitly. To support this we must document ptg7913109 264 Chapter 6. Object-Oriented Programming the type as having an invalid zero value and provide one or more construction functions that can be used to create valid values. We can use a similar approach when it comes to fields that must be validated. We can make such fields unexported and provide exported accessor methods in which we can put the necessary validation.★ Let’s look at a small but complete custom type that illustrates these points. type Place struct { latitude, longitude float64 Name string } func New(latitude, longitude float64, name string) *Place { return &Place{saneAngle(0, latitude), saneAngle(0, longitude), name} } func (place *Place) Latitude() float64 { return place.latitude } func (place *Place) SetLatitude(latitude float64) { place.latitude = saneAngle(place.latitude, latitude) } func (place *Place) Longitude() float64 { return place.longitude } func (place *Place) SetLongitude(longitude float64) { place.longitude = saneAngle(place.longitude, longitude) } func (place *Place) String() string { return fmt.Sprintf("(%.3f°, %.3f°) %q", place.latitude, place.longitude, place.Name) } func (original *Place) Copy() *Place { return &Place{original.latitude, original.longitude, original.Name} } The Place type is exported (from the place package), but its latitude and longi- tude fields are unexported because they require validation. We have provided a construction function, New(), to ensure that we always create valid *place.Places. It is a Go convention to call construction functions New(), or if there is more than one, for their names to begin with “New”. (We haven’t shown the saneAngle() function since it is beside the point—it takes the old angle and the new angle and returns the new if the new is in range; otherwise it returns the old.) And ★ Recall that in Go, identifiers are unexported (i.e., only visible within the package in which they are defined) if they start with a lowercase letter, and identifiers are exported (i.e., visible in any package that imports the package in which they are defined) if they start with an uppercase letter. ptg7913109 6.2. Custom Types 265 by providing getters and setters for the unexported fields, we have ensured that only valid values can be set. The String() method means that *Place values fulfill the fmt.Stringer interface, so *Places will print the way we want rather than using Go’s default formatting. We have also provided a Copy() method—and not used any validation for it since we know that the original is already valid. newYork := place.New(40.716667,-74,"New York")// newYork is a *Place fmt.Println(newYork) baltimore := newYork.Copy() // baltimore is a *Place baltimore.SetLatitude(newYork.Latitude() - 1.43333) baltimore.SetLongitude(newYork.Longitude() - 2.61667) baltimore.Name = "Baltimore" fmt.Println(baltimore) (40.717°, -74.000°) "New York" (39.283°, -76.617°) "Baltimore" We put the Place type in the place package and call place.New() to create *Places. Once we have a *Place we can call its methods in the same way as we do for any of the standard library’s custom types. 6.3. Interfaces In Go an interface is a custom type that specifies a set of one or more method signatures. Interfaces are wholly abstract, so it is not possible to instantiate an interface. However, it is possible to create a variable whose type is that of an interface—and which can then be assigned a value of any concrete type that has the methods the interface requires. The interface{} type is the interface that specifies the empty set of methods. Every value satisfies the interface{} type whether the value has methods or not—after all, if a value does have methods,its set of methods includes the emp- ty set of methods as well as the methods it actually has. This is why the inter- face{} type can be used for anyvalue. We cannot call methodsdirectly on a value passed as an interface{} (even if it has some), since the interface it is fulfilling has no methods. So, in general, it is better to pass values either as their actual type or as an interface that hasthe methodsthat we want to use. Of course,if we do use the interface{} type for values with methods, we can access those meth- ods by using a type assertion (§5.1.2, 191 ➤ ) or a type switch (§, 197 ➤ ), or even by using introspection (§9.4.9, ➤ 427). Here is a very simple interface. ptg7913109 266 Chapter 6. Object-Oriented Programming type Exchanger interface { Exchange() } The Exchanger interface specifies a single method, Exchange(), which takes no ar- guments and returns nothing. We have followed the Go convention for interface names, which is that they should end with er. It is quite common to have inter- faces with only one method—for example, the standard library’s io.Reader and io.Writer interfaces each specify a single method. Notice that an interface is re- ally specifying an API(ApplicationProgramming Interface),that is,zero or more methods—although it says nothing about what those methods actually do. A nonempty interface on its own is not usually of any use; to make it useful we must create some custom types that have the methods the interface requires.★ Here are two such types. type StringPair struct{ first, second string } func (pair *StringPair) Exchange() { pair.first, pair.second = pair.second, pair.first } type Point [2]int func (point *Point) Exchange() { point[0], point[1] = point[1], point[0]} The custom StringPair and Point types are completely different, but since both provide an Exchange() method, both satisfy the Exchanger interface. This means that we can create StringPair and Point values and pass them to functions that accept Exchangers. Notice that although both the StringPair and Point types fulfill the Exchanger interface, nowhere have we said this explicitly—there are no “implements” or “inherits” statements. The mere fact that the StringPair and Point types provide the methods (in this case just one method) that the interface specifies, is sufficient for Go to know that they satisfy the interface. The methods’ receivers are specified as pointers to their types so that we can change the (pointed to) values the methods are called on. Although Go is smart enough to print custom types in a sensible way, we usually prefer to take control over their string representation. This can easily be done by adding a method that fulfills the fmt.Stringer interface, that is, a method with the signature String() string. ★ If we were creating a framework we might create interfaces but no types that implement them, and require users of the framework to create such types for use with the framework. ptg7913109 6.3. Interfaces 267 func (pair StringPair) String() string { return fmt.Sprintf("%q+%q", pair.first, pair.second) } This method returnsa string that consistsof each of the stringsin double quotes with a “+” sign between them. With this method defined, Go’s fmt package’s print methods will use it to print StringPair values—and also *StringPairs, since Go will dereference such pointers to get the pointed-to value. Here is a code snippet that shows the creation of some custom Exchanger values, some calls to the Exchange() method, and a call to a custom exchangeThese() function that accepts Exchanger values. jekyll := StringPair{"Henry","Jekyll"} hyde := StringPair{"Edward","Hyde"} point := Point{5,-3} fmt.Println("Before: ", jekyll, hyde, point) jekyll.Exchange() // Treated as: (&jekyll).Exchange() hyde.Exchange() // Treated as: (&hyde).Exchange() point.Exchange() // Treated as: (&point).Exchange() fmt.Println("After #1:", jekyll, hyde, point) exchangeThese(&jekyll, &hyde, &point) fmt.Println("After #2:", jekyll, hyde, point) Before: "Henry"+"Jekyll" "Edward"+"Hyde" [5 -3] After #1: "Jekyll"+"Henry" "Hyde"+"Edward" [-3 5] After #2: "Henry"+"Jekyll" "Edward"+"Hyde" [5 -3] All the variables are created as values, yet the Exchange() methods require pointer receivers. This isn’t a problem since, as we noted earlier, Go is smart enough to pass a value’s address when we call a method on it that requires a pointer—providing the value is addressable. So, in the snippet, jekyll.Ex- change() is automatically treated as if it were written (&jekyll).Exchange(), and the same for the others. In the call to the exchangeThese() function,we must passthe values’ addressesex- plicitly. If, for instance, we passed the hyde value of type StringPair, the Go com- piler would notice that StringPairs do not fulfill the Exchanger interface—since there is no Exchange() method with a StringPair receiver—and would stop com- piling and report the problem. However, if we pass a *StringPair (e.g., &hyde), the compilation will complete successfully. This works because there is an Ex- change() method which takes a *StringPair receiver which means that *String- Pairs fulfill the Exchanger interface. Here is the exchangeThese() function. ptg7913109 268 Chapter 6. Object-Oriented Programming func exchangeThese(exchangers ...Exchanger) { for _, exchanger := range exchangers { exchanger.Exchange() } } This function doesn’t know or care that we passed it two *StringPairs and one *Point;all that it requiresis that the parametersit is passed are Exchangers—and this requirement is enforced by the compiler, hence the duck typing used here is type-safe. In addition to satisfying our own custom interfaces, we can satisfy those in the standard library or any others we want, as we saw when we defined the String- Pair.String() method to fulfill the fmt.Stringer interface. Another example is the io.Reader interface which specifies a single method with the signature Read([]byte) (int, error) that when called writes the data of the value it is called on to the given []byte.Thewriting is destructive,that is,each byte written is removed from the called-on value. func (pair *StringPair) Read(data []byte) (n int, err error){ if pair.first == "" && pair.second == "" { return 0, io.EOF } if pair.first != "" { n = copy(data, pair.first) pair.first = pair.first[n:] } if n c:", a.Less(c), a.Equal(c), c.Less(a)) fmt.Println("Bool: ", a.Bool(), b.Bool(), c.Bool(), d.Bool()) fmt.Println("Float: ", a.Float(), b.Float(), c.Float(), d.Float()) } Original: 0% 25% 75% 100% Not: 100% 75% 25% 0% Not Not: 0% 25% 75% 100% 0.And(.25)→0% .25.And(.75)→25% .75.And(1)→75% 0.And(.25,.75,1)→0% 0.Or(.25)→25% .25.Or(.75)→75% .75.Or(1)→100% 0.Or(.25,.75,1)→100% a < c, a == c, a > c: true false false Bool: false false true true Float: 0 0.25 0.75 1 [0% 25% 75% 100%] The custom type is called FuzzyBool. We will start by looking at the type defini- tion, and then the construction function. And we will finish up by looking at the methods. type FuzzyBool struct{ value float32 } The FuzzyBool type is based on a struct that contains a single float32. The value is unexported so anyone who imports the fuzzybool package must use the construction function (which we have called New(), in accordance with Go ptg7913109 284 Chapter 6. Object-Oriented Programming convention),to create fuzzy Booleans. This means,of course,that we can ensure that only valid fuzzy Booleans are created. Since the FuzzyBool type is based on a struct that contains a value whose type is unique inside the struct, we could simplify the type to type FuzzyBool struct { float32 }. This would mean changing the code that accesses the value—and which we will see in some of the following methods—from fuzzy.value to fuz- zy.float32. We preferred to use a named variable partly because we found it more æsthetically pleasing, and partly because if we wanted to change the un- derlying type (to, say, float64), we would have far fewer changes to make. Further variationsare also possible since the struct contains only a single value. For example, we could change the type to type FuzzyBool float32 to make it based directly on a float32. Doing this works perfectly well, but requires slightly more code and is a bit trickier to implement than the struct-based approach used here. However, if we constrain ourselves to creating immutable fuzzy Booleans (where the only difference is that instead of setting a new value with the Set() method we would have to assign a new fuzzy Boolean), we can greatly simplify the code by basing the type directly on a float32. func New(value interface{}) (*FuzzyBool, error){ amount, err := float32ForValue(value) return &FuzzyBool{amount}, err } For the convenience of fuzzy Boolean users, rather than accept only a float32 as the initializing value we also accept float64s (Go’s default floating-point type), ints (the default integer type), and bools. This flexibility is achieved by using a custom float32ForValue() function which returns a float32 and nil for the given value—or which returns 0.0 and an error if the value is not of a type it can handle. If we passed a value of an invalid type we have made a programming error and want to know about it straight away. But we don’t want the application to crash on our users. So, instead of just returning a *FuzzyBool, we also return an error. If we pass a known valid literal to New() (as in the earlier snippet; 283 ➤ ), we can safely ignore the error; but if we pass a variable, we should check the error that is returned in case it isn’t nil. The New() function returns a pointer to a FuzzyBool rather than a value because we have chosen to make the fuzzy Booleans in this implementation mutable. This means that those methods which modify a fuzzy Boolean (in this exam- ple, just one, Set()) must take a pointer receiver, not a value.★ ★ In fact, we could return a FuzzyBool value and still have a mutable type, as the book’s source code’s fuzzy_value example illustrates. ptg7913109 6.5. Examples 285 A reasonable rule of thumb is to create methods that take receiver values for immutable types, and methods that take pointers for mutable types. (For mutable types, having some methods take values and others take pointers is perfectly possible—but can be inconvenient in practice.) It is also best to use pointers for large struct types (e.g., those with two or more fields), to keep the cost of passing them to a single pointer. func float32ForValue(value interface{}) (fuzzy float32, err error){ switch value := value.(type){// shadow variable case float32: fuzzy = value case float64: fuzzy = float32(value) case int: fuzzy = float32(value) case bool: fuzzy = 0 if value { fuzzy = 1 } default: return 0, fmt.Errorf("float32ForValue(): %v is not a "+ "number or Boolean", value) } if fuzzy < 0 { fuzzy = 0 } else if fuzzy > 1 { fuzzy = 1 } return fuzzy, nil } This unexported helper function is used by the New() and Set() methods to convert a value into a float32 in the range [0.0,1.0].Handling the different types is easily accomplished by using a type switch (§, 197 ➤ ). If the function is called with a value of an invalid type we return a non-nil error. This gives the caller responsibility for checking the return value and responding appropriately if an error has occurred. The caller could panic on error and cause the application to crash with a traceback,or it could handle the problem itself. It is often better for low-level functions like this to return an error when they hit a problem because they don’t know enough about the program’s logic to know how or whether to handle it, and to push the problem up to the caller which should be in a better position to know what to do. ptg7913109 286 Chapter 6. Object-Oriented Programming While we consider passing in a value of an invalid type to be a programming error and therefore worthy of returning a non-nil error, we have chosen to treat out-of-range values much more leniently, and simply clamp them to the nearest valid value. func (fuzzy *FuzzyBool) String() string { return fmt.Sprintf("%.0f%%", 100*fuzzy.value) } This method satisfies the fmt.Stringer interface. This means that fuzzy Booleans will be printed as we have specified, and that a fuzzy Boolean can be passed wherever an fmt.Stringer value is expected. We have chosen the string representation of fuzzy Booleans to be whole number percentages. (Recall that the "%.0f" string format specifies a floating-point number with no decimal point or decimal digits and the "%%" format specifies a literal % character. String formatting was covered earlier; §3.5, 93 ➤ .) func (fuzzy *FuzzyBool) Set(value interface{}) (err error){ fuzzy.value, err = float32ForValue(value) return err } The presence of this method is what makes our fuzzy Booleans mutable. The method is very similar to the New() function, only here we work on an existing *FuzzyBool rather than creating a new one. If the returned error is not nil then the fuzzy Boolean’s value will be invalid, so we expect our callers to check the return value. func (fuzzy *FuzzyBool) Copy() *FuzzyBool { return &FuzzyBool{fuzzy.value} } For custom types that are passed around as pointers, it is often convenient to provide a Copy() method. Here, we simply create a new FuzzyBool, with the same value as that of the receiver, and return a pointer to it. There is no need to do any validation since we know that the receiver’s value is valid. This assumes, of course, that the original value had a nil error when it was created with New(), and similarly for any subsequent calls to the Set() method. func (fuzzy *FuzzyBool) Not() *FuzzyBool { return &FuzzyBool{1 - fuzzy.value} } This is the first of the logical operator methods, and like all the methods, it works on a *FuzzyBool receiver. ptg7913109 6.5. Examples 287 There are three sensible ways we could have designed this method. One way would be to have it change the value it is called on and return nothing. Another way is to change the value and also return the value—this is the approach taken by many of the big.Int and big.Rat types’ methods in the standard library. This approach means that operations can be chained (e.g., b.Not().Not()). It can also save memory (since values are reused rather than created), but can easily catch us off guard when we forget that the returned value is the same as the one the method is called on—and that it has been changed. The other way is to do what we have done here: Leave the value untouched and return a new fuzzy Boolean whose value is the result of the logical operation. This is easy to understand and use, and supports chaining—but at the price of creating more values. We have taken this last approach with all the fuzzy Boolean logical operator methods. Incidentally, the fuzzy NOT logic is simple, returning 1.0 for 0.0 and 0.0 for 1.0, 0.75 for 0.25 and 0.25 for 0.75, 0.5 for 0.5, and so on. func (fuzzy *FuzzyBool) And(first *FuzzyBool, rest ...*FuzzyBool) *FuzzyBool { minimum := fuzzy.value rest = append(rest, first) for _, other := range rest { if minimum > other.value { minimum = other.value } } return &FuzzyBool{minimum} } The logic for fuzzy AND is to return the minimum of the given fuzzy values. The method’s signature guarantees that when the method is called it will be given at least one other *FuzzyBool to work with (first), and will accept zero or more additional ones (rest). The method simply appends the first onto the (possibly empty) rest slice, and then iterates over the slice setting the minimum if its value is bigger than the iteration’scurrent other value. And just like the Not() method, we return a new *FuzzyBool, and leave the fuzzy Boolean the method was called on unchanged. The logic for fuzzy OR is to return the maximum of the given fuzzy values. We have not shown the Or() method since it is structurally identical to the And() method. The only differences are that the Or() method uses a maximum variable instead of a minimum variable and compares using the < less than operator instead of the > greater than operator. func (fuzzy *FuzzyBool) Less(other *FuzzyBool) bool { return fuzzy.value < other.value } ptg7913109 288 Chapter 6. Object-Oriented Programming func (fuzzy *FuzzyBool) Equal(other *FuzzyBool) bool { return fuzzy.value == other.value } These two methods allow us to compare fuzzy Booleans in terms of the float32s they contain; both methods produce bool results. func (fuzzy *FuzzyBool) Bool() bool { return fuzzy.value >= .5 } func (fuzzy *FuzzyBool) Float() float64 { return float64(fuzzy.value) } The fuzzybool.New() construction function can be seen as a conversion function since given a float32, float64, int, or bool, it produces a *FuzzyBool. These two methods perform similar conversions going the other way. The FuzzyBool type provides a complete fuzzy Boolean data type that can be used like any other custom data type. So, *FuzzyBools may be stored in slices, and in maps as keys or values or both. Of course, if we use *FuzzyBools as map keys we will be able to store multiple fuzzy Booleans even if they have the same value because they will each have a unique address. One solution is to use a value-based fuzzy Boolean (such as the fuzzy_value example in the book’s source code). Alternatively, we could use a custom collection type that stored pointers but which uses their values for comparisons: The custom omap.Map type can do this, providing we supply a suitable less than function (§6.5.3, ➤ 302). In addition to the fuzzy Boolean type shown in this subsection, the book’s exam- ples also include three alternative fuzzy Boolean implementations for compar- ison: These are not shown in the book or discussed beyond this paragraph. The first two alternatives are in the files fuzzy_value/fuzzybool/fuzzybool.go and fuzzy_mutable/fuzzybool/fuzzybool.go—these have exactly the same function- ality as the version described in this subsection (in file fuzzy/fuzzybool/fuzzy- bool.go).The fuzzy_value version works in terms of FuzzyBool values rather than *FuzzyBools, and the fuzzy_mutable version is based directly on a float32 rather than on a struct. The fuzzy_mutable’s code is slightly longer and trickier than the struct-based version shown here. The third alternative provides slightly less functionality because it provides an immutable fuzzy Boolean type. Again, it is directly based on a float32; this version is in file fuzzy_immutable/fuzzy- bool/fuzzybool.go. It is the simplest of the three implementations. ptg7913109 6.5. Examples 289 6.5.2. Example: Shapes—A Family of Custom Types When we have a set of related classes—such as shapes—upon which we might want to apply some generic operations (e.g., asking a shape to draw itself), there are two broad approacheswe can take to implementing them. The one most like- ly to be familiar to C++, Java, and Python programmersis to use a hierarchy—in Go’s case,of embedded interfaces. However,it is often more convenient and ver- satile to create independent interfaces that can be freely composed. In this sub- section we will show both approaches, the first in file shaper1/ shapes/shapes.go and the second in file shaper2/shapes/shapes.go.(Note that when the packages’ types, functions, and methods are the same—which most of them are—we will simply refer to the “shapes package”. Naturally, we will distinguish them as the “shaper1 shapes package” or the “shaper2 shapes package” when discussing code that is specific to one of them.) Figure 6.3 shows an example of what the shapes package can do—in this case it is used to create a white rectangle and to draw on it a circle and some polygons with different numbers of sides and using various colors. Figure 6.3 The shaper example’s shapes.png file The shapes package provides three exported functions for working with images and three types for creating shapes values—two of which are exported. The hierarchical shapes1 shapes package provides three exported interfaces and the compositional shapes2 shapes package provides five exported interfaces. We will begin with the image-related code—the convenience functions—then we will look at the interfaces (in two separate subsubsections), and finally, we will review the concrete shape-related code. Package-Level Convenience Functions The standard library’s image package providesthe image.Image interface.Thisin- terface specifies three methods: image.Image.ColorModel() to return the image’s color model (as a color.Model), image.Image.Bounds() to return the image’s bound- ing box (as an image.Rectangle), and image.Image.At(x, y) which returns the col- or.Color value for the given pixel. Notice that there is no image.Image method for setting a pixel—even though several image types provide a Set(x, y int, fill ptg7913109 290 Chapter 6. Object-Oriented Programming color.Color) method. However, the image/draw package provides the draw.Image interface which embeds the image.Image interface and also has a Set() method. The draw.Image interface is fulfilled by the standard library’s image.Gray and im- age.RGBA types, among others. func FilledImage(width, height int, fill color.Color) draw.Image { if fill == nil {// We silently treat a nil color as black fill = color.Black } width = saneLength(width) height = saneLength(height) img := image.NewRGBA(image.Rect(0, 0, width, height)) draw.Draw(img, img.Bounds(), &image.Uniform{fill}, image.ZP, draw.Src) return img } This is an exported convenience function that creates an image of the given size uniformly filled with the given color. We begin by silently replacing a nil color with black, and ensuring that both dimensions are sensible. Then we create an image.RGBA value (an image that defines its colors using red, green, blue, and alpha—transparency—values), and return it as a draw.Image, since we are only concerned with what we can do with it, not with what its actual type happens to be. The draw.Draw() function takes a destination image (of type draw.Image), a rect- angle that specifies where the drawing should be done (in this case the entire destination image), a source image to copy from (in this case an infinite-sized image filled with the given color), the point where the destination rectangle to be drawn in should go (image.ZP is the zero point, i.e., point (0, 0)), and how the drawing should be done. Here we have specified draw.Src so the function will simply copy from the source onto the destination. So, the effect we achieve here is to copy the given color to every pixel in the target image. (The draw package also has a draw.DrawMask() function that supports some of the Porter-Duff com- positing operations.) var saneLength, saneRadius, saneSides func(int) int func init() { saneLength = makeBoundedIntFunc(1, 4096) saneRadius = makeBoundedIntFunc(1, 1024) saneSides = makeBoundedIntFunc(3, 60) } We have defined three unexported variables to hold helper functionsall of which take an int and return an int.And we have given the package an init() function in which the variables are assigned suitable anonymous functions. ptg7913109 6.5. Examples 291 func makeBoundedIntFunc(minimum, maximum int) func(int) int { return func(x int) int { valid := x switch { case x < minimum: valid = minimum case x > maximum: valid = maximum } if valid != x { log.Printf("%s(): replaced %d with %d\n", caller(1), x, valid) } return valid } } This function returns a function that given a value x, returns x if it is between the minimum and maximum (inclusive), or returns the closest bounding value. If x is out of range, in addition to returning a valid alternative, we also log the problem. However, we don’t want to report the problem as existing in the function created here (i.e., in saneLength(), saneRadius(),orsaneSides()), because the problem belongs to their caller. So, instead of logging the name of the function created here, we log the name of the function’s caller using a custom caller() function. func caller(steps int) string { name := "?" if pc, _, _, ok := runtime.Caller(steps + 1); ok { name = filepath.Base(runtime.FuncForPC(pc).Name()) } return name } The runtime.Caller() function returnsinformation about the functionsthat have been called and not yet returned in the current goroutine. The int argument says how far (i.e., how many functions) back we want to look. An argument of 0 looks at the current function (i.e., this function, shapes.caller()), and an ar- gument of 1 looks at this function’s caller, and so on. We add the 1 so as to start from this function’s caller. We are given four pieces of information by the runtime.Caller() function: the program counter (which we have stored in variable pc), the filename and line number at which the call took place (both of which we have ignored by using blank identifiers), and a Boolean flag (which we have stored in the ok variable) that reports on whether the information could be retrieved. ptg7913109 292 Chapter 6. Object-Oriented Programming If we successfully retrieve the program counter we then call the runtime.Func- ForPC() function which returns a *runtime.Func value and on which we call the runtime.Func.Name() method to retrieve the name of the called function. The name is returned like a file path, for example, /home/mark/goeg/src/shaper1/ shapes.FilledRectangle for a function, or /home/mark/goeg/src/shaper1/shapes. *shape•SetFill for a method. The path is unnecessary for a small project, so we have stripped it off using the filepath.Base() function. We then return the name. For example, if we called the shapes.FilledImage() function and passed it an out-of-range width or height such as 5000, the problem will be corrected in the saneLength() function. In addition, since a problem occurred, a log output will be produced, in this case saying "shapes.FilledRectangle(): replaced 5000 with 4096". This works because the saneLength() function calls caller() with an argument of 1, to which caller() adds 1 to produce 2,socaller() goes three functions up—itself (0), saneLength() (1), and FilledImage() (2). func DrawShapes(img draw.Image, x, y int, shapes ...Shaper) error { for _, shape := range shapes { if err := shape.Draw(img, x, y); err != nil { return err } } return nil } This is another exported convenience function, and the only one to differ be- tween the two shapes package implementations. The one shown here is from the hierarchical shapes1 shapes package. The compositional shapes2 shapes pack- age differs only in the function’s signature where it accepts Drawers, that is, val- ues which satisfy the Drawer interface (i.e., have a Draw() method), rather than Shapers which must have Draw(), Fill(), and SetFill() methods. So, in this case, the compositional approach means that we use a more specific and less demand- ing argument type (Drawer) than the hierarchical approach requires (Shaper). We will review these interfaces in the next two subsubsections. The body and behavior of the function is the same in both cases. The function takes a draw.Image to draw on, a position (as an x, y coordinate), and zero or more Shaper (or Drawer) values. Inside the loop each shape is told to draw itself on the image at the given position. The x, y coordinates are checked in the lower-level shape-specific Draw() methods and if they are invalid we will get a non-nil error which we immediately return to the caller. For Figure 6.3 (289 ➤ ), we used a modified version of this function that draws each shape three times, once at the given x, y position, once offset by one pixel to the right, and once offset one pixel down. This was done to make the lines thicker for the screenshot. ptg7913109 6.5. Examples 293 func SaveImage(img image.Image, filename string) error { file, err := os.Create(filename) if err != nil { return err } defer file.Close() switch strings.ToLower(filepath.Ext(filename)) { case ".jpg",".jpeg": return jpeg.Encode(file, img, nil) case ".png": return png.Encode(file, img) } return fmt.Errorf("shapes.SaveImage(): '%s' has an unrecognized "+ "suffix", filename) } This is the last of the exported convenience functions. Given an image that fulfills the image.Image interface—which includes any that fulfill the draw.Image interface since that embeds an image.Image—the function attempts to save the image to a file with the given name. If the os.Create() call fails (e.g., due to an empty filename or an I/O error), or if the filename has an unrecognized suffix, or if the image encoding fails, the function will return a non-nil error. At the time of this writing, Go’s standard library had support for reading and writing two image file formats: .png (Portable Network Graphics) and .jpg (Joint Photographic Experts Group). Packages that support additional image formats are available from The jpeg.Encode() function has an extra parameter which can be used to fine-tune how the image is saved—we have passed nil which means that the default settings will be used. These encoders can panic—for example, if passed a nil image.Image—so if we want to protect the program against our own mistakes we would need to put a deferred function with a call to recover(), either in this function or farther up the call chain (see §5.5.1, 213 ➤ ). We have opted not to add such protection since the test suite (not shown) has sufficient calls of the function for there to be confidence that such a programming error would immediately be triggered and lead to a termination—and therefore would be almost impossible to miss. Given the draw.Image interface we can specify image values whose individual pixels can be set to colors of our choice. And with the DrawShapes() function we can draw shapes (that fulfill the Shaper—or Drawer—interface) on such an image. We can save images to disk using the SaveImage() function. With these convenience functions in place, what we need is to create the interfaces (e.g., Shaper or Drawer, etc.), and the concrete types and their methods to satisfy the interfaces. ptg7913109 294 Chapter 6. Object-Oriented Programming A Hierarchy of Embedded Interfaces Programmers with a conventional inheritance-based object-oriented back- ground are likely to use Go’s ability to embed interfaces to create a hierarchy of interfaces. The recommended way is to use the compositional approach which we will cover in the next subsubsection. Here are the interfaces used in the hi- erarchical shapes1 shapes package. type Shaper interface { Fill() color.Color SetFill(fill color.Color) Draw(img draw.Image, x, y int) error } type CircularShaper interface { Shaper // Fill(); SetFill(); Draw() Radius() int SetRadius(radius int) } type RegularPolygonalShaper interface { CircularShaper // Fill(); SetFill(); Draw(); Radius(); SetRadius() Sides() int SetSides(sides int) } We have created a hierarchy—by embedding, not inheritance—of three inter- faces to specify the methods we want our shape values to have. The Shaper interface defines methods to get and set a fill color of type color. Color and a method for drawing itself onto a draw.Image at a given position. The CircularShaper interface embeds an anonymous Shaper and adds a getter and setter for a radius of type int. Similarly, the RegularPolygonalShaper interface embeds an anonymous CircularShaper (and therefore also a Shaper) and adds a getter and setter for a number of sides of type int. Although creating hierarchies like this may be familiar—and works—it is probably not the best way to do things in Go. This is because it locks us into a hierarchy when we don’t need a hierarchy at all: What we really need is to just say that this particular shape supports some relevant interfaces. This gives us much more flexibility, as we will see in the next subsubsection. Freely Composable Independent Interfaces For the shapes we mostly want to say what specific things they can do (draw, get/set a fill color, get/set a radius, etc.), with a little bit of genericity. Here are the interfaces in the compositional shapes2 shapes package. ptg7913109 6.5. Examples 295 type Shaper interface { Drawer // Draw() Filler // Fill(); SetFill() } type Drawer interface { Draw(img draw.Image, x, y int) error } type Filler interface { Fill() color.Color SetFill(fill color.Color) } type Radiuser interface { Radius() int SetRadius(radius int) } type Sideser interface { Sides() int SetSides(sides int) } This package’s Shaper interface is a convenient way of specifying shapes generi- cally, that is, shapes which can be drawn and which can get/set a fill color. Each of the other interfaces specifies one very specific behavior (counting getting and setting as one). Having many independent interfaces is much more flexible than using a hierar- chy. For example, we were able to be much more specific about the shapespassed to the DrawShapes() function than was possible using a hierarchy (292 ➤ ). Also, with no hierarchy to preserve we can add other interfacesmuch more freely,and, of course, with these more fine-grained interfaces we can compose with them much more readily—as we did to create the Shaper interface. The two different versions of the shapes package have completely different interfaces (although both have a Shaper interface, its body differs between them).Nonetheless, since interfaces and concrete types are completely separate and independent, these differences have no effect whatsoever on the concrete implementations that fulfill them. Concrete Types and Methods This is the last subsubsection covering the shapes package; it is here that we will review the concrete implementationsthat satisfy the interfaces described in the previous two subsubsections. ptg7913109 296 Chapter 6. Object-Oriented Programming type shape struct{ fill color.Color } func newShape(fill color.Color) shape { if fill == nil {// We silently treat a nil color as black fill = color.Black } return shape{fill} } func (shape shape) Fill() color.Color { return shape.fill } func (shape *shape) SetFill(fill color.Color) { if fill == nil {// We silently treat a nil color as black fill = color.Black } shape.fill = fill } This simple type is unexported so it is only accessible inside the shapes package. This means that no shape value can be created outside the package. In the hierarchical shaper1 shapes package this type does not fulfill any of the interfaces because it does not have a Draw() method. But in the compositional shaper2 shapes package it fulfills the Filler interface. In the code as it stands, only the Circle type (which we will cover in a moment) directly embedsa shape.So,in theory,we could have incorporated the color.Color value into the Circle type and made the color getter and setter take *Circles instead of shapes, and eliminated the shape type altogether. However, we prefer to keep the shape type because it gives us the flexibility to add additional shape interfaces and types that might be based directly on the shape type (i.e., to have a color), rather than on the Circle type (e.g., because they don’t have a notion of radius). This flexibility will come in handy in one of the exercises. type Circle struct { shape radius int } func NewCircle(fill color.Color, radius int) *Circle { return &Circle{newShape(fill), saneRadius(radius)} } func (circle *Circle) Radius() int { return circle.radius } func (circle *Circle) SetRadius(radius int) { circle.radius = saneRadius(radius) ptg7913109 6.5. Examples 297 } func (circle *Circle) Draw(img draw.Image, x, y int) error { // ... ~30 lines elided ... } func (circle *Circle) String() string { return fmt.Sprintf("circle(fill=%v, radius=%d)", circle.fill, circle.radius) } This is the complete implementation of the Circle type. Although we can create concrete *Circle values, we can pass them around as interfaces, which gives us a lot of flexibility. For example, the DrawShapes() function (292 ➤ ) accepts Shapers (or Drawers), no matter what their underlying concrete type is. In the hierarchical shaper1 shapes package this type satisfies the CircularShaper and Shaper interfaces. In the compositional shaper2 shapes package it satisfies the Filler, Radiuser, Drawer, and Shaper interfaces. And in both cases the type also satisfies the fmt.Stringer interface. Since Go doesn’t have constructors and we have unexported fields, we must provide construction functions which must be called explicitly. The Circle’s construction function is NewCircle(); later on we will see that the package also has a New() function that can create a value of any of the package’s shapes. The saneRadius() helper function returns the given int if it is in a specified range or a sensible value otherwise; we saw it created earlier (290 ➤ ). The code for the Draw() method has been elided (although it is in the book’s source code),since our concern in this chapter is with creating custom interfaces and types rather than with graphics. type RegularPolygon struct { *Circle sides int } func NewRegularPolygon(fill color.Color, radius, sides int) *RegularPolygon { return &RegularPolygon{NewCircle(fill, radius), saneSides(sides)} } func (polygon *RegularPolygon) Sides() int { return polygon.sides } func (polygon *RegularPolygon) SetSides(sides int) { polygon.sides = saneSides(sides) } ptg7913109 298 Chapter 6. Object-Oriented Programming func (polygon *RegularPolygon) Draw(img draw.Image, x, y int) error { // ... ~55 lines including two helper functions elided ... } func (polygon *RegularPolygon) String() string { return fmt.Sprintf("polygon(fill=%v, radius=%d, sides=%d)", polygon.Fill(), polygon.Radius(), polygon.sides) } Here is the complete implementation of the RegularPolygon type used to provide regular polygon shapes. This type is very similar to the Circle type, only it has a more complicated Draw() method (whose body has been elided). Since the Reg- ularPolygon embeds a *Circle we have populated that field using the NewCircle() function (which handles the validation). The saneSides() helper function is just like the saneRadius() and saneLength() functions (290 ➤ ). In the hierarchical shaper1 shapes package this type satisfies the RegularPolyg- onalShaper, CircularShaper, Shaper, and fmt.Stringer interfaces. In the compo- sitional shaper2 shapes package it satisfies the Filler, Radiuser, Sideser, Drawer, Shaper, and fmt.Stringer interfaces. The NewCircle() and NewRegularPolygon() functions allow us to create *Circle and *RegularPolygon values, and since their types fulfill the Shaper and other interfaces, we can pass them around as Shapers or as values of any of the other interfaces that they fulfill. We can call any Shaper method on such values (i.e., Fill(), SetFill(), and Draw()). And if we want to call a non-Shaper method on a Shaper value, we can do so using a type assertion or type switch to access the value as a value of the interface whose method we want to call. We will see an example when we look at the showShapeDetails() functions (➤ 300). It is easy to imagine that we could create many other shape types, some building on shape, others on Circle or RegularPolygon.Furthermore,there may be situations when we would like to create shapes whose shape was determined at runtime, for example, by using a shape name. For this purpose we can create a factory function, that is, a function which returns shape type values where the specific type of the value returned depends upon an argument. type Option struct { Fill color.Color Radius int } func New(shape string, option Option) (Shaper, error){ sidesForShape := map[string]int{"triangle": 3,"square": 4, "pentagon": 5,"hexagon": 6,"heptagon": 7,"octagon": 8, "enneagon": 9,"nonagon": 9,"decagon": 10} if sides, found := sidesForShape[shape]; found { ptg7913109 6.5. Examples 299 return NewRegularPolygon(option.Fill, option.Radius, sides), nil } if shape != "circle" { return nil, fmt.Errorf("shapes.New(): invalid shape '%s'", shape) } return NewCircle(option.Fill, option.Radius), nil } This factory function requires two arguments: the name of the shape to be cre- ated and a custom Option value in which optional shape-specific parameters can be specified. (The use of structs to make functions capable of handling mul- tiple optional arguments was covered in Chapter 5; §, 222 ➤ .) The func- tion returns a shape that fulfills the Shaper interface and nil, or if an invalid shape name is given, nil and an error.(Recall that the Shaper interface differs between the two shapes packages’ implementations;§,294 ➤ and §, 294 ➤ .) The particular shape created depends on the shape string that is passed. There is no need to validate the color or radius since that is handled by the shapes.shape.SetFill() method and the shapes.saneRadius() function that are ultimately called by the NewRegularPolygon() and NewCircle() methods—and sim- ilarly for the sides. polygon := shapes.NewRegularPolygon(color.RGBA{0, 0x7F, 0, 0xFF}, 65, 4) showShapeDetails(polygon) ➊ y=30 for i, radius := range []int{60, 55, 50, 45, 40}{ polygon.SetRadius(radius) polygon.SetSides(i + 5) x += radius y += height / 8 if err := shapes.DrawShapes(img, x, y, polygon); err != nil { fmt.Println(err) } } This little snippet shows how some of the polygons shown in Figure 6.3 (289 ➤ ) were created using the DrawShapes() function (292 ➤ ). The showShapeDetails() function (above, ➊; ➤ 300) is used to print the details of any kind of shape. This is possible because the function accepts any value that satisfies the Shaper interface (i.e., any of our shapes), rather than a specific concrete shape type (such as a *Circle or *RegularPolygon). Since the Shaper interface differs between the two shapes packages there are two different showShapeDetails() implementations. Here is the one for the hierarchical shaper1 version. ptg7913109 300 Chapter 6. Object-Oriented Programming Embedding Is Not Inheritance i In this subsection, the shaper example illustrates how to use struct embed- ding to achieve an inheritance-like effect. This technique may appeal to those porting C++ or Java code to Go (or to those learning Go from a C++ or Java background). However, although this approach works: The Go way is not to simulate inheritance,but to avoid it altogether. In the context of the example this would mean having independent structs: type Circle struct { color.Color Radius int } type RegularPolygon struct { color.Color Radius int Sides int } This still allows us to pass generic shape values—after all, if both shapes have Draw() methods that satisfy a Drawer interface, then both Circles and RegularPolygons can be passed as Drawer values. Another point to note is that here we have made all the fields exported, with no validation at all. This means that we must validate the fields when they are used, rather than when they are set. Both approaches to validation are sensible: Which is best depends on the circumstances. The book’s shaper3 example uses the structs shown above and has the same functionality as the shaper1 and shaper2 examples shown in this section. However, shaper3 is written in a more Go-like style, with no embedding and doing its validation at the point of use. func showShapeDetails(shape shapes.Shaper) { fmt.Print("fill=", shape.Fill(), "")// All shapes have a fill color if shape, ok := shape.(shapes.CircularShaper); ok {// shadow variable fmt.Print("radius=", shape.Radius(), "") if shape, ok := shape.(shapes.RegularPolygonalShaper); ok {//shadow fmt.Print("sides=", shape.Sides(), "") } } fmt.Println() } In the shaper1 shapes package’s interface hierarchy,the Shaper interface specifies Fill() and SetFill() methods, so these can be used immediately. But for other methods we must use checked type assertions to see if the shape passed in sat- isfies the interfaces that have the methods we want to call. Here, for example, we only access the Radius() method if the shape fulfills the CircularShaper inter- ptg7913109 6.5. Examples 301 face, and similarly for the RegularPolygonalShaper’s Sides() method. (Recall that RegularPolygonalShaper embeds a CircularShaper.) The shaper2 version of the showShapeDetails() function is similar to the shaper1 version. func showShapeDetails(shape shapes.Shaper) { fmt.Print("fill=", shape.Fill(), "")// All shapes have a fill color if shape, ok := shape.(shapes.Radiuser); ok {// shadow variable fmt.Print("radius=", shape.Radius(), "") } if shape, ok := shape.(shapes.Sideser); ok {// shadow variable fmt.Print("sides=", shape.Sides(), "") } fmt.Println() } The compositional shaper2 shapes package has a convenient Shaper interface that embeds the Drawer and Filler interfaces, so we know that the passed-in shape has a Fill() method. And unlike shaper1’s hierarchical interfaces, here we are able to use very specific type assertions to access the Radius() and Sides() methods for shapes that support them. If new methods or fields are added to shape, Circle,orRegularPolygon, our code will continue to work unchanged. But if we add new methods to any of the interfaces, then we would have to update the affected shape types to provide the methods or our code would break. A better alternative is to create new interfaces with the additional methodsand embed the existing interfaces inside them. This won’t break any existing code and gives us the option to add or not add the new methods to the existing types,depending on whether we want them to fulfill the new interfaces as well as the ones they already fulfill. We recommend using the compositional approach to interfaces rather than a hi- erarchical approach. And as for struct embedding, we recommend doing things the Go way, that is,having independent structs and not trying to simulate inher- itance. Of course, once sufficient Go experience has been gained, such choices can be made on the basis of technical merit rather than to ease translation or out of habit. In addition to the shaper1 and shaper2 examples shown in this subsection, the book’s examples include shaper3 which shows a “purer” Go approach. The shaper3 version has just one interface, Drawer, and independent Circle and Reg- ularPolygon structs (as shown in the sidebar “Embedding Is Not Inheritance”, 300 ➤ ).Also, shaper3 uses shape values rather than pointers,and does its valida- tion at the point of use. It is worth looking at the shaper2/shapes/shapes.go and shaper3/shapes/shapes.go files, to compare and contrast the two approaches. ptg7913109 302 Chapter 6. Object-Oriented Programming 6.5.3. Example: Ordered Map—A Generic Collection Type This chapter’s final example is of a generic ordered map type that stores key– value pairs like Go’s built-in map type, only with all the pairs stored in key order. The ordered map uses a left-leaning red-black tree so it is very fast, with a lookup time complexity of O(log2 n).★ By comparison, an unbalanced binary tree’s performance can degrade to that of a linked list (O(n)) if items are added in order. Balanced trees do not suffer from this defect because they maintain the tree’s balance as items are added and removed and thereby preserve their excellent performance characteristics. Programmers with a background in inheritance-based object orientation (e.g., C++, Java, Python) are likely to want to make the ordered map’s keys support the < less than operator or a Less(other) bool method. This can easily be done by defining a Lesser interface that requires such a method, and providing tiny wrapper types for int, string, MyType, and so on, that implement the method. However, the right way to do this in Go is somewhat different. For our Go ordered map we will impose no direct constraint on the key type. Instead, we give each map a “less than” function to use for comparing its keys. This means that it doesn’t matter whether our keys support the < operator or not—so long as we can provide a suitable less than comparison function for them. Before looking at the implementation, let’s look at some examples of use, starting with the creation and population of an ordered map. words := []string{"Puttering","About","in","a","Small","Land"} wordForWord := omap.NewCaseFoldedKeyed() for _, word := range words { wordForWord.Insert(word, strings.ToUpper(word)) } Our ordered map is in package omap and is of type Map. To create a Map we must use the omap.New() function, or one of the other Map construction functions, such as the omap.NewCaseFoldedKeyed() function we have used here, since the Map’s zero value is not usable. This particular construction function creates an empty Map with a predefined less than function that compares string keys case-insensitive- ly, and returns a pointer to it (i.e., a *Map). ★ Our ordered map implementation is based on the left-leaning red-black trees described by Robert Sedgewick in and LLRB/RedBlack.pdf. At the time of this writing, the Java implementations presented in these papers were incomplete and had some errors, so we used ideas from Lee Stanza’s C++ code at www.teach- to complete our implementation. ptg7913109 6.5. Examples 303 Every key–valuepair is added using the omap.Map.Insert() method which accepts two interface{} values,that is,a key and a value of any types. (However,the key must be of a type that is compatible with the less than function, so in this exam- ple keys must be strings.) The Insert() method returns true if a new item was inserted, and false if an item with the given key already existed (in which case that item’s value is replaced by the new given value—this is the same behavior as the built-in map type). wordForWord.Do(func(key, value interface{}) { fmt.Printf("%v→%v\n", key, value) }) a→A About→ABOUT in→IN Land→LAND Puttering→PUTTERING Small→SMALL The omap.Map.Do() method takes a function with the signature func(interface{}, interface{}) as argument and calls this function on every item in the ordered map—in key order—and passing in the key and value as the arguments for each call. Here we have used the Do() method to print all of the wordForWord’s keys and values. In addition to inserting items and calling a method on all the map’s items, we can also query how many items are in the map, search for items, and delete items. fmt.Println("length before deleting:", wordForWord.Len()) _, containsSmall := wordForWord.Find("small") fmt.Println("contains small:", containsSmall) for _, key := range []string{"big","medium","small"}{ fmt.Printf("%t ", wordForWord.Delete(key)) } _, containsSmall = wordForWord.Find("small") fmt.Println("\nlength after deleting: ", wordForWord.Len()) fmt.Println("contains small:", containsSmall) length before deleting: 6 contains small: true false false true length after deleting: 5 contains small: false The omap.Map.Len() method returns the number of items in the ordered map. ptg7913109 304 Chapter 6. Object-Oriented Programming The omap.Map.Find() method returns the value of the item with the given key as an interface{} and true,ornil and false if there is no such item. The omap. Map.Delete() method deletes the item with the given key and returns true—or safely does nothing if there is no such item, and returns false. If we want to store keys of a custom type we can do so by creating the Map with the omap.New() function and supplying it with a suitable less than method. For example, here is an implementation of a very simple custom type. type Point struct{ X, Y int } func (point Point) String() string { return fmt.Sprintf("(%d, %d)", point.X, point.Y) } Now we will see how to create an ordered map that stores *Points as keys and their distance from the origin as values. In the following snippet we have created an empty Map and given it a less than function for comparing *Point keys. Then we have created a slice of *Points and populated the map from the points. And finally, we have used the omap.Map.Do() method to print the map’s keys and values in key order. distanceForPoint := omap.New(func(a, b interface{}) bool { α, β := a.(*Point), b.(*Point) if α.X != β.X { return α.X < β.X } return α.Y < β.Y }) points := []*Point{{3, 1}, {1, 2}, {2, 3}, {1, 3}, {3, 2}, {2, 1}, {2, 2}} for _, point := range points { distance := math.Hypot(float64(point.X), float64(point.Y)) distanceForPoint.Insert(point, distance) } distanceForPoint.Do(func(key, value interface{}) { fmt.Printf("%v → %.2v\n", key, value) }) (1, 2) → 2.2 (1, 3) → 3.2 (2, 1) → 2.2 (2, 2) → 2.8 (2, 3) → 3.6 (3, 1) → 3.2 (3, 2) → 3.6 ptg7913109 6.5. Examples 305 Recall from Chapter 4 (155 ➤ ) that Go is smart enough to allow us to drop inner type names and ampersands when creating slice literals, so here the creation of the points slice is really a shorthand for points := []*Point{&Point{3, 1}, &Point{1, 2}, …}. Although not shown, we can use the ordered map Delete(), Find(), and Len() methods on the distanceForPoint map in exactly the same way as we did for the wordForWord map, but using *Point keys for the first two (since the less than function works in terms of *Points, not Points). Now that we have seen the ordered map in use we will review its implementa- tion. We won’t cover the Delete() method’s helper method and functions, be- cause some of them are rather tricky and covering them won’t really add to our knowledge of Go programming. (All these functions are in the book’s source code, of course, in the file We will start by looking at the two types used to implement the ordered map (Map and node), and then the con- struction functions. Then we will look at the Map’s methods and some of their helper functions. As is common in Go programming, most methods are quite short with more complicated processing passed on to helper functions. type Map struct { root *node less func(interface{}, interface{}) bool length int } type node struct { key, value interface{} red bool left, right *node } The ordered map is implemented in terms of two custom struct types. The first struct type is the Map struct which holds the root of the left-leaning red-black tree, a less than function that is used to compare keys, and a length which holds the number of items in the map. This type’s fields are all unexported and the less function’szero value is nil,so creating a Map variable will produce an invalid Map. The Map type’s documentation explains this and directs users to use one of the omap package’s construction functions to create valid Maps. The second struct type is the node struct which represents a single key–value item. In addition to its key and value fields, the node struct has three additional fields needed to implement the tree. The red field of type bool is used to identify whether a node is “red” (true) or “black” (false)—this is used when portions of the tree are rotated to maintain its balance. The left and right fields of type *node hold pointers to a node’s left and right subtrees (which can be nil). ptg7913109 306 Chapter 6. Object-Oriented Programming The omap package provides several construction functions; here, we will look at the generic omap.New() function and a couple of others. func New(less func(interface{}, interface{}) bool) *Map { return &Map{less: less} } This is the package’s generic function for creating an ordered map for any built-in or custom type for which we can provide a suitable less than function. func NewCaseFoldedKeyed() *Map { return &Map{less: func(a, b interface{}) bool { return strings.ToLower(a.(string)) < strings.ToLower(b.(string)) }} } This construction function creates an empty ordered map that has string keys which are compared case-insensitively. func NewIntKeyed() *Map { return &Map{less: func(a, b interface{}) bool { return a.(int) < b.(int) }} } This construction function creates an empty ordered map that has int keys. The omap package also has an omap.NewStringKeyed() function for creating an ordered map with case-sensitive string keys (the implementation is almost identical to omap.NewCaseFoldedKeyed() but without the strings.ToLower() calls), and an omap.NewFloat64Keyed() function which is just like the omap.NewIntKeyed() function except that it uses float64s instead of ints. func (m *Map) Insert(key, value interface{}) (inserted bool) { m.root, inserted = m.insert(m.root, key, value) = false if inserted { m.length++ } return inserted } This method is structurally typical of many Go methods in that it passes most of its work on to a helper, in this case the unexported insert() method. The tree’s root might change as the result of an insertion,either because the tree was ptg7913109 6.5. Examples 307 empty and now contains a single node which must become the root, or because the insertion resulted in rotations to keep the tree balanced and which involved the root. The insert() method returns the tree’sroot whether it changed or not,and also a Boolean. The Boolean is true if a new item was inserted, and in this case we in- crement the map’s length. If the Boolean is false it means that an item with the given key was already in the map, so all that was done was to replace the item’s current value with the given value, and the map’s length is left unchanged. (We won’t explain why nodes are made red or black or why they are rotated. These matters are fully explained in Robert Sedgewick’s papers—see the footnote; 302 ➤ .) func (m *Map) insert(root *node, key, value interface{}) (*node, bool) { inserted := false if root == nil {// If the key was in the tree it would belong here return &node{key: key, value: value, red: true}, true } if isRed(root.left) && isRed(root.right) { colorFlip(root) } if m.less(key, root.key) { root.left, inserted = m.insert(root.left, key, value) } else if m.less(root.key, key) { root.right, inserted = m.insert(root.right, key, value) } else {// The key is already in the tree so just replace its value root.value = value } if isRed(root.right) && !isRed(root.left) { root = rotateLeft(root) } if isRed(root.left) && isRed(root.left.left) { root = rotateRight(root) } return root, inserted } This is a recursive function that traverses the tree to find the place where the given key belongs, and rotates subtrees as necessary to maintain the tree’s balance. When the Insert() method calls this method the root that is passed in is the root of the entire tree (or nil if the tree is empty), but on subsequent recursive calls the root is the root of a subtree (which could be nil). If the new key isn’t the same as any existing key the traversal will reach the right place to insert the new key—and this will be a nil leaf. At this point we create and return a new *node to be the leaf and with its own leaves both nil. ptg7913109 308 Chapter 6. Object-Oriented Programming We don’t have to explicitly initialize the new node’s left and right fields (i.e., its leaves) since Go will automatically set them to their zero values (i.e., nil), so we use the struct’s key: value syntax to initialize just those fields that have nonzero values. If the new key is the same as an existing key we reuse the existing key’s node, and simply replace its value with the new value. (This is the same behavior as the built-in map type.) A consequence of this is that every item in an ordered map has a unique key. func isRed(root *node) bool { return root != nil && } This tiny helper function returns whether the given node is red; it treats nil nodes as black. func colorFlip(root *node) { = ! if root.left != nil { = ! } if root.right != nil { = ! } } This helper function flips the colors of the given node and of its non-nil leaves. func rotateLeft(root *node) *node { x := root.right root.right = x.left x.left = root = = true return x } func rotateRight(root *node) *node { x := root.left root.left = x.right x.right = root = = true return x } These functions rotate the root’s subtree to maintain the subtree’s balance. func (m *Map) Find(key interface{}) (value interface{}, found bool) { root := m.root for root != nil { if m.less(key, root.key) { root = root.left } else if m.less(root.key, key) { root = root.right } else { ptg7913109 6.5. Examples 309 return root.value, true } } return nil, false } Since this method’simplementation is straightforwardand uses iteration rather than recursion there is no need to create a helper function. The Find() method identifies the item to find by comparing for equality the current root’s key (as the method traverses the tree), with the given key, using the less() function. This is done by using the logical equivalence x = y ⇔ ¬ (x < y ∨ y < x). This is valid for ints, float64s, strings, the custom Point type,and many other types,but is not true for all types. It would be easy to extend the omap.Map type to accept a separate equals function if it were needed. Notice that we have used named return values although we never explicitly assign to them. Of course, they are assigned to implicitly in the return state- ments. Naming return values like this can be a useful supplement to a function or method’s documentation. Here, for instance, it is obvious from the signature Find(key interface{}) (value interface{}, found bool) what is being returned—it would not be quite so obvious if the signature was Find(key interface{}) (inter- face{}, bool). func (m *Map) Delete(key interface{}) (deleted bool) { if m.root != nil { if m.root, deleted = m.remove(m.root, key); m.root != nil { = false } } if deleted { m.length-- } return deleted } Deleting an item from a left-leaning red-black tree is tricky, so we have passed the work on to an unexported remove() method plus that method’s helper functions, none of which are shown. If the ordered map is empty—or if no item in the map has the given key—then Delete() safely does nothing and returns false.If the tree has a single item,and that is the one deleted,then the *omap.Map receiver’s root will be set to nil (and the tree will be empty). If a deletion takes place we return true and decrement the map’s length. Incidentally, the remove() method identifies the item to be deleted using the same equality equivalence as the Find() method uses. ptg7913109 310 Chapter 6. Object-Oriented Programming func (m *Map) Do(function func(interface{}, interface{})) { do(m.root, function) } func do(root *node, function func(interface{}, interface{})) { if root != nil { do(root.left, function) function(root.key, root.value) do(root.right, function) } } The Do() method and its do() helper function are used to traverse all the items in the ordered map—in key order—and call the given function for each item, passing in the item’s key and value each time. func (m *Map) Len() int { return m.length } This method simply returns the map’s length. The length is incremented and decremented in the omap.Map.Insert() and omap.Map.Delete() methods we have already seen. This completes our review of the ordered map custom collection type, and brings us to the end of our coverage of object-oriented Go programming. When it comes to custom types for which any value is valid we can simply create the type (e.g., using a struct) and make the type and its fields exported (starting with uppercase letters), and that is sufficient. (For examples, see the standard library’s image.Point and image.Rectangle types.) For custom types that require validation (e.g., those that are based on structs with one or more fields and which require at least one of the fields to be validat- ed), Go has a particular programming idiom. The fields that must be validated are made unexported (start with a lowercase letter) and we provide getter and setter accessor methods for them. In the case of types whose zero values are invalid, we make the relevant fields unexported and provide accessor methods. We also document the fact that the zero value is invalid, and provide an exported construction function (typically called New()). The construction function normally returns a pointer to a value of the type with all the fields set to valid values. We can pass values and pointers to values with exported—and unexported— fields, and in the case of types which satisfy one or more interfaces, we can, of course, pass them as interfaces when this is useful, that is, when we care only about how they behave, not what they are. ptg7913109 6.5. Examples 311 Clearly those programmerscoming from a more inheritance-based object-orient- ed background (such as C++, Java, or Python) will need to adapt their thinking. However, the power and convenience of Go’s duck typing and interfaces, and the lack of painful-to-maintain inheritance hierarchies,make the investment in learning well worthwhile. Go’s approach to object-oriented programming works extremely well, providing it is done in the spirit of the Go way. 6.6. Exercises There are three exercises for this chapter. The first involves the creation of a small but complete custom type whose fields must be validated. The second involves adding new functionality to one of the custom types we discussed in this chapter. The third requires the creation of a small custom collection type. The first two exercises are not difficult; however, the third exercise is quite chal- lenging. 1. Create a new package called font (e.g., in my_font/font.go). The purpose of the package is to provide values that represent font properties (e.g., a font’s family and size). There should be a New() function that takes a family and size (both of which must be validated) and returns a *Font (with valid un- exported fields).Also provide getters and validating setters. For validation, don’t allow empty family names and only allow font sizes between 5 and 144 points inclusive: If invalid values are given set valid values (or the previous values for setters), and log the problem. Be sure to provide a method that satisfies the fmt.Stringer interface. Here is an example of how to create, manipulate, and print a font using the package. titleFont := font.New("serif", 11) titleFont.SetFamily("Helvetica") titleFont.SetSize(20) fmt.Println(titleFont) {font-family: "Helvetica"; font-size: 20pt;} Once the package is ready, copy the example’s font/font_test.go file into the my_font directory and run go test to do some very basic testing. A solution is given in font/font.go. The entire package is around 50 lines of code. Incidentally, we chose to make the String() method return the font’s details in CSS (Cascading Style Sheet) style. It would be straightforward,if a little tedious, to extend this package to handle all the CSS font attributes, such as weight, style, and variant. 2. Copy an entire shaper example (the hierarchical shaper1, the compositional shaper2, or the Go-style shaper3, whichever you prefer—although we recom- ptg7913109 312 Chapter 6. Object-Oriented Programming mend shaper2 or shaper3—including its subdirectory) into a new directory, say, my_shaper. Edit the my_shaper/shaper[123].go file: Delete the imports except for image and shapes, and delete all the statements inside the main() function. Edit the my_shaper/shapes/shapes.go file to add support for a new shape called Rectangle.This shape should have a point of origin and a width and height (all of which the image.Rectangle type provides),a fill color, and a Boolean indicating whether the shape should be filled. Add the Rectangle in the same style as the other shapes, that is, with unexported fields and with an interface (e.g., a hierarchical RectangularShaper or a compositional Rect- angler and Filleder), or with no interface and exported fields (in Go style), to specify the type’s API. The Draw() method isn’t difficult, especially if you use the unexported drawLine() function that is inside the shapes package, and the draw.Draw() function. Remember also to update the New() function so that it can create rectangles—and extend the Option type accordingly. Once the rectangle has been added, fill in the my_shaper/shaper[123].go file’s main() function to create and save an image like that shown in Figure 6.4. Figure 6.4 An image created using the Rectangle type Three solutions are given; a hierarchical one in the shaper_ans1 directory, a compositional one in the shaper_ans2 directory, and a Go-style one in the shaper_ans3 directory. Here is the RectangularShaper interface used in the shaper_ans1 solution: type RectangularShaper interface { Shaper // Fill(); SetFill(); Draw() Rect() image.Rectangle SetRect(image.Rectangle) Filled() bool SetFilled(bool) } For the shaper_ans2 solution we have the Rectangler and Filleder inter- faces: ptg7913109 6.6. Exercises 313 type Rectangler interface { Rect() image.Rectangle SetRect(image.Rectangle) } type Filleder interface { Filled() bool SetFilled(bool) } No new interface is needed for the Go-style version. The code for the concrete Rectangle type itself is the same for the hierarchi- cal and compositional approaches and features unexported fields with get- ters and setters. But for the Go-style version, use exported fields and only validate at the point of use. type Rectangle struct { color.Color image.Rectangle Filled bool } In shaper_ans1/shapes/shapes.go, the Rectangle type and its supporting methods are less than 50 lines. The Option type needs a couple more lines and the New() function five more lines. In shaper_ans1/shaper1.go, the new main() function is less than 20 lines. It is very similar for the shaper_ans2 solution’s Go files. The shaper_ans3 solution requires the least extra code. More ambitious readers might like to take the example further by providing separate fill and outline colors with a nil color signifying that the fill or outline should not be drawn, and a non-nil color being the color to fill or outline with. 3. Create a custom collection type in package my_oslice called Slice. This type must implement an ordered slice. Provide several construction functions, for example, New(func(interface{}, interface{}) bool), which accepts a less than function, and some others with predefined less than functions such as NewStringSlice() and NewIntSlice().The*oslice.Slice type should provide a Clear() method to empty the slice, an Add(interface{}) method for inserting an item into the correct place in the slice, a Remove(interface{}) bool method for removing the first occurrence of the specified item and which reports if the item was removed, an Index(interface{}) int method to return the index position of the first occurrence of the specified item (or -1), an At(int) interface{} method to return the item at the given index position (and which panics if the index is out of range), and a Len() int method which reports how many items are in the slice. ptg7913109 314 Chapter 6. Object-Oriented Programming func bisectLeft(slice []interface{}, less func(interface{}, interface{}) bool, x interface{}) int { left, right := 0, len(slice) for left < right { middle := int((left + right) / 2) if less(slice[middle], x) { left = middle + 1 } else { right = middle } } return left } The bisectLeft() function is used in the solution and might prove useful. If it returns len(slice), the item isn’t in the slice and belongs at the end. Any other value means either that the item is in the slice at the returned position, or that it isn’t in the slice but belongs at the returned position. Some readers might like to copy the oslice/oslice_test.go file to the my_oslice directory to test their solution. A solution is given in oslice/ oslice.go and is less than 100 lines. The Add() method is quite tricky, but the code from Chapter 4’s InsertStringSlice() function (158 ➤ ) should prove helpful. ptg7913109 7 Concurrent Programming §7.1. Key Concepts ➤ 317 §7.2. Examples ➤ 322 §7.2.1. Example: Filter ➤ 322 §7.2.2. Example: Concurrent Grep ➤ 326 §7.2.3. Example: Thread-Safe Map ➤ 334 §7.2.4. Example: Apache Report ➤ 341 §7.2.5. Example: Find Duplicates ➤ 349 Concurrent programming allows developers to implement parallel algorithms and to write programs that take advantage of multiple processors and multiple cores. The downside is that in most mainstream programming languages (such as C, C++, and Java) it is much harder to write, maintain, and debug concurrent programs than single-threaded programs. Furthermore, it isn’t always possible to split up processing to make using multiple threads worthwhile. And in any case, the hoped-for performance benefits are not always achieved due to the overhead of threading itself, or simply because it is much easier to make mistakes in threaded programs. One solution is to avoid threading altogether. For example, we can pass the bur- dens on to the operating system by using multiprocessing. However, this has the disadvantages that it leaves us responsible for handling all the inter-process communication ourselves, and usually has more overhead than shared-memory concurrency. The Go solution is threefold. First, Go provides high-level support for concur- rent programming that makes it much easier to do correctly;second, concurrent processing is done in goroutines that are much more lightweight than threads; and third, automatic garbage collection relieves programmers of the sometimes fiendishly complex memory management required by concurrent programs. Go’s built-in high-level API for writing concurrent programs is based on CSP (Communicating Sequential Processes). This means that explicit locking—and all the care required to lock and unlock at the right times—can be avoided, with synchronization achieved by sending and receiving data via thread-safe chan- nels. This greatly simplifies the writing of concurrent programs. And whereas 315 ptg7913109 316 Chapter 7. Concurrent Programming dozens or scores of threads could overburden a typical desktop computer, the same machine could happily cope with hundreds, thousands, or even tens of thousands of goroutines. Go’s approach makes it easier for programmers to rea- son about their concurrent programs in terms of what they want the programs to achieve rather than in terms of locking and other low-level details. While most other languages have support for very low-level concurrent opera- tions (atomic adds and compare and swaps), and some low-level facilities such as mutexes, no other mainstream language has the kind of built-in high-level concurrency support that Go offers (except, perhaps, as add-on libraries that are not an integral part of the language). In addition to the high-level concurrency support that is the subject of this chap- ter,Go also providesthe same low-level functionality as other languagesprovide. At the lowest level the standard library’s sync/atomic package providesfunctions for performing atomic additions and compare and swap operations. These ad- vanced functions are designed to support the implementation of thread-safe syn- chronization algorithms and data structures—they are not intended for use by application programmers. Go’s sync package providesthe conventional low-level concurrency primitives:wait conditions and mutexes. These are as high-level as it gets in most other languages, so application programmers are often forced to use them. Go application programmers are expected to use Go’s high-level facilities— channels and goroutines—for concurrent programming. In addition, the sync. Once type can be used to call a function once only, no matter how many times the call is made, and the sync.WaitGroup type provides a high-level synchronization mechanism, as we will see later. We have already covered the basic syntax and usage of channels and goroutines in Chapter 5 (§5.4, 205 ➤ ). None of that material is repeated here, although knowledge of it is assumed, so it might be helpful to reread or at least skim that section before continuing. This chapter begins with an overview of some key conceptsin Go concurrent pro- gramming. Then the chapter presents five complete working programs that il- lustrate Go concurrent programming and which between them show some stan- dard patterns of use. The first example shows how to create a pipeline where each section of the pipe executes in its own goroutine to maximize throughput. The second example shows how to split work over a fixed number of goroutines that output their results independently of each other. The third example shows how to create a thread-safe data structure—without a lock or low-level primitive in sight. The fourth example shows how to perform independent pieces of work in a fixed number of goroutines and whose results must be merged together, with three different approaches shown. The fifth example shows how to create a processing-dependent number of goroutines and how to merge the work from these goroutines into a single set of results. ptg7913109 7.1. Key Concepts 317 7.1. Key Concepts In concurrent programming we typically want to split up the processing that needs doing over one or more goroutines (in addition to the main orchestrating goroutine), and either output results as soon as they are computed or gather the results for outputting at the end. Even with Go’s high-level approach to concurrency there are pitfalls that we must avoid. One such pitfall is when the program finishes almost immediately but produces no results. Go programs automatically terminate when the main goroutine terminates—even if other goroutines are processing at the time—so we must be careful to keep the main goroutine alive long enough for all the work to be done. Another pitfall we must avoid is deadlock. One form of this problem is essential- ly the opposite of the first pitfall: The main goroutine and all the processing go- routines are alive even though all the work has been done. This is typically due to a failure to report the completion of processing. Another cause of deadlock is when two different goroutines (or threads) are using locks to protect resources and try to acquire the same locks at the same time, as illustrated in Figure 7.1. This kind of deadlock can only occur when locking is used, so is a common risk in other languages but quite rare in Go, since Go applications can avoid the use of locks by using channels. Thread #1 Thread #2 AB Figure 7.1 Deadlock: two or more blocked threads trying to acquire each other’s locks The most common way to avoid premature termination and nontermination is to make the main goroutine wait for a “done” channel to report that the work is finished (as we will see in a moment,and also in §7.2.2,➤ 326 and §7.2.4,➤ 341). (It is also possible to use a sentinel value sent as the last “result”, but this is rather clumsy compared to the other approaches.) Another way to avoid the pitfalls is to wait for all the processing goroutines to report that they are finished using a sync.WaitGroup.However,using a sync.Wait- Group itself can cause a deadlock, particularly if the sync.WaitGroup.Wait() call occursin the main goroutine when all the processing goroutinesare blocked (e.g., waiting to receive on a channel). We will see how to use a sync.WaitGroup later (§7.2.5, ➤ 349). It is still possible to get deadlocks in Go, even if we use only channels, and don’t use locks. For example, suppose we have a set of goroutines that can ask each ptg7913109 318 Chapter 7. Concurrent Programming other to execute functions (e.g., by sending requests to each other).Now if one of the requested functionsdoes a send to the goroutine in which it is executing—for example, to pass it some data—we will get a deadlock. This is illustrated in Figure 7.2. (Later on we will see an example where this kind of deadlock is possible; ➤ 337 and ➤ 340.) goroutine function() Response #1 Request #1 Request #2 Cannot respond until request #1 is serviced, but request #1 cannot respond until request #2 is serviced—deadlock! Figure 7.2 Deadlock: a goroutine that tries to service a request with a request to itself Channels provide a lock-free means of communication between concurrently running goroutines. (Under the hood locks may be used, but these are an imple- mentation detail that we don’t have to concern ourselves with.) When a channel communication takes place, at the moment of communication the sending and receiving channels (and their respective goroutines) are synchronized. By default channels are bidirectional, that is, we can send values into them and we can receive values from them. However,it is quite common for a channel that is a field in a struct or a channel that is passed as a parameter to be used unidi- rectionally, that is, only to be sent to, or only to be received from. In such cases we can express the semantics (and force the compiler to check for us) by speci- fying the channel’s direction. For example, the type chan<- Type is a send-only channel and the type <-chan Type is a receive-only channel. We have not used this syntax in earlier chapters, because it is never required—we can always use chan Type instead—and there was plenty of other material to learn. But from now on we will use unidirectional channels wherever they are appropriate, since they provide additional compile-time checking and are best practice. Sending valuessuch as bools, ints, and float64sthrough channelsisintrinsically safe since these are copied, so there is no risk of inadvertent concurrent access to the same value. Similarly, sending strings is safe since they are immutable. Sending pointers or references (e.g., slices or maps) through channels is not in- trinsically safe since the pointed-to or referred-to value could be changed by the sending goroutine and by the receiving goroutine at the same time—with unpredictable results. So, when it comes to pointers and references, we must make sure that they can only ever be accessed by one goroutine at any one time, that is, accesses must be serialized. The exception is where the documentation specifically says it is safe to pass a pointer—for example, the same *regexp.Reg- exp can safely be used in as many goroutines as we like, because none of the methods called on the pointed-to value change the value’s state. ptg7913109 7.1. Key Concepts 319 One way to serialize accesses is to use mutexes. Another way is to apply a poli- cy that a pointer or reference is only ever sent once, and once sent the sender never accesses it again. This leaves the receiver free to access the pointed-to or referred-to value—and to send on the pointer or reference providing both sender and receiver follow the same policy. (We will see an example of using this policy- based approach later; §, ➤ 347.) A downside of policy-based approaches is that they require discipline. A third way to work safely with pointers or refer- ences is to provide them with exported methods that cannot change the pointed- to or referred-to value and unexported methods that can perform changes. Such pointers or references could then be passed around and accessed concurrently through their exported methods, with only one goroutine allowed to use their unexported methods (e.g., inside their own package; packages are covered in Chapter 9). It is also possible to send interface values—that is, values that meet a particular interface—through channels. Values of read-only interfaces can safely be used in any number of goroutines (unless the documentation says otherwise), but values of interfaces that include methods for changing a value’s state must be treated in the same way as pointers, with accesses to them serialized. For instance,if we create a new image using the image.NewRGBA() function we will get an *image.RGBA. This type fulfills both the image.Image interface (which has only getter methods,and so is read-only) and the draw.Image interface (which has all the image.Image methods plus a Set() method). So, it is safe to pass the same *image.RGBA value to as many goroutinesas we like—providing we pass it to func- tions that accept an image.Image.(Unfortunately, the safety could be subverted by the receiving method using a type assertion to, say, a draw.Image interface,so it is wise to have a policy that disallows such things.) And if we want to use the same *image.RGBA value in multiple goroutines that might change it, we should either send it as an *image.RGBA or as a draw.Image, and in either case we must ensure that all accesses to it are serialized. One of the simplest ways to use concurrency is to use one goroutine to prepare jobs to do and another goroutine to do the jobs, leaving the main goroutine and some channels to orchestrate everything. For example, here is how we might create a “jobs” channel and a “done” channel in the main goroutine. jobs := make(chan Job) done := make(chan bool, len(jobList)) Here we have created an unbuffered jobs channel to pass values of some custom Job type. We have also created a buffered done channel whose buffer size corresponds to the number of jobs to be done based on the jobs available in variable jobList of type []Job (whose initialization isn’t shown). With the channels and job list set up, we can begin. ptg7913109 320 Chapter 7. Concurrent Programming go func() { for _, job := range jobList { jobs <- job // Blocks waiting for a receive } close(jobs) }() This snippet creates the first additional goroutine. It iterates over the jobList slice and sends each job to the jobs channel. Because the channel is unbuffered the goroutine is immediately blocked and will remain blocked until another goroutine tries to receive from the jobs channel. Once all the jobs have been sent to the jobs channel the channel is closed so that receivers will know when there are no more jobs to do. The semantics of this snippet are not entirely obvious! The for loop runs to completion and then closes the jobs channel—but this takes place concurrently with any other goroutines in the program. Furthermore, the go statement will return immediately, leaving the code to be executed in its own goroutine—and, of course, with no one trying to receive jobs at this time, the goroutine blocks. So, right after this go statement, the program has two goroutines, the main go- routine which continues to the next statement,and this newly created goroutine that is blocked waiting for another goroutine to do a receive on the jobs channel. Hence, it will take some time before the for loop completes and the channel is closed. go func() { for job := range jobs {// Blocks waiting for a send fmt.Println(job) // Do one job done <- true } }() This snippet creates the second additional goroutine. This goroutine iterates over the jobs channel, and for each job it receives, it processes the job (here, just prints it), and sends true on the done channel for every job that it completes. (We could just as well send false, since we only care about how many sends are performed on the done channel, not about the actual values sent.) Just like the first go statement, this statement returns immediately, and the for statement blocks waiting for a send. So, at this point, three concurrent gorou- tines are executing—the main goroutine and the two additional goroutines, as Figure 7.3 illustrates. Since we have already got a send waiting (in goroutine #1), that job is received straight away (by goroutine #2) and processed. Meanwhile goroutine #1 is again blocked, this time waiting to send its second job. Once goroutine #2 has finished ptg7913109 7.1. Key Concepts 321 Main goroutine (wait until done) Goroutine #1 (send jobs) Goroutine #2 (receive jobs) jobs channeldone channel Results Figure 7.3 Concurrent independent preparation and processing processing it sends to the done channel—this channel is buffered and so doesn’t block sends. Control then returns to goroutine #2’s for loop and the next job is sent from goroutine #1 and received by goroutine #2, and so on, until all the jobs are done. for i:=0;i \n", filepath.Base(os.Args[0])) os.Exit(1) } if lineRx, err := regexp.Compile(os.Args[1]); err != nil { log.Fatalf("invalid regexp: %s\n", err) } else { grep(lineRx, commandLineFiles(os.Args[2:])) } } The program’s main() function’s first statement tells the Go runtime system to use as many processors (cores) as the system has available. Calling runtime.GO- MAXPROCS(0) returns the number of processors and changes nothing; calling it with a positive number setsthe number of processorsthe Go runtime should use. The runtime.NumCPU() function returns how many logical processors/cores the machine has.★ We put this line at the start of most concurrent Go programs,but eventually it will be redundant because Go’s runtime system will become smart enough to automatically adapt to the machine it is running on. ★ Some processors claim to have more cores than they really have; see threading . ptg7913109 328 Chapter 7. Concurrent Programming The main() function handles the command-line arguments (a regular ex- pression and a list of files) and calls the grep() function to do—or rather, to orchestrate—the work. (We saw the custom commandLineFiles() function in an earlier chapter; 176 ➤ .) The lineRx (of type *regexp.Regexp; §3.6.5, 120 ➤ ) that is created here is passed to the grep() function and will eventually be shared by all the worker goroutines. This should be a cause of concern since, in general, we must assume that any shared pointed-to value is not thread-safe. In such cases we must provide the safety ourselves,for example, by using a mutex. Alternatively,we could sacrifice a little bit of memory and provide one value (e.g., one regexp) per worker goroutine rather than share a single value. Fortunately, in the specific case of *regexp.Regexp,Go’s documentation says that the pointed-to value is thread-safe. This means that it is safe to share a *regexp.Regexp with as many goroutines as need to use it. var workers = runtime.NumCPU() func grep(lineRx *regexp.Regexp, filenames []string) { jobs := make(chan Job, workers) results := make(chan Result, minimum(1000, len(filenames))) done := make(chan struct{}, workers) go addJobs(jobs, filenames, results) // Executes in its own goroutine for i:=0; i < workers; i++ { go doJobs(done, lineRx, jobs) // Each executes in its own goroutine } go awaitCompletion(done, results) // Executes in its own goroutine processResults(results) // Blocks until the work is done } This function creates the three bidirectional channels needed by the program. The jobs are spread over as many worker goroutinesas the number of processors the machine has, so we make the jobs and done channels’ buffer sizes this number to minimize needless blocking. (Of course, it would be easy to add a command-line option to let the user specify how many worker goroutines to use, regardless of the number of processors.) For the results channel we use a much larger buffer, just as we did for the previous subsection’s filter example, and using a custom minimum() function (not shown; see §, 221 ➤ for possible implementations, or the cgrep.go source code for the one used here). Instead of making the done channel’s type chan bool and not caring whether true or false is sent, since all that matters is whether something is sent, we have made the channel’s type chan struct{} (i.e., an empty struct) to more clearly express our semantics. The only thing we can send on a channel of this type is an empty struct value (struct{}{}), which is just what we need to signify a send where the value doesn’t matter. ptg7913109 7.2. Examples 329 Main goroutine processResults() Goroutine #6 awaitCompletion() Goroutine #1 addJobs() Goroutine #2 doJobs() Goroutine #3 doJobs() Goroutine #4 doJobs() Goroutine #5 doJobs() jobs channel results channel done channel Results Figure 7.5 Multiple independent concurrent jobs With the channels in place we call the addJobs() function to start adding jobs to the jobs channel—this function does its work in a goroutine. Next we call the doJobs() function to perform the actual work: In fact, we call the function four times, so we get four separate executions, each doing their work in their own goroutine. Then we call the awaitCompletion() function which waits—in its own goroutine—for all the work to be done and then closes the results channel. Fi- nally, we call the processResults() function which executes in the main gorou- tine. This function processesresults received from the results channel,blocking when no results are available, and finishing only when all the results have been received. Figure 7.5 shows a schematic view of the program’s concurrency pat- tern. func addJobs(jobs chan<- Job, filenames []string, results chan<- Result) { for _, filename := range filenames { jobs <- Job{filename, results} } close(jobs) } This function sends every filename to the jobs channel (one by one) as a Job value. The jobs channel has a buffer size of four (to match the number of worker goroutines), so the first four jobs are added immediately and then the goroutine in which this function is executed is blocked waiting for a job to be received to free up room in the jobs channel to send another job. Once all the jobs have been sent—which will depend on how many filenames are to be processed and how long the processing takes—the jobs channel is finally closed. ptg7913109 330 Chapter 7. Concurrent Programming Although both of the actual channels passed into the function are bidirectional, we have specified them both to be unidirectional send-only channels since that is how the jobs channel is used inside the function and how the Job.results channel is defined in the Job struct. func doJobs(done chan<- struct{}, lineRx *regexp.Regexp, jobs <-chan Job) { for job := range jobs { job.Do(lineRx) } done <- struct{}{} } This function is called four times in four separate goroutines, so there are four invocations to share the work. Each invocation iterates over the same shared jobs channel (declared as a receive-only channel), and each is blocked (i.e., blocks the goroutine it is executing in) until a job becomes available for it to receive. For each job that an invocation gets to do, the function calls the job’s Job.Do() method (which we will review further on). When an invocation runs out of jobs it signifies that it has finished by sending an empty struct to the done channel (which is declared as a send-only channel). Incidentally, it is a Go convention that functions that have channel parameters have the destination channels first, followed by the source channels. func awaitCompletion(done <-chan struct{}, results chan Result) { for i:=0; i < workers; i++ { <-done } close(results) } This function (along with the processResults() function) ensures that the main goroutine waits until all the processing is done before terminating,thusavoiding the pitfalls mentioned in the previous section (§7.1, 317 ➤ ). It is executed in its own goroutine and waits for as many receives on the done channel as there are workers—blocking its goroutine while it waits. Once the loop terminates the results channel is closed so that the receiver will know when it has received the last result. Note that we cannot pass the results channel as a receive-only chan- nel (<-chan Result), since Go disallows closing such channels. We don’t bother closing the done channel since we never use it in a context where it is checked for being closed. func processResults(results <-chan Result) { for result := range results { fmt.Printf("%s:%d:%s\n", result.filename, result.lino, result.line) ptg7913109 7.2. Examples 331 } } This function executes in the main goroutine, iterating over the results channel —or blocking waiting for results. Once all the results have been received and processed (i.e., printed), the loop will finish, the function will return, and the program will terminate. Go’s concurrency support is so flexible that the approach used here for wait- ing for the jobs to be done, closing the channels, and outputting the results can be done in various other ways. For example, the cgrep2 program (in file cgrep2/cgrep.go) is a variation on the cgrep1 program discussed in this subsec- tion which has no awaitCompletion() or processResults() functions, but instead has a single waitAndProcessResults() function. func waitAndProcessResults(done <-chan struct{}, results <-chan Result) { for working := workers; working > 0;{ select {// Blocking case result := <-results: fmt.Printf("%s:%d:%s\n", result.filename, result.lino, result.line) case <-done: working-- } } DONE: for { select {// Nonblocking case result := <-results: fmt.Printf("%s:%d:%s\n", result.filename, result.lino, result.line) default: break DONE } } } The function begins with a for loop that will execute so long as there are active worker goroutines. Each time the for loop’s select is executed, it blocks waiting to receive a result or a done value. (If we had used a nonblocking select, i.e., one with a default case,we would have effectively created a CPU-wasting spin-lock.) The for loop ends when there are no more active worker goroutines,that is, after all the workers have sent a value to the done channel. Once all the workers have finished, we start a second for loop. Inside this loop we use a nonblocking select. If the results channel has any unprocessed results ptg7913109 332 Chapter 7. Concurrent Programming left, the first case matches, a result is output, and then the for loop executes the select statement again. This repeats until all the unprocessed results have been output. But as soon as there is no result to receive (which could be straight away if the results channel is empty when the for loop is entered), we break to the DONE label. (A bare break is not sufficient since that would only break out of the select statement.) This second for loop isn’t CPU-wasting because at each iteration either there is a result to output or we are finished, so there is no needless waiting. For this particular example the waitAndProcessResults() function is longer and more complicated than the original code’s awaitCompletion() and processRe- sults() functions. However, using select statements can be beneficial when there are several different channels to handle. For example, we could stop pro- cessing after a specified amount of time has passed even if the results are in- complete at that point, by using a select statement. Here is our third and final variation, cgrep3 (in file cgrep3/cgrep.go). func waitAndProcessResults(timeout int64, done <-chan struct{}, results <-chan Result) { finish := time.After(time.Duration(timeout)) for working := workers; working > 0;{ select {// Blocking case result := <-results: fmt.Printf("%s:%d:%s\n", result.filename, result.lino, result.line) case <-finish: fmt.Println("timed out") return // Time's up so finish with what results there were case <-done: working-- } } for { select {// Nonblocking case result := <-results: fmt.Printf("%s:%d:%s\n", result.filename, result.lino, result.line) case <-finish: fmt.Println("timed out") return // Time's up so finish with what results there were default: return } } } ptg7913109 7.2. Examples 333 This is a variation of cgrep2, the difference being that we pass in a timeout value. The time.After() function takes a time.Duration value (essentially a number of nanoseconds), and returns a channel on which it returns the current time after at least time.Duration nanoseconds have passed. Here, we have assigned the returned channel to the finish variable and include a receive case for it in both for loops’ select statements. If we get a timeout (i.e.,if the finish channel sends a value), we return from the function and the program finishes, even if work is still being done. If all the results are gathered before the timeout occurs (i.e., all the workers are done), the first for loop is terminated and the second for loop is started the same as for the cgrep2 example. The only differenceshere are that instead of breaking out of the second for loop, we have simply done a return from the second select’s default case; and we have included a timeout case. Now that we have seen how the concurrency is handled, we will finish our coverage of the cgrep examples by reviewing how each job is processed. func (job Job) Do(lineRx *regexp.Regexp) { file, err := os.Open(job.filename) if err != nil { log.Printf("error: %s\n", err) return } defer file.Close() reader := bufio.NewReader(file) for lino := 1;; lino++ { line, err := reader.ReadBytes('\n') line = bytes.TrimRight(line, "\n\r") if lineRx.Match(line) { job.results <- Result{job.filename, lino, string(line)} } if err != nil { if err != io.EOF { log.Printf("error:%d: %s\n", lino, err) } break } } } This method is used to process each file. It is passed a *regexp.Regexp, a pointer which—unusually for a pointer—is thread-safe, so it doesn’t matter how many different goroutines make use of it. Practically all of the function should be familiar by now: We open the file for reading, handle any error, and if there is no error we defer closing the file. Then we create a buffered reader to make it easy ptg7913109 334 Chapter 7. Concurrent Programming to iterate over the file’s contents line by line. Whenever we get a matching line we send a Result value to the results channel: The send will block if the results channel’s buffer is full. Any file that is processed could produce any number of results, including zero if none of the file’s lines match the regexp. As is common when processing a text file in Go, if an error occurs when reading a line we handle it after working on the line. If the bufio.Reader.ReadBytes() method encounters an error (including end of file), it returns any bytes read prior to the error along with the error. Sometimes the very last line of a text file doesn’t end with a newline, so to make sure that we process the very last line (whether or not it ends in a newline), we handle the error after processing the line. The disadvantage of handling the error afterward is that if the regexp can match an empty string and we get a non-nil not-io.EOF error, we will get a spurious match. (This can be worked around, of course.) The bufio.Reader.ReadBytes() method returns the bytes up to and including the specified byte (or up to the end of the file if the specified byte isn’t present). We don’t want the newline, so we remove it using the bytes.TrimRight() method which removesfrom the right of the given line,the given character or characters. (This is just like the strings.TrimRight() function; 109 ➤ .) To make our program work cross-platform we trim both newline and carriage return characters. Another small detail to note is that we read the lines as byte slices and do the matching using the regexp.Regexp.Match() method rather than regexp.Reg- exp.MatchString(). So we only do the (very cheap) conversion from []byte to string for matching lines. Also, we count line numbersfrom one rather than zero since that is conventional. A particularly nice aspect of the cgrep programs’design isthat their concurrency frameworks are both simple and separate from the actual processing (which is done by the Job.Do() method), with the only connection between these two as- pects being the results channel. This separation of concerns is common in con- current Go programs and compares very favorably with the use of lower-level concurrency constructs (such as mutexes) where the locking and unlocking code is often needed throughout the program and can obscure and complicate the program’s logic. 7.2.3. Example: Thread-Safe Map Go’s sync and sync/atomic packages provide the low-level operations needed to create concurrent algorithms and data structures. However, it is also possible to take an existing data structure—such as a map or slice (or an omap.Map; §6.5.3, 302 ➤ )—and make it thread-safe by ensuring that all accesses are serialized using Go’s high-level channels. In this subsection we will develop a thread-safe map that has string keys and interface{} values (i.e., any values) and that can safely be shared by as many ptg7913109 7.2. Examples 335 goroutines as we like—and without a lock in sight. (Of course, if we store and retrieve pointer or reference values, they must be treated as read-only or access- es to them must be serialized.) The thread-safe map is in file safemap/safemap.go and consists of an exported SafeMap interface that specifies the methods that the safe map supports, and an unexported safeMap concrete type that fulfills the interface. We will see the safe map in action in the next subsection (§7.2.4, ➤ 341). The safe map is implemented by executing an unexported method that captures a map inside a goroutine. The only way to accessthe map is via channels,and this in itself is sufficient to ensure that all map accesses are serialized. The method runs an infinite loop on an incoming channel that blocks waiting for commands (i.e., “insert this”, “delete that”, etc.). We will begin by looking at the SafeMap interface, then at the safeMap type’s exported methods, then at the safemap package’s New() function, and finally at the unexported method. type SafeMap interface { Insert(string, interface{}) Delete(string) Find(string) (interface{}, bool) Len() int Update(string, UpdateFunc) Close() map[string]interface{} } type UpdateFunc func(interface{}, bool) interface{} All these methods are provided by the safeMap type. (We discussed the pattern of having an exported interface and an unexported concrete type in the previous chapter.) The UpdateFunc type makes it convenient to specify the signature of an update function: We will discuss this when we cover the Update() method further on. type safeMap chan commandData type commandData struct { action commandAction key string value interface{} result chan<- interface{} data chan<- map[string]interface{} updater UpdateFunc } type commandAction int ptg7913109 336 Chapter 7. Concurrent Programming const ( remove commandAction = iota end find insert length update ) The safeMap type is based on a channel that can send and receive values of the custom commandData type. Each commandData value specifies the action that is to be taken and also the data necessary to perform the action—for example, most of the methods require a key to identify which item to work on. We will see all of the fields in use as we review the safeMap’s methods. Notice that both the result and data channels are declared as unidirectional send-only channels; in other words, the safe map itself can send values to them, but cannot receive from them. As we will see, the methods that create these channels create them as bidirectional channels and so these channels are able to receive whatever the safe map sends to them. func (sm safeMap) Insert(key string, value interface{}) { sm <- commandData{action: insert, key: key, value: value} } This method is the safe map equivalent of m[key]=value where m is a map[string] interface{}. It creates a commandData value with the insert action and with the given key and value and sends the command to the safe map—which as we have just seen is a type based on a chan commandData.(We covered creating structs with some values initialized and the others set to their zero values by Go earlier;§6.4, 275 ➤ .) When we review the safemap package’s New() function we will see that the safeMap returned by the New() function (as a SafeMap) is associated with a goroutine. The method is executed in the goroutine, and is a closure which has captured the safeMap channel. The method also contains the underlying map that is used to store the safe map’s items, and has a for loop which iterates over the safeMap channel and that performs each command it receives from the channel on the underlying map. func (sm safeMap) Delete(key string) { sm <- commandData{action: remove, key: key} } This method tells the safe map to delete the item with the given key—or to safely do nothing. ptg7913109 7.2. Examples 337 type findResult struct { value interface{} found bool } func (sm safeMap) Find(key string) (value interface{}, found bool) { reply := make(chan interface{}) sm <- commandData{action: find, key: key, result: reply} result := (<-reply).(findResult) return result.value, result.found } The safeMap.Find() method creates its own reply channel so that it can receive a response from the safe map—which may only send to the reply channel—and then sends a find command with the given key and with its own reply channel to the safe map. Since none of the channelsare buffered,when we send a command the send is blocked until no other goroutines are sending to the safe map. Once the command has been sent we immediately receive back the reply (which for a find command comes packaged up as a findResult struct), whose constituents we return to the caller. Incidentally, the use of names for the return values is purely to make their purpose obvious. func (sm safeMap) Len() int { reply := make(chan interface{}) sm <- commandData{action: length, result: reply} return (<-reply).(int) } This method has a similar structure to the Find() method in that it creates and sends its own reply channel and passes on the received reply to its caller. func (sm safeMap) Update(key string, updater UpdateFunc) { sm <- commandData{action: update, key: key, updater: updater} } This method seems somewhat incongruous with its unusual signature whose second argument is of type func(interface{}, bool) interface{} (335 ➤ ). The method sends an update command with the given key and with an updater function to the safe map. Once the command is received the updater function is called with the value of the item with the given key (or nil if there is no such item) and a bool indicating whether the item exists. The item’s value is set to the updater function’s return value (with a new item with the given key and the returned value being created if necessary). ptg7913109 338 Chapter 7. Concurrent Programming It is important to note that if the updater function calls a safeMap method, then we will get a deadlock! We will explain why when we cover the method further on. But why do we need this strange method, and how do we use it? When it comes to inserting, deleting, or finding items from the safe map, we can use the Insert(), Delete(), and Find() methods perfectly well. But what happens when we want to update an existing item? For example, what if we are using the map to keep the prices of various parts and want to increase the price of a part by 5%? With a normal map we can simply write m[key]*= 1.05,knowing that thanks to Go’s automatic zero-value setting, if an item with the given key exists its value will be incremented by 5%, and otherwise a new item with the key will be created with a value of zero. Here is how we might try to achieve the same thing using a safe map that holds float64 values. if price, found := priceMap.Find(part); found {// WRONG! priceMap.Insert(part, price.(float64)*1.05) } The problem with the code shown here is that it is possible that one or more other goroutines sharing the same priceMap could change the map in between the Find() and Insert() calls shown here, so there is no guarantee that at the time of the Insert() the price we are inserting really is exactly 5% more than the original price. What we need to do is an atomic update, that is, to retrieve and update the value as a single uninterruptible operation. This is what the Update() method allows us to do. priceMap.Update(part, func(price interface{}, found bool) interface{} { if found { return price.(float64) * 1.05 } return 0.0 }) This code snippet shows how to do an atomic update. If there is no item with the given key a new item with a value of 0.0 will be created; otherwise the existing item’s value will be incremented by 5%. Since this update takes place in the safe map’sgoroutine in response to an update command,no other command (e.g., from another goroutine) can be executed in the middle. func (sm safeMap) Close() map[string]interface{} { reply := make(chan map[string]interface{}) sm <- commandData{action: end, data: reply} ptg7913109 7.2. Examples 339 return <-reply } The Close() method works in a similar way to the Find() and Len() methods, only it serves two different purposes. First, it closes the safeMap channel (inside the method), so that no further updates can take place. This will cause the for … range loop in the method to terminate and will therefore free up the goroutine in which it is executing for garbage collection. Second, it returns the underlying map[string]interface{} which the caller can keep or ignore. The Close() method can only ever be called once per safe map, no matter how many goroutines are accessing the safe map, and once called no other method can be called. This means that if we keep the returned map we can safely access it just like a normal map (i.e., in a single goroutine). We have now completed our review of the safeMap’s exported methods. The last things we need to look at are the safemap package’s New() function in which a safeMap is created and returned as a SafeMap for use outside the package, and the method which captures the channel, provides the map[string] interface{} to store the data, and handles all the communications. func New() SafeMap { sm := make(safeMap) // type safeMap chan commandData go return sm } The safeMap is of type chan commandData, so we must use the built-in make() function to create the channel and return a reference to it. Once we have the safe map we call its unexported run() method to create the map used for storage and to receive commands. The run() method is executed in its own goroutine, and as usual the go statement returns immediately. At the end, the function returns the safeMap as a SafeMap interface to the New() function’s caller. func (sm safeMap) run() { store := make(map[string]interface{}) for command := range sm { switch command.action { case insert: store[command.key] = command.value case remove: delete(store, command.key) case find: value, found := store[command.key] command.result <- findResult{value, found} case length: ptg7913109 340 Chapter 7. Concurrent Programming command.result <- len(store) case update: value, found := store[command.key] store[command.key] = command.updater(value, found) case end: close(sm) <- store } } } Once the storage map is created, the run() method starts an effectively infinite loop that iterates over the safe map channel, blocking if there is no command to receive. Since the store is no more than an ordinary map, all the actions that correspond to each command that isreceived are straightforwardto understand. One slight- ly tricky case is update where the item’s value is set to the return value of the command.updater() function (as we saw earlier; 337 ➤ ). The end case corresponds to a Close() call; this first closes the safe map channel to prevent any further commands being received, and then sends the storage map back to the caller. Note that if the command.updater() function were to call a safeMap method we will get a deadlock. This is because the update case cannot finish until the com- mand.updater() function returns, but if the function has called a safeMap method, that call will be blocked waiting to be processed until the update case has fin- ished, so neither will be able to finish. This kind of deadlock was illustrated ear- lier in Figure 7.2 (318 ➤ ). Clearly, there is a certain amount of overhead using a safe map compared with an ordinary map.Eachcommand requiresthe creation of a commandData struct and for this to be sent through a channel, with sends from however many goroutines we create to access the safe map being automatically serialized under the hood by Go. One alternative is to use an ordinary map but protect accesses to it with a sync.Mutex or a sync.RWMutex. Another alternative is to create a custom thread- safe data structure such as those described in the literature (e.g., see The Art of Multiprocessor Programming in Appendix C, ➤ 441). Another alternative is to keep goroutine-specific maps so that no synchronization is needed at all—and then to just merge the maps at the end. Nonetheless, the safe map described here is easy to use and may prove sufficient for many purposes. We will see the safe map in action in the next subsection, along with a couple of the alternative approaches for comparison. ptg7913109 7.2. Examples 341 7.2.4. Example: Apache Report A frequent requirement in concurrent programs is for the concurrent processing to update a shared data structure. One common solution is to use a mutex to serialize accesses to the data structure. In Go, we can either use a mutex or use channels to serialize accesses. In this subsection we will begin by showing an approach that uses channels and the generic safe map developed in the previous subsection. Then we will look at how to achieve the same thing using a shared map protected by a mutex. And finally, we will review how to use local maps that are independent and therefore don’t need serialized accesses so as to maximize throughput, with channels used to serialize the updating of a common map at the end. The apachereport programs all do the same thing: They read an Apache web server’s access.log file specified on the command line and output the number of accesses to each unique HTML page that the log has recorded. These log files can easily grow huge, so we have used one separate goroutine to read lines from the file and three additional goroutines to share the processing of the lines. Each HTML page must be added to a map with a count of 1 the first time it is seen, and this count must be incremented each time the page is seen again. So, although multiple goroutines are independently processing lines from the log file, they must all update the same map. Each version of the program handles the updating of the map in a different way. Synchronizing with a Shared Thread-Safe Map In this subsubsection we will review the apachereport1 program (in file apachere- port1/apachereport.go). This program uses the safe map developed in the pre- vious section to provide a shared thread-safe map. The program’s concurrency structure is illustrated in Figure 7.6 (➤ 342). In the figure, goroutine #2 is used to populate the work channel with lines from the log file and goroutines #3 to #5 process each line and update the shared safeMap. The operations on the safeMap itself take place in yet another goroutine, so the program uses six goroutines in total. var workers = runtime.NumCPU() func main() { runtime.GOMAXPROCS(runtime.NumCPU()) // Use all the machine's cores if len(os.Args) != 2 || os.Args[1]=="-h" || os.Args[1]=="--help" { fmt.Printf("usage: %s \n", filepath.Base(os.Args[0])) os.Exit(1) } lines := make(chan string, workers*4) done := make(chan struct{}, workers) ptg7913109 342 Chapter 7. Concurrent Programming Main goroutine waitUntil() showResults() Goroutine #1 safeMap Goroutine #2 readLines() Goroutine #3 processLines() • anon. #1 Goroutine #4 processLines() • anon. #2 Goroutine #5 processLines() • anon. #3 lines channel done channel safeMap channel Results Figure 7.6 Multiple inter-dependent concurrent jobs with synchronized results pageMap := safemap.New() go readLines(os.Args[1], lines) processLines(done, pageMap, lines) waitUntil(done) showResults(pageMap) } The main() function begins by making sure that the Go runtime system will use all of the machine’s processors. Then it creates the two channels needed to organize the processing. The lines channel will be sent each line from the log file and the worker goroutineswill each receive lines from this channel. We have given the lines channel a small buffer to reduce the likelihood of the worker goroutines being blocked waiting for lines. The done channel is used to keep track of when all the work is done, and since we only care about the occurrence of sends and receives rather than their values, we use empty structs. The done channel is buffered so that it doesn’t block when a goroutine wants to announce that it has finished its work. With the channels in place we create a new unexported safeMap which is returned by the safemap.New() function as an exported SafeMap interface that can freely be passed around. We then start off the goroutine for reading lines from the files and then the goroutines for processing the lines. Then we wait for all the worker goroutines to be finished, after which we can output the results. func readLines(filename string, lines chan<- string) { file, err := os.Open(filename) if err != nil { log.Fatal("failed to open the file:", err) ptg7913109 7.2. Examples 343 } defer file.Close() reader := bufio.NewReader(file) for { line, err := reader.ReadString('\n') if line != "" { lines <- line } if err != nil { if err != io.EOF { log.Println("failed to finish reading the file:", err) } break } } close(lines) } This function should be very familiar since it is so similar to ones we have seen before. The first key aspect is that each line is sent to the lines send-only channel—this will block if the channel’s buffer becomes full, until another go- routine receives from the channel. Naturally, even if there is blocking, it only affects this goroutine; all the others will continue unaffected. The second key aspect is that once all the lines have been sent we close the lines channel; this tells prospective receivers that there is no more data to receive. Keep in mind, though, that this goroutine executes concurrently with the program’s other goroutines—in particular,the line processing worker goroutines—so the close() statement isn’t reached until most of the work has been done. func processLines(done chan<- struct{}, pageMap safemap.SafeMap, lines <-chan string) { getRx := regexp.MustCompile(`GET[ \t]+([^ \t\n]+[.]html?)`) incrementer := func(value interface{}, found bool) interface{} { if found { return value.(int) + 1 } return 1 } for i:=0; i < workers; i++ { go func() { for line := range lines { if matches := getRx.FindStringSubmatch(line); matches != nil { pageMap.Update(matches[1], incrementer) } ptg7913109 344 Chapter 7. Concurrent Programming } done <- struct{}{} }() } } This function follows the Go parameter ordering convention of having destina- tion channels first (in this case the done channel), followed by the source chan- nels (in this case the lines channel). The function creates the goroutines (three in this example) that do the work. Each goroutine shares the same *regexp.Regexp (since, unusually for pointed-to values, this is documented to be thread-safe), and the same incrementer() function (since it has no side effects and doesn’t access any shared data). They also share the same pageMap (of type SafeMap interface), since even the methods that modify a SafeMap are thread-safe. The regexp.Regexp.FindStringSubmatch() function returns nil if there is no match, or a []string where the first item is the entire matching text and each subsequent item corresponds to the text that matches a parenthesized subex- pression in the regular expression. In this case we have one such subexpression, so if we get a match at all, the []string will have exactly two items in it, the en- tire matching text and the text matched by the parenthesized subexpression, in this case the filename of an HTML page. Each worker goroutine receives different lines that are read by the goroutine created in the readLines() function from the lines receive-only channel. If a line matches the regular expression that identifies GET requests on HTML files, the safeMap.Update() method is called with the page filename (matches[1]) and with the incrementer() function. The incrementer() function is executed inside the safe map’s goroutine and returns an incremented value for existing pages and a value of 1 for pages that are new to the map. (Recall from the previous subsection that if the function passed to the safeMap.Update() method calls any other SafeMap method we will get a deadlock.) When there are no more pages to process, each worker goroutine sends an empty struct to the done send-only channel to signify that it has finished its work. func waitUntil(done <-chan struct{}) { for i:=0; i < workers; i++ { <-done } } This function executes in the main goroutine and blocks waiting on the done receive-only channel. When every worker goroutine has sent an empty struct to the done channel, the for loop finishes. As usual, we don’t bother to close the done ptg7913109 7.2. Examples 345 channel since we never use it in a context where it is checked for being closed. By blocking, this function ensures that the processing gets done before the main goroutine terminates. func showResults(pageMap safemap.SafeMap) { pages := pageMap.Close() for page, count := range pages { fmt.Printf("%8d %s\n", count, page) } } Once all the lines have been read and all the matches added to the safe map, this function is called to output the results. It begins by calling the safemap. safeMap.Close() method which closes the safe map’s channel, terminates the method executing in a goroutine, and returns the underlying map[string]interface{} to the caller. The returned map can no longer be accessed using the safe map’s channel and so can safely be used in a single goroutine (or multiple goroutines if we serialize accesses using a mutex). From this point, we only access the map in the main goroutine, so no serialization is necessary. We simply iterate over the map’s key–value pairs and print them to the console. Using a value of type SafeMap interface providesthread safety and simple syntax with no need to concern ourselveswith locks. One disadvantageof thisapproach is that the safe map’s values are generic interface{} values rather than of a spe- cific type, hence the need to use a type assertion in the incrementer() function. (We will discuss another disadvantage further on; §, ➤ 347.) Synchronizing with a Mutex-Protected Map We will now compare and contrast the clean channel-based approach to the tra- ditional mutex-based approach. To do this we will briefly discuss the apachere- port2 program (in file apachereport2/apachereport.go). This is a variation of the apachereport1 program that uses a custom data type which encapsulates a map and a mutex instead of using a SafeMap. The two programs do the same job but are different in that the apachereport2 program stores int values in its map rather than the interface{}s used by the generic SafeMap, and also only provides the minimal and specific functionality required to do the job—an Increment() method—compared to the safe map’s complete set of methods. type pageMap struct { countForPage map[string]int mutex *sync.RWMutex } One advantage of using a specific custom type is that we can use the exact data types that we need rather than having to use the generic interface{} type. ptg7913109 346 Chapter 7. Concurrent Programming func NewPageMap() *pageMap { return &pageMap{make(map[string]int), new(sync.RWMutex)} } This function returns a ready-to-use *pageMap.(Incidentally, we could have created the read-write mutex pointer using the syntax &sync.RWMutex{} instead of new(sync.RWMutex); we discussed this equivalence in §4.1, 140 ➤ .) func (pm *pageMap) Increment(page string) { pm.mutex.Lock() defer pm.mutex.Unlock() pm.countForPage[page]++ } Every method that modifies the countForPage map must serialize its accesses to the map using the mutex. The pattern used here is canonical: Lock the mutex; defer unlocking the mutex so that whenever we return, the unlocking is guaranteed (even in the face of a panic); and perform the access—ideally for as short a time as possible. Thanks to Go’s automatic zero initialization, the first time a page is accessed in the countForPage map (i.e., when it isn’t in the map), it is added to the map with a value of 0, which is then immediately incremented. Correspondingly, on subsequent accesses to a page that is already in the map, its existing value will be incremented. Every method that accesses the countForPage map must serialize that access us- ing a mutex. For updates to the map, the sync.RWMutex.Lock() and sync.RWMutex. Unlock() methods shown here must be used, but for read-only accesses, we can use different read-only methods. func (pm *pageMap) Len() int { pm.mutex.RLock() defer pm.mutex.RUnlock() return len(pm.countForPage) } This method is included purely to show how to use a read lock. The pattern of use is the same as for a normal lock, but read locks are potentially more efficient (since we are promising to read but not update the protected resource). For example, if we have multiple goroutines all reading the same countForPage map, they can all safely execute concurrently using a read lock. But if even one of them acquires a normal (read-write) lock—and hence the ability to change the map—then no other lock can be granted. pageMap.Increment(matches[1]) ptg7913109 7.2. Examples 347 With the pageMap type in place the worker goroutines can update the shared map using the statement shown here. Synchronizing by Merging Local Maps via Channels Whether we use a safe map or a plain map protected by a mutex, we might reasonably expect that by increasing the number of worker goroutines we would increase the overall speed of the program. However, since accesses to the safe map or to the plain map are serialized (behind the scenes in the case of the safe map), as we add goroutines, contention will increase. As is often the case, we can trade some memory in exchange for increased speed. So, for example, we could let every worker goroutine have its own plain map. This would maximize throughput since there would be no contention at all during processing, but at the price of using extra memory (since most likely every map would have some or even all pages in common). At the end, of course, we would have to merge these maps—and that could be a bottleneck since,while one map was being merged, any others that were ready to merge would have to wait. The apachereport3 program (in file apachereport3/apachereport.go) uses local goroutine-specific plain maps and merges them into a single overall map at the end. The code is almost identical to the apachereport1 and apachereport2 programs, so we will very briefly review the key differences. The program’s concurrency structure is illustrated by Figure 7.7. Main goroutine merge() showResults() Goroutine #1 readLines() Goroutine #2 processLines() Goroutine #3 processLines() Goroutine #4 processLines() lines channelresults channel Results Figure 7.7 Multiple inter-dependent concurrent jobs with synchronized results // ... lines := make(chan string, workers*4) results := make(chan map[string]int, workers) ptg7913109 348 Chapter 7. Concurrent Programming go readLines(os.Args[1], lines) getRx := regexp.MustCompile(`GET[ \t]+([^ \t\n]+[.]html?)`) for i:=0; i < workers; i++ { go processLines(results, getRx, lines) } totalForPage := make(map[string]int) merge(results, totalForPage) showResults(totalForPage) // ... This is an extract from apachereport3’s main() function. Instead of having a done channel we have a results channel through which the local goroutine-specific maps are sent once they have been fully populated. We also create an overall map (totalForPage) into which all the results will be merged. func processLines(results chan<- map[string]int, getRx *regexp.Regexp, lines <-chan string) { countForPage := make(map[string]int) for line := range lines { if matches := getRx.FindStringSubmatch(line); matches != nil { countForPage[matches[1]]++ } } results <- countForPage } This function is almost the same as the version shown before. The key differ- ences are first that we have created a local map to hold page counts, and second that we return the local map to the results channel (rather than sending, say, a struct{}{} to a done channel), once the function has finished processing lines. func merge(results <-chan map[string]int, totalForPage map[string]int) { for i:=0; i < workers; i++ { countForPage := <-results for page, count := range countForPage { totalForPage[page] += count } } } This function is structurally the same as the waitUntil() function we saw earlier, only this time we make use of the received value, using it to update the overall totalForPage map. Note that the maps received here are never accessed by their sending goroutines after they have been sent, so no locking is necessary. ptg7913109 7.2. Examples 349 The showResults() function is almost the same as before (so isn’t shown), only it accepts the totalForPage map as its argument and iterates over this map printing counts and pages. The apachereport3 program’s code is simpler and cleaner than that needed by apachereport1 and apachereport2, and the concurrency model it uses—each goroutine populating its own uncontended data structure and merging results at the end—should prove useful in many different contexts. Of course, it is only natural for programmers used to the locking paradigm to be tempted to use mutexes to serialize accesses in Go programs. However, the Go documentation strongly recommends the use of goroutines and channels with its mantra of “don’t communicate by sharing memory,instead, share memory by communicating”,and Go compilers are being optimized above all to support this concurrency model. 7.2.5. Example: Find Duplicates This chapter’s final concurrency example is a program that attempts to find duplicate files using file sizes and SHA-1 values rather than filenames.★ The program we will review is called findduplicates (in file findduplicates/find- duplicates.go).The program usesthe standard library’sfilepath.Walk() function to iterate over all the files and directories in a given path, including subdirecto- ries, and subsubdirectories, and so on. The program uses a variable number of goroutines depending on the work it does. For each “big” file, a new goroutine is created on the fly to compute that file’s SHA-1, while each “small” file’s SHA-1 is computed in the current goroutine. This means that we don’t know in advance how many goroutines any particular run of the program will use, although we can—and do—set an upper limit. One way of handling a variable number of goroutines is to use a “done” channel just like in earlier examples, only this time keeping a running count of how many goroutines are created. An easier way is to use a sync.WaitGroup which achieves the same thing but passes on the counting to Go. const maxGoroutines = 100 func main() { runtime.GOMAXPROCS(runtime.NumCPU()) // Use all the machine's cores if len(os.Args) == 1 || os.Args[1]=="-h" || os.Args[1]=="--help" { fmt.Printf("usage: %s \n", filepath.Base(os.Args[0])) os.Exit(1) } ★ The SHA-1 secure hash algorithm produces a 20-byte value for any given chunk of data—such as a file. Identical files will have the same SHA-1 values and different files will almost always have different SHA-1 values. ptg7913109 350 Chapter 7. Concurrent Programming infoChan := make(chan fileInfo, maxGoroutines*2) go findDuplicates(infoChan, os.Args[1]) pathData := mergeResults(infoChan) outputResults(pathData) } The main() function accepts a directory to start the processing from and orches- trates all the work. It begins by creating a channel for passing fileInfo values (which we will look at shortly). We have buffered the channel because experi- ments showed that this consistently improved performance. Next, the function executes the findDuplicates() function in a goroutine, and then calls the mergeResults() function that reads the infoChan channel until it is closed. Once the merged data is returned, the results are output. The program’s goroutines and flow of communications are illustrated in Fig- ure 7.8. The figure’s results channel is for values of type fileInfo; these values are sent to the infoChan channel by a “walker” function (of type filepath.Walk- Func) that we pass to the filepath.Walk() function. The filepath.Walk() func- tion is itself called in the findDuplicates() function. The results are received by the mergeResults() function. The goroutines in the figure are created by the findDuplicates() function and by the walker function. In addition, the standard library’s filepath.Walk() function might create its own goroutines (e.g., one to process each directory), although how it actually works is an implementation detail. type fileInfo struct { sha1 []byte size int64 path string } This type is used to summarize the data for each file. If two files’ SHA-1s and sizes are the same they are considered to be duplicates—no matter what their paths or filenames are. func findDuplicates(infoChan chan fileInfo, dirname string) { waiter := &sync.WaitGroup{} filepath.Walk(dirname, makeWalkFunc(infoChan, waiter)) waiter.Wait() // Blocks until all the work is done close(infoChan) } This function calls the filepath.Walk() function to walk the directory’s tree (starting at dirname), and for every file and directory it calls the filepath.Walk- Func function it is passed as its second argument. ptg7913109 7.2. Examples 351 Main goroutine mergeResults() outputResults() WaitGroup Goroutine #1 findDuplicates() Goroutine #2 processFile() Goroutine #3 processFile() … Goroutine #N processFile() fileInfo channel Figure 7.8 Multiple independent concurrent jobs with synchronized results Our walker function will create an arbitrary number of goroutines, so we need to make sure that we wait until they have all finished before the findDupli- cates() function returns. We have done this by creating a sync.WaitGroup. Each time we create a new goroutine we call sync.WaitGroup.Add(), and each time a goroutine completes its work we call sync.WaitGroup.Done(). Once all the go- routines have been set running we then wait for them all to finish by calling sync.WaitGroup.Wait()—which blocks until the number of done announcements equals the number added. Once all the worker goroutines are finished there will be no more fileInfo values sent to the infoChan,so we close the channel.Naturally,the mergeResults() function will still be able to read the channel, until it has read all the channel’s items. const maxSizeOfSmallFile = 1024 * 32 func makeWalkFunc(infoChan chan fileInfo, waiter *sync.WaitGroup) func(string, os.FileInfo, error) error { return func(path string, info os.FileInfo, err error) error { if err == nil && info.Size() > 0 && (info.Mode()&os.ModeType == 0){ if info.Size() < maxSizeOfSmallFile || runtime.NumGoroutine() > maxGoroutines { processFile(path, info, infoChan, nil) } else { waiter.Add(1) go processFile(path, info, infoChan, func() { waiter.Done() }) ptg7913109 352 Chapter 7. Concurrent Programming } } return nil // We ignore all errors } } This function creates and returns an anonymous function of type filepath.Walk- Func (i.e., that has signature func(string, os.FileInfo, error) error). This func- tion will be called for every file and directory that the filepath.Walk() function encounters. The path is the file or directory name, the info has the results of a stat call on the path, and the err either is nil or has the details of any problem that was encountered regarding the path. We can skip directories by returning an error value of filepath.SkipDir, and we can stop the walk by returning any other non-nil error. We have chosen to process only regular files of nonzero size. (Of course, all files of size 0 bytes are the same, but we ignore such files.) The os.ModeType is a bitmask that has bits set for directories, symbolic links, named pipes, sockets, and devices—so if none of these are set we have a regular file. If the file is “small” (in this case of up to 32KiB), we compute its SHA-1 imme- diately using a custom processFile() function. But for all other files we create a new goroutine in which the processFile() function is executed asynchronously. This means that small files will block (until we’ve computed their SHA-1), but big files will not since their computation is done in a separate goroutine. Either way, when the computation is complete the resultant fileInfo value is sent to the infoChan channel. We need only call the sync.WaitGroup.Add() method when we create a new goroutine, but whenever we do so we must be sure to call the corresponding sync.WaitGroup.Done() method once the goroutine has finished its work. We do this by taking advantage of Go’s support for closures (§5.6.3, 225 ➤ ). If we call the processFile() function in a new goroutine we pass an anonymous function as the last argument—when this anonymous function is called it will call the sync.WaitGroup.Done() method. The processFile() function is expected to do a deferred call on this anonymous function to ensure that Done() gets called when the goroutine finishes. If we call the processFile() method in the current gorou- tine, we pass nil instead of an anonymous function. Why didn’t we simply process every file in a new goroutine? Go would have no problem with that since we can create hundreds or thousands of goroutines without hitting any problems. Unfortunately, most operating systems impose a limit as to how many files may be open at the same time. For Windows the default limit could be as small as 512 although this can be increased to 2048. Mac OS X systems may have a default limit as low as 256 files, and Linux sys- tems a default limit of 1024—however,Unix-like systems like these can usually ptg7913109 7.2. Examples 353 have their limit set to tens or hundreds of thousands or more. Clearly, if we cre- ated one goroutine per file we could easily exceed the limit on some platforms. We have avoided the “too many open files” problem by combining two tactics. First, we process small files in one goroutine (or perhaps in just a few goroutines if the filepath.Walk() function happens to spread its work over multiple gorou- tines and calls the walker function in them concurrently). This ensures that if we hit a directory with thousands of tiny files we don’t have too many open at once because only one or a few goroutines can process them at the same time. We should be able to afford to process big files in their own goroutines since the very fact that they are large means that we are unlikely to be able to be fast enough to open too many of them at the same time. But if we do end up processing too many big files, that is, if we start to have too many goroutines, the second tactic will kick in. (The runtime.NumGoroutine() function tells us how many goroutines exist at the time the function is called.) If we have too many goroutines we stop creating new goroutines for processing big files, and instead process every file—no matter what size it is—in the cur- rent goroutine. This forces the program to use the same goroutine (or same few goroutines) to process each subsequent file, and thus keeps a lid on the number of goroutines, and as a consequence also limits the number of files that we have open at the same time. Meanwhile,as each of the goroutines already processing big files finish and are removed by Go’s runtime system, the number of gorou- tines in use will decrease. So, at some point, there will be fewer goroutines than our limit, and we will once again start creating new goroutines to process big files. func processFile(filename string, info os.FileInfo, infoChan chan fileInfo, done func()) { if done != nil { defer done() } file, err := os.Open(filename) if err != nil { log.Println("error:", err) return } defer file.Close() hash := sha1.New() if size, err := io.Copy(hash, file); size != info.Size() || err != nil { if err != nil { log.Println("error:", err) } else { log.Println("error: failed to read the whole file:", filename) ptg7913109 354 Chapter 7. Concurrent Programming } return } infoChan <- fileInfo{hash.Sum(nil), info.Size(), filename} } This function is called from the current goroutine or in a newly created addition- al goroutine to compute the given file’s SHA-1 and to send the file’s details to the infoChan channel. If the done variable is not nil, this function has been called in a new goroutine, so we defer a call to the done() function (which simply contains a call to the sync.WaitGroup.Done() method;352 ➤ ).This is to ensure that for every sync.Wait- Group.Add() call, there is a corresponding Done() call, since this is essential to the correct operation of the sync.WaitGroup.Wait() function. If the variable is nil we safely ignore it. Next, we open the given file for reading, and defer closing it, in the usual way. The standard library’s crypto/sha1 package provides the sha1.New() function which returns a value that satisfies the hash.Hash interface. This interface provides a Sum() method that returns the hash value (i.e., the 20-byte SHA-1 hash in this case), and also fulfills the io.Writer interface.(We pass in nil to make the Sum() method give us a new []byte; alternatively, we could pass it an existing []byte to which it will append the hash sum.) We could have read in the entire contents of the file and then called the sha1.Write() method to write the file into the hash, but we have opted to take a much more efficient approach and used the io.Copy() function instead. This function takes a writer to write to (in this case the hash), and a reader to read from (here, the open file), and copies from the latter to the former. When the copy has finished, io.Copy() returns the number of bytes copied and either nil or an error if a problem occurred. Since the SHA-1 hash can work on chunks of data at a time, the maximum amount of memory used by the io.Copy() call will be the size of the buffer used by the SHA-1 plus some fixed overhead. If we had read the entire file into memory first, we would have used the same amount of buffer and overhead memory—plus enough memory to store the whole file. So, especially for larger files, using io.Copy() achieves real savings. Once the computation is done we send a fileInfo value to the infoChan channel with the SHA-1 value, the file’s size (available from the os.FileInfo passed to the processFile() function from the walker function), and the filename (including the full path). type pathsInfo struct { size int64 paths []string } ptg7913109 7.2. Examples 355 This struct type is used to store the details of each duplicate file, that is, its size, and all the file’s paths and filenames. It is used by the mergeResults() and outputResults() functions. func mergeResults(infoChan <-chan fileInfo) map[string]*pathsInfo { pathData := make(map[string]*pathsInfo) format := fmt.Sprintf("%%016X:%%%dX", sha1.Size*2)// == "%016X:%40X" for info := range infoChan { key := fmt.Sprintf(format, info.size, info.sha1) value, found := pathData[key] if !found { value = &pathsInfo{size: info.size} pathData[key] = value } value.paths = append(value.paths, info.path) } return pathData } This function begins by creating a map to store duplicate files. The keys are strings composed of the file’s size, a colon, and the file’s SHA-1, and the values are *pathsInfos. To produce the keys we have created a format string which has 16 zero-padded hexadecimal digits to represent the file’s size and enough hexadecimal digits to represent the file’s SHA-1.We have used leading zeros for the file size part of the key so that we can sort the keys by size later on. The sha1.Size constant holds the number of bytes occupied by an SHA-1 (i.e., 20). Since one byte represented in hexadecimal has two digits, we must use twice as many characters as there are bytes for the SHA-1 in the format string. (Incidentally,we could have created the format string with format = "%016X:%" + fmt.Sprintf("%dX", sha1.Size*2).) Although multiple goroutines send to the infoChan channel, this function (in the main goroutine) is the only one that receives from the channel. The for loop receives fileInfo values—or blocks waiting. The loop ends when all the values have been received and the infoChan channel is closed. For each fileInfo value received, a map key string is computed. If the map doesn’t have an item with the key, we create a suitable value with the given file size, and with an empty slice of paths, and add it to the map under the new key. Then, with the new or existing item in the map, we append to the item’s paths the path of the file in the fileInfo value. And at the end, this will result in duplicate files having path slices with more than one path, and nonduplicates having path slices with exactly one path. Once all the fileInfo values have been processed and the map populated, the function returns the map ready for further processing. ptg7913109 356 Chapter 7. Concurrent Programming func outputResults(pathData map[string]*pathsInfo) { keys := make([]string, 0, len(pathData)) for key := range pathData { keys = append(keys, key) } sort.Strings(keys) for _, key := range keys { value := pathData[key] if len(value.paths) > 1 { fmt.Printf("%d duplicate files (%s bytes):\n", len(value.paths), commas(value.size)) sort.Strings(value.paths) for _, name := range value.paths { fmt.Printf("\t%s\n", name) } } } } The pathData map’skeys are stringsthat begin with each file’s size in 16 hexadec- imal digits padded with leading zeros. (16 digits was chosen since this is large enough to represent an int64.) This means that by retrieving and sorting the keys we can get the files in size order from smallest to biggest. So, the function begins by creating a keys slice to which each of the map’s keys are appended, after which the keys are sorted. Then, the function iterates over the size-sorted slice of keys, retrieving the corresponding pathsInfo value. For those that have more than one path the files’ size is output followed by an indented list of the duplicate files in path-sorted order, as shown below. $ ./findduplicates $GOROOT 2 duplicate files (67 bytes): /home/mark/opt/go/test/fixedbugs/bug248.dir/bug0.go /home/mark/opt/go/test/fixedbugs/bug248.dir/bug1.go ... 4 duplicate files (785 bytes): /home/mark/opt/go/doc/gopher/gophercolor16x16.png /home/mark/opt/go/favicon.ico /home/mark/opt/go/misc/dashboard/godashboard/static/favicon.ico /home/mark/opt/go/src/pkg/archive/zip/testdata/gophercolor16x16.png ... 2 duplicate files (1,371,249 bytes): /home/mark/opt/go/bin/ebnflint /home/mark/opt/go/src/cmd/ebnflint/ebnflint ptg7913109 7.2. Examples 357 We have, of course, elided many lines as the ellipses indicate. func commas(x int64) string { value := fmt.Sprint(x) for i:=len(value) - 3;i>0;i-=3 { value = value[:i] + "," + value[i:] } return value } Most humans find it difficult to parse long numbers (e.g., 1371249), so we have used this simple commas() function to insert grouping commas to make the sizes easier to read. The function accepts a single int64 value, so if we have a plain int or an integer of some other size, we must convert it—for example, commas(int64(i)).★ This completes our review of the findduplicates program and of Go concurrent programming generally. Go’s support for concurrency (<-, chan, go, select) is very flexible and there are many more variations on how to do things than we have had the space to show here. Nonetheless, the examples—and the exercises that follow—should be sufficient to provide a good understanding of how to do concurrent programming in Go, and offer a useful starting point for creating new concurrent programs. Of course,it is not possible to state definitively which of the approaches we have covered (or the other possible approaches) is the best to use, since each one may prove best in different circumstances. The performance profile of each approach is affected by the particular machine, the number of goroutines, and whether the processing is purely in-memory or involves external data (such as accessing files or network communications). A reliable means of finding out which is the best approach for a particular program is to time (or profile) the program using real (or realistic) data using different approaches and different numbers of goroutines. 7.3. Exercises This chapter has three exercises. The first involves creating a thread-safe data structure. The second and third require the creation of small but complete concurrent programs, with the third exercise being fairly challenging. 1. Create a thread-safe slice type called safeSlice and the following exported SafeSlice interface: ★ At the time of this writing, Go has no locale support. ptg7913109 358 Chapter 7. Concurrent Programming type SafeSlice interface { Append(interface{}) // Append the given item to the slice At(int) interface{} // Return the item at the given index position Close() []interface{} // Close the channel and return the slice Delete(int) // Delete the item at the given index position Len() int // Return the number of items in the slice Update(int, UpdateFunc) // Update the item at the given index position } It will also be necessary to create a method in which the un- derlying slice (of type []interface{}) is created,and which runs an “infinite” loop iterating over the communications channel, and a New() function in which the safe slice is created and the method executed in a goroutine. The safe slice implementation can easily be modeled on the safe map cov- ered earlier (§7.2.3, 334 ➤ ), and has the same deadlock risk in the safe- Slice.Update() method. A solution is given in the safeslice/safeslice.go file and is under 100 lines. (The apachereport4 program can be used for testing since it uses the safeslice package.) 2. Create a program that accepts one or more image filenames on the com- mand line, and for each one prints a line on the console as an HTML tag of the form . The program should process the images concurrently using a fixed number of worker goroutines, and the order of output doesn’t matter (so long as each line is output complete!).The filenamesshould be output without paths. Allow any files to be given on the command line, but ignore those that aren’t regular files or that aren’t images, and ignore all errors. The standard library’s image.DecodeConfig() function can retrieve an im- age’swidth and height from an io.Reader (as returned by os.Open()),without having to read the entire image. For this function to be able to recognize various image formats, (.jpg,.png, etc.), it is necessary to import the corre- sponding packages. However, we don’t need to access these packages di- rectly, so to avoid the Go compiler giving us “imported and not used” errors, we must import the packages to the blank identifier (e.g., _"image/jpeg";_ "image/png", etc.). We discuss these kinds of import in the next chapter. The book’s examples include two implementations, imagetag1 which is single-threaded (no additional goroutines, no channels), and imagetag2 which is concurrent in a similar way to this chapter’s cgrep2 example. If you just want to focus on the concurrency aspects you might like to copy the imagetag1/imagetag1.go file to, say, the my_imagetag directory and convert it from being single-threaded to concurrent. This will involve modifying the main() and process() functions, and the addition of around 40 lines of code. Windows users might like to use the commandLineFiles() function (176 ➤ )to ptg7913109 7.3. Exercises 359 handle file globbing; both implementations already include this. The more confident reader might prefer to try writing the program entirely from scratch. The solution in imagetag2/imagetag2.go takes one of many different possible approaches and is around 100 lines. 3. Create a concurrent program that uses a fixed number of worker goroutines and that acceptsone or more HTML files on the command line.The program should read each HTML file and every time it finds an tag it should check to see if the tag has width and height attributes, and if not, it should ascertain the image’s width and height and add the missing attribute or attributes. The concurrency structure could be done in a similar way to one of the apachereport examples (but not the mutex-using apachereport2 version), except that there is no need for a results channel. Since Go 1 doesn’t have an HTML parser, for this exercise we use the reg- exp and strings packages.★ We searched for image tags using the regular expression string `<[iI][mM][gG][^>]+>`, and we identified image filenames within image tags using the regular expression `src=["']([^"']+)["']`. (These regexeps are not very sophisticated since we want to focus on con- currency, not regular expressions, for which see §3.6.5, 120 ➤ .) Use the im- age.DecodeConfig() function discussed in the previous exercise (along with all the imports)to get each image’ssize. As usual,Windows users might like to use the commandLineFiles() function (176 ➤ ) to handle file globbing. The concurrency aspects of this program are quite straightforward, but the processing is quite tricky. This is a welcome change from concurrent programming in some other languages where the concurrency aspects overwhelm the actual processing that needs to be done. Two solutions are provided. The first solution, sizeimages1 (in file size- images1/sizeimages1.go), does everything it should, but has one disadvan- tage: It can only find the image files if it is run in the same directory as the HTML files. This limitation is due to the fact that we replace each tag with an updated tag using the regexp.Regexp.ReplaceAllStringFunc() method. This method expects to be passed a replacer function with signa- ture func(string) string with the string passed in being the matched text and the returned string being the replacement. A typical string passed in might be ``. The replacer function has no idea what the path to the splash.png file is, and so must assume the current directory, and hence the limitation that sizeimages1 must be run in the same directory as the HTML files. It might be tempting to try to solve the problem by using a global direc- tory string and assigning the current HTML file’s path to it before using the replacer function—but this won’t work in all situations. Why? The ★ An HTML parser is in development and may be available by the time you read this. ptg7913109 360 Chapter 7. Concurrent Programming second solution, sizeimages2 (in file sizeimages2/sizeimages2.go), solves the problem by creating a fresh replacer function as a closure that captures the HTML file’s directory each time it is needed. Then, the replacer function uses the captured directory as the image’s path for image files that have rel- ative paths. This is probably the most challenging exercise in the book so far, and will require looking up functions in the image, regexp, and strings packages. The sizeimages1.go file is around 160 lines, with sizeimages2.go under 170 lines. ptg7913109 8 File Handling §8.1. Custom Data Files ➤ 362 §8.1.1. Handling JSON Files ➤ 365 §8.1.2. Handling XML Files ➤ 371 §8.1.3. Handling Plain Text Files ➤ 377 §8.1.4. Handling Go Binary Files ➤ 385 §8.1.5. Handling Custom Binary Files ➤ 387 §8.2. Archive Files ➤ 397 §8.2.1. Creating Zip Archives ➤ 397 §8.2.2. Creating Optionally Compressed Tarballs ➤ 399 §8.2.3. Unpacking Zip Archives ➤ 401 §8.2.4. Unpacking Optionally Compressed Tarballs ➤ 403 In earlier chapters we saw several examples of reading, creating, and writing text files. In this chapter we will look in more detail at Go’s file handling facilities and in particular how to read and write files in standard formats (such as XML and JSON), as well as custom plain text and binary formats. Since we have now covered all of Go’s language features (apart from creating programs with our own custom packages and with third-party packages—which is covered in the next chapter), we are free to use all the facilities that Go pro- vides. We will take advantage of this freedom and make use of closures (§5.6.3, 225 ➤ ) to avoid repetitive code, and in some cases make more advanced use of Go’s support for object orientation, in particular adding methods to functions. This chapter’s focus is on files rather than directories or the file system gen- erally. For directories, the previous chapter’s findduplicates example (§7.2.5, 349 ➤ ) shows how to iterate over the files and subdirectories of a directo- ry using the filepath.Walk() function. In addition, the standard library’s os package’s os.File type provides methods for reading the names in a directory (os.File.Readdirnames()), and for retrieving os.FileInfo values for each item in a directory (os.File.Readdir()). 361 ptg7913109 362 Chapter 8. File Handling This chapter’s first section shows how to write and read files using standard and custom file formats. The second section covers Go’s support for handling archive files and compression. 8.1. Custom Data Files It is quite common for programs to maintain internal data structures and to provide import/export functionality to support data interchange, and also to facilitate the processing of their data by external tools. Since our concern here is with file handling, we will focus purely on how to write and read data to and from standard and custom formats from and into a program’s internal data structures. For this section we will use the same data for all the examples so that we can get a direct comparison between the various file formats. All the code is taken from the invoicedata program (in the invoicedata directory in the files invoicedata.go, gob.go, inv.go, jsn.go, txt.go, and xml.go). The program takes two filenames as command-line arguments, one to read and one to write (and which must be different). The program then reads the data from the first file (in whatever format its suffix indicates) and writes the same data to the second file (again, using whatever format its suffix indicates). The files created by invoicedata work cross-platform, that is, a file created on, say, Windows, is readable on Mac OS X and Linux, and vice versa, no matter which format it is. Gzip-compressed files (e.g., invoices.gob.gz) can be read and written seamlessly; compression is covered in the second section (§8.2, ➤ 397). The data consists of an []*Invoice, that is, a slice of pointers to Invoice values. Each invoice is held in an Invoice value, and each invoice holds zero or more items in its Items field of type []*Item (slice of pointers to Item). type Invoice struct { Id int CustomerId int Raised time.Time Due time.Time Paid bool Note string Items []*Item } type Item struct { Id string Price float64 Quantity int Note string } These two structs are used to hold the data. Table 8.1 shows some informal com- parisons of how long it took to read and write the same 50000 random invoices, and the size of the file occupied by those invoicesin each format. The timingsare in seconds rounded up to the nearest tenth of a second—they should be taken as unitless since they will undoubtedly vary on different hardware under different ptg7913109 8.1. Custom Data Files 363 Table 8.1 Format Speed and Size Comparisons Suffix Read Write Size (KiB) Read/Write LOC Format .gob 0.3 0.2 7 948 .gob.gz 0.5 1.5 2 589 21 +11= 32 Go Binary .jsn 4.5 2.2 16 283 .jsn.gz 4.5 3.4 2 678 32 +17= 49 JSON .xml 6.7 1.2 18 917 .xml.gz 6.9 2.7 2 730 45 +30= 75 XML .txt 1.9 1.0 12 375 .txt.gz 2.2 2.2 2 514 86 +53= 139 Plain text (UTF-8) .inv 1.7 3.5 7 250 .inv.gz 1.6 2.6 2 400 128 +87= 215 Custom binary loads. The size column shows the sizes in kibibytes—these should be very con- sistent across machines. For this data set,the compressed sizes are surprisingly similar,even though theuncompressedsizes vary considerably. The linesof code exclude code that is common to all formats (e.g., the code for decompressing and compressing and the structs). The timings and file sizes are what we would reasonably expect—apart from the extremely fast reading and writing of the plain text format. This is thanks to the fmt package’s excellent print and scan functions in conjunction with a custom text format that we designed to be easy to parse. For the JSON and XML formats, instead of using the default storage for time.Time values (an ISO-8601 date/time string),we have simply stored the date part—thishas slightly reduced the file size at the expense of some processing speed and some additional code. For example, the JSON code would run faster, and be about the same number of lines as the Go binary code, if we allowed it to handle time.Time values itself. For binary data, the Go binary format is the most convenient to use—it is very fast, is extremely compact, requires very little code, and is relatively easy to adapt to data changes. However, if we use custom types that are not initially gob-encodable and have to make them satisfy the gob.Encoder and gob.Decoder interfaces, this can considerably slow down writing and reading in gob format, and also bloat out the file size. For human-readable data, XML is probably the best format to use—particularly since it is so useful as a data interchange format. More lines are required to handle XML format than JSON format. This is because Go 1 doesn’t have an xml.Marshaler interface (a lack that is expected to be made good in a later Go 1.x release), and also because we have used parallel data types (XMLInvoice and XMLItem) to help map between the XML data and the invoice data (Invoice and Item). Applications that use XML as their external storage format would not ptg7913109 364 Chapter 8. File Handling need the parallel data types or the conversions that the invoicedata program needs, so are likely to be faster and require less code than the invoicedata exam- ple suggests. In addition to reading and writing speed, file size, and lines of code, there is one other issue that we should consider: format robustness. For example, if we add one field to the Invoice struct and one field to the Item struct, then we will need to change our file formats. How easy is it to adapt our code to write and read the new format—and also be able to continue reading the old format? Providing we take care to version our file formats such changes are pretty straightforward to cater to (as one of the chapter’s exercises will demonstrate)—with the exception of JSON format for which adapting the code to be able to read both the old and new formats is slightly tricky. In addition to the Invoice and Item structs, all the file formats share some constants: const ( fileType = "INVOICES" // Used by text formats magicNumber = 0x125D // Used by binary formats fileVersion = 100 // Used by all formats dateFormat = "2006-01-02" // This date must always be used ) The magicNumber is used to uniquely identify invoice files.★ The fileVersion indicates the invoice file version—using this makes it easier to change the program later on to accommodate changes to the data. The dateFormat shows how we want dates formatted in human-readable formats and is discussed later on (➤ 368). We have also created a couple of interfaces. type InvoicesMarshaler interface { MarshalInvoices(writer io.Writer, invoices []*Invoice) error } type InvoicesUnmarshaler interface { UnmarshalInvoices(reader io.Reader) ([]*Invoice, error) } The purpose of these is to make it easy to use a reader or writer for a specific format in a generic way. For example, here is the function the invoicedata program uses to read invoices from an open file. ★ There is no universal repository of magic numbers, so we cannot be certain that any particular magic number hasn’t been used before. ptg7913109 8.1. Custom Data Files 365 func readInvoices(reader io.Reader, suffix string) ([]*Invoice, error){ var unmarshaler InvoicesUnmarshaler switch suffix { case ".gob": unmarshaler = GobMarshaler{} case ".inv": unmarshaler = InvMarshaler{} case ".jsn",".json": unmarshaler = JSONMarshaler{} case ".txt": unmarshaler = TxtMarshaler{} case ".xml": unmarshaler = XMLMarshaler{} } if unmarshaler != nil { return unmarshaler.UnmarshalInvoices(reader) } return nil, fmt.Errorf("unrecognized input suffix: %s", suffix) } The reader is any value that satisfies the io.Reader interface, such as an open file (of type *os.File), or a gzip decompressor (of type *gzip.Reader), or a string.Reader. The suffix string is the file’s suffix (after any .gz suffix has been stripped off). The GobMarshaler, InvMarshaler, and so on, are custom types which provide the MarshalInvoices() and UnmarshalInvoices() methods (and therefore satisfy the InvoicesMarshaler and InvoicesUnmarshaler interfaces), as we will see in the following subsections. 8.1.1. Handling JSON Files According to, JSON (JavaScript Object Notation) is a lightweight data interchange format that is easy for humans to read and write, and easy for machines to parse and generate. JSON is a plain text format using the UTF-8 encoding. JSON has become increasingly popular—particularly for transferring data over network connections—because it is more convenient to write, is (usually) much more compact,and requires much less processing power to parse than XML. Here is the data for a single invoice in JSON format, but with most of the invoice’s second item’s fields elided. { "Id": 4461, "CustomerId": 917, "Raised": "2012-07-22", ptg7913109 366 Chapter 8. File Handling "Due": "2012-08-21", "Paid": true, "Note": "Use trade entrance", "Items": [ { "Id": "AM2574", "Price": 415.8, "Quantity": 5, "Note": "" }, { "Id": "MI7296", ... } ] } Normally, the encoding/json package writes JSON data without any unnec- essary whitespace, but we have shown it here with indentation and spaces to make it easier to see the data’s structure. Although the encoding/json package supports time.Times, we have chosen to implement our own custom MarshalJSON() and UnmarshalJSON() Invoice methods to handle the raised and due dates our- selves. This allows us to store shorter date strings (since for our data the time elements are always zero),such as "2012-09-06", rather than full date/times such as "2012-09-06T00:00:00Z". Writing JSON Files We have created a type based on an empty struct to give us something with which to associate the JSON-specific MarshalInvoices() and UnmarshalInvoices() methods. type JSONMarshaler struct{} This type fulfills the generic InvoicesMarshaler and InvoicesUnmarshaler inter- faces that we saw earlier (365 ➤ ). Here is a method for writing an entire data set of []*Invoice items to an io.Writer in JSON format using the encoding/json package’s standard Go to JSON marshaling conversions. The writer could be an *os.File returned by the os.Create() function, or a *gzip.Writer returned by the gzip.NewWriter() function—or anything else that fulfills the io.Writer interface. func (JSONMarshaler) MarshalInvoices(writer io.Writer, invoices []*Invoice) error { ptg7913109 8.1. Custom Data Files 367 encoder := json.NewEncoder(writer) if err := encoder.Encode(fileType); err != nil { return err } if err := encoder.Encode(fileVersion); err != nil { return err } return encoder.Encode(invoices) } The JSONMarshaler type has no data so we don’t need to assign its value to a receiver variable. We begin by creating a JSON encoder that wraps the io.Writer and provides an encoder to which we can write our JSON-encodable data. We write the data using the json.Encoder.Encode() method. This method copes perfectly with our slice of invoices each of which contains its own slice of items. The method returns an error or nil; if we get an error we immediately return it to the caller. Writing a file type and a file version isn’t strictly necessary, but as one of the exercises will illustrate, doing so makes it much easier to change the file format later on (e.g., to accommodate additional fields in the Invoice and Item structs), and be able to read both this format and the new format. Notice that the method isn’t really concerned with the types of the data it en- codes, so it is trivial to create similar functions for writing other JSON-encod- able data. Furthermore, the JSONMarshaler.MarshalInvoices() method will not require any changes to write a new format providing that all the new fields are exported and JSON-encodable. If the code shown here was all of the program’sJSON-related code,then it would work perfectly well as it is. However, since we want to exercise finer control over the JSON output—in particular the formatting of time.Time values—we have provided the Invoice type with a MarshalJSON() method that satisfies the json.Marshaler interface. The json.Encode() function is smart enough to check whether a value it is asked to encode is a json.Marshaler, and if it is, the function uses the value’s own MarshalJSON() method rather than using its own built-in encoding code. type JSONInvoice struct { Id int CustomerId int Raised string // time.Time in Invoice struct Due string // time.Time in Invoice struct Paid bool ptg7913109 368 Chapter 8. File Handling Note string Items []*Item } func (invoice Invoice) MarshalJSON() ([]byte, error){ jsonInvoice := JSONInvoice{ invoice.Id, invoice.CustomerId, invoice.Raised.Format(dateFormat), invoice.Due.Format(dateFormat), invoice.Paid, invoice.Note, invoice.Items, } return json.Marshal(jsonInvoice) } The custom Invoice.MarshalJSON() method takes an existing Invoice and returns a JSON-encoded version of it. The function’s first statement simply copies the invoice’s fields into a custom JSONInvoice struct, converting the two time.Time values into strings. Since the JSONInvoice struct’s fields are all Booleans, numbers, or strings, the struct can be JSON-encoded using the json.Marshal() function, so we use this function to do almost all of the work. To write date/times (i.e., time.Time values) as strings,we must use the time.Time. Format() method. This method takes a format string which indicates how the date/time must be written. The format string is rather unusual in that it must be a string representation of Unix time 1136243045, that is, the pre- cise date/time 2006-01-02T15:04:05Z07:00, or, as here, some subset of that date/time. The choice of this particular date/time is arbitrary,but fixed—no oth- er value will do for specifying date, time, and date/time formats. If we want to create our own date/time formats they must always be written in terms of the Go date/time. For example, if we wanted to write a date in the form weekday,month,day,year,we must use a format such as "Mon, Jan 02, 2006", or "Mon, Jan _2, 2006" if we want to suppress leading zeros. The time package’s documentation has full details—and also lists some predefined format strings. Reading JSON Files Reading JSON data is just as easy as writing it—especially if we read it back into top-level variables of the same types as the ones written. The JSON- Marshaler.UnmarshalInvoices() method takes an io.Reader which could be an *os.File returned by the os.Open() function, or a *gzip.Reader returned by the gzip.NewReader() function—or anything else that fulfills the io.Reader inter- face. ptg7913109 8.1. Custom Data Files 369 func (JSONMarshaler) UnmarshalInvoices(reader io.Reader) ([]*Invoice, error){ decoder := json.NewDecoder(reader) var kind string if err := decoder.Decode(&kind); err != nil { return nil, err } if kind != fileType { return nil, errors.New("cannot read non-invoices json file") } var version int if err := decoder.Decode(&version); err != nil { return nil, err } if version > fileVersion { return nil, fmt.Errorf("version %d is too new to read", version) } var invoices []*Invoice err := decoder.Decode(&invoices) return invoices, err } We have three items of data to read in: the file type, the file version, and the en- tire invoices data. The json.Decoder.Decode() method takes a pointer to the val- ue it must populate with the JSON data it decodes, and returns an error or nil. We use the first two variables (kind and version) to check that we have a JSON invoices file and that the file version is one we can handle. Then we read the invoices, in the process of which the json.Decoder.Decode() method will increase the size of the invoices slice to accommodate the invoices it reads and will popu- late the slice with pointers to Invoices (and their Items) that the function creates on the fly as necessary. At the end, the method returns the invoices and nil—or it returns nil and an error if a problem occurred. If we had relied purely on the json package’s built-in functionality and left the raised and due dates to be marshaled in the default way, the code shown here would be sufficient on itsown to unmarshal a JSON-format invoicesfile. Howev- er, since we have chosen to marshal the invoice data’s raised and due time.Times in a custom way (i.e., storing only the date parts), we must provide our own cus- tom unmarshaling method that understands our custom marshaling. func (invoice *Invoice) UnmarshalJSON(data []byte) (err error){ var jsonInvoice JSONInvoice if err = json.Unmarshal(data, &jsonInvoice); err != nil { return err } ptg7913109 370 Chapter 8. File Handling var raised, due time.Time if raised, err = time.Parse(dateFormat, jsonInvoice.Raised); err != nil { return err } if due, err = time.Parse(dateFormat, jsonInvoice.Due); err != nil { return err } *invoice = Invoice{ jsonInvoice.Id, jsonInvoice.CustomerId, raised, due, jsonInvoice.Paid, jsonInvoice.Note, jsonInvoice.Items, } return nil } This method uses the same JSONInvoice struct as before, and relies on the standard json.Unmarshal() function to populate it with an invoice’s data. We then create and assign an Invoice with the unmarshaled data and with the dates converted into time.Time values. Naturally, the json.Decoder.Decode() method is smart enough to check if a value it is being asked to decode into satisfies the json.Unmarshaler interface, and if it does, it uses the value’s own UnmarshalJSON() method. If the invoices data is changed by the addition of exported fields, this method will continue to work as is—but only if we make our Invoice.UnmarshalJSON() method version-sensitive. Furthermore, if the new fields’ zero values are not acceptable, then when reading files in the original format we must post-process the data to give the new fields sensible values. (One of the exercises involves the addition of new fields and just this kind of post-processing.) JSON is an easy format to work with, especially if we create suitable structs with exported fields, although it can be tricky to support two or more versions of a JSON file format. Also, the json.Encoder.Encode() and json.Decoder.Decode() functions (and the json.Marshal() and json.Unmarshal() functions) are not perfect inverses of each other—this means, for example, that it is possible to marshal data that cannot then be unmarshaled back into the same form as the original data. So, we must take care to check that they work correctly for our particular data. Incidentally, there is a JSON-like format called BSON (Binary JSON) which is much more compact than JSON and much faster to write and read. A Go ptg7913109 8.1. Custom Data Files 371 package that provides BSON support (gobson) is available from the godash- web page. (Installing and using third-party packages is covered in Chapter 9.) 8.1.2. Handling XML Files The XML (eXtensible Markup Language) format is widely used both as a data interchange format and as a file format in its own right. XML is a much more complex and sophisticated format than JSON,but is generally more verbose and tedious to write by hand. The encoding/xml package can encode and decode structs to and from XML for- mat in a similar way to the encoding/json package. However, the XML encoding and decoding functionality is much more demanding than for the encoding/json package. This is partly because the encoding/xml package requires struct fields to have suitably formatted tags (whereas they are not always needed for JSON). Also,Go 1’s encoding/xml package does not have an xml.Marshaler interface,sowe must write more code to handle XML than for the JSON and Go binary formats. (This is expected to be resolved in a later Go 1.x release.) Here is a single example invoice in XML format with newlines and extra whitespace included to make it fit the page and be easier to read. See special Terms & Conditions "Blue" ordered but will accept "Navy" Having raw character data in tags (e.g., for the invoices’ and items’ Note fields) is rather tricky to handle when using the xml package’s encoder and decoder, so the invoicedata example uses explicit tags. Writing XML Files The encoding/xml package requires us to use structs whose fields have encod- ing/xml package-specific tags. In view of this, we cannot use the Invoice and Item structs directly for XML. So we have created the XML-specific XMLInvoices, XMLInvoice, and XMLItem structs to solve this. And since the invoicedata program requires us to have parallel sets of structs, we must also provide a means of con- verting between them. Of course, applications that use XML as their primary ptg7913109 372 Chapter 8. File Handling storage format would need only one struct (or one set of structs) and would add the necessary encoding/xml package tags directly to those structs’ fields. Here is the XMLInvoices struct that will be used to hold the entire data set. type XMLInvoices struct { XMLName xml.Name `xml:"INVOICES"` Version int `xml:"version,attr"` Invoice []*XMLInvoice `xml:"INVOICE"` } No struct tag has any intrinsic semantics in Go—they are just strings acces- sible using Go’s reflection interface (§9.4.9, ➤ 427). However, the encoding/xml package requires us to use such tags to provide it with information about how we want it to map our structs’ fields to or from XML. An xml.Name field is used to name the tag that will contain the struct that the field is in. Fields tagged with `xml:",attr"` will become attributes of the tag using the field name as the at- tribute name. We can force another name to be used if we wish,by preceding the comma with the name we want to use. We have done so here to make the Version field become an attribute called version, rather than accepting the default name of Version. If the tag just contains a name, this name is used to indicate nested tags, in this example. One important subtlety to note is that instead of calling the XMLInvoices’s invoices field Invoices, we have called it Invoice to (case- insensitively) match the tag name. Here is the original Invoice struct and the parallel XML-equivalent XMLInvoice. type Invoice struct { Id int CustomerId int Raised time.Time Due time.Time Paid bool Note string Items []*Item } type XMLInvoice struct { XMLName xml.Name `xml:"INVOICE"` Id int `xml:",attr"` CustomerId int `xml:",attr"` Raised string `xml:",attr"` Due string `xml:",attr"` Paid bool `xml:",attr"` Note string `xml:"NOTE"` Item []*XMLItem `xml:"ITEM"` } We have used the default names for the attributes here—for example, field Cus- tomerId will become an attribute with exactly the same name. There are two nested tags, and , and just as for the XMLInvoices struct, instead of calling the XML invoice’s items field Items, we have called it Item to (case-insen- sitively) match the tag name. We have made the XMLInvoice’s Raised and Due fields strings since we want to handle their values ourselves (just storing dates), rather than allow the encoding/xml package to store full date/time strings. ptg7913109 8.1. Custom Data Files 373 Here is the original Item struct and the parallel XML-equivalent XMLItem. type Item struct { Id string Price float64 Quantity int Note string } type XMLItem struct { XMLName xml.Name `xml:"ITEM"` Id string `xml:",attr"` Price float64 `xml:",attr"` Quantity int `xml:",attr"` Note string `xml:"NOTE"` } The XMLItem’s fields are tagged to make them attributes, except for the Note field which will become a nested tag, and the XMLName field which holds the item’s own XML tag name. Just as we did for the JSON format, for XML we have created an empty struct to give us something with which to associate the XML-specific MarshalInvoices() and UnmarshalInvoices() methods. type XMLMarshaler struct{} This type fulfills the generic InvoicesMarshaler and InvoicesUnmarshaler inter- faces that we saw earlier (365 ➤ ). func (XMLMarshaler) MarshalInvoices(writer io.Writer, invoices []*Invoice) error { if _, err := writer.Write([]byte(xml.Header)); err != nil { return err } xmlInvoices := XMLInvoicesForInvoices(invoices) encoder := xml.NewEncoder(writer) return encoder.Encode(xmlInvoices) } This method takes an io.Writer (i.e., anything that satisfies the io.Writer inter- face such as an open file or open compressed file), into which it can write XML. The method begins by writing the standard XML header, (the xml.Header constant also includes a trailing newline).Then it converts all the invoices and their items into the equivalent XML structs. Al- though this looks like it will use as much memory again as the original data, be- cause Go’s strings are immutable, under the hood only references to the original strings will be copied into the XML structs, so the overhead will not be as great as it appears. And for applications that use structs with XML tags directly, no conversion is necessary. Once we have populated the xmlInvoices (of type XMLInvoices), we create a new xml.Encoder, passing it the io.Writer we want it to write to. Then we encode all ptg7913109 374 Chapter 8. File Handling the data as XML and return the encoder’s return value—which will be either an error or nil. func XMLInvoicesForInvoices(invoices []*Invoice) *XMLInvoices { xmlInvoices := &XMLInvoices{ Version: fileVersion, Invoice: make([]*XMLInvoice, 0, len(invoices)), } for _, invoice := range invoices { xmlInvoices.Invoice = append(xmlInvoices.Invoice, XMLInvoiceForInvoice(invoice)) } return xmlInvoices } This function takes an []*Invoice and returns an *XMLInvoices that has all the data converted to *XMLInvoices (and containing *XMLItems rather than *Items), relying on the XmlInvoiceForInvoice() function to do all the work. We never have to populate an xml.Name field ourselves (unless we want to use namespaces),so here when we create the *XMLInvoices we need only populate the Version field to ensure that our tag has a version attribute—for exam- ple, . We have also set the Invoice field to be an empty slice with sufficient capacity for all the invoices. This isn’t strictly necessary,but is potentially more efficient than leaving the field’s initial value as nil, since it meansthe built-in append() function will never have to allocate memory and copy data to grow the slice behind the scenes. func XMLInvoiceForInvoice(invoice *Invoice) *XMLInvoice { xmlInvoice := &XMLInvoice{ Id: invoice.Id, CustomerId: invoice.CustomerId, Raised: invoice.Raised.Format(dateFormat), Due: invoice.Due.Format(dateFormat), Paid: invoice.Paid, Note: invoice.Note, Item: make([]*XMLItem, 0, len(invoice.Items)), } for _, item := range invoice.Items { xmlItem := &XMLItem{ Id: item.Id, Price: item.Price, Quantity: item.Quantity, Note: item.Note, } ptg7913109 8.1. Custom Data Files 375 xmlInvoice.Item = append(xmlInvoice.Item, xmlItem) } return xmlInvoice } This function takes an Invoice and returns the equivalent XMLInvoice. The conversion is straightforward:We simply copy most of the Invoice field values to the corresponding XMLInvoice fields. Since we have chosen to handle the raised and due dates ourselves (so that we store only dates and not full date/times), we convert them to strings. And for the Invoice.Items field we append each item as an XMLItem to the XMLInvoice.Item slice. We used the same potential optimization as before,creating the Item slice to have sufficient capacity to avoid append() ever having to allocate memory and copy data. We discussed the writing of time.Time values when covering the JSON format earlier (368 ➤ ). One final point to note is that nowhere in our code do we do any XML-escaping— this is taken care of automatically by the xml.Encoder.Encode() method. Reading XML Files Reading XML files is slightly more involved than writing, especially if we have to parse some fields ourselves (such as dates); but it isn’t difficult if we use suitable XML-tagged structs. func (XMLMarshaler) UnmarshalInvoices(reader io.Reader) ([]*Invoice, error){ xmlInvoices := &XMLInvoices{} decoder := xml.NewDecoder(reader) if err := decoder.Decode(xmlInvoices); err != nil { return nil, err } if xmlInvoices.Version > fileVersion { return nil, fmt.Errorf("version %d is too new to read", xmlInvoices.Version) } return xmlInvoices.Invoices() } This method takes an io.Reader (i.e., anything that satisfies the io.Reader interface such as an open file or open compressed file), from which to read XML. The method begins by creating a pointer to an empty XMLInvoices struct and an xml.Decoder for reading the io.Reader. The entire XML file is then parsed by the xml.Decoder.Decode() method, and if the parse is successful the *XMLInvoices struct is populated with the XML file’s data. If the parse failed (e.g.,if there was an XML syntax error or if the file isn’t an invoices file), the decoder will return an error which we immediately pass to the caller. If the parse is successful we ptg7913109 376 Chapter 8. File Handling check the version, and if it is one we can handle we convert all the XML structs back to the structs used internally in our program. Naturally, this conversion step would be unnecessary if we used only XML-tagged structs. func (xmlInvoices *XMLInvoices) Invoices() (invoices []*Invoice, err error){ invoices = make([]*Invoice, 0, len(xmlInvoices.Invoice)) for _, xmlInvoice := range xmlInvoices.Invoice { invoice, err := xmlInvoice.Invoice() if err != nil { return nil, err } invoices = append(invoices, invoice) } return invoices, nil } The XMLInvoices.Invoices() method converts an *XMLInvoices into an []*Invoice. It is the inverse of the XmlInvoicesForInvoices() function (374 ➤ ), and passes all the conversion work onto the XMLInvoice.Invoice() method. func (xmlInvoice *XMLInvoice) Invoice() (invoice *Invoice, err error){ invoice = &Invoice{ Id: xmlInvoice.Id, CustomerId: xmlInvoice.CustomerId, Paid: xmlInvoice.Paid, Note: strings.TrimSpace(xmlInvoice.Note), Items: make([]*Item, 0, len(xmlInvoice.Item)), } if invoice.Raised, err = time.Parse(dateFormat, xmlInvoice.Raised); err != nil { return nil, err } if invoice.Due, err = time.Parse(dateFormat, xmlInvoice.Due); err != nil { return nil, err } for _, xmlItem := range xmlInvoice.Item { item := &Item{ Id: xmlItem.Id, Price: xmlItem.Price, Quantity: xmlItem.Quantity, Note: strings.TrimSpace(xmlItem.Note), } invoice.Items = append(invoice.Items, item) ptg7913109 8.1. Custom Data Files 377 } return invoice, nil } This XMLInvoice.Invoice() method is used to return the *Invoice that’s equiva- lent to the *XMLInvoice it is called on. The method begins by creating an Invoice with most of the fields populated from the XMLInvoice, and with the Items field set to an empty slice with sufficient capacity for the items. Then the two date/time valued fields are populated manually, since we opted to handle these ourselves. The time.Parse() function takes a date/time for- mat string (which, as noted earlier, must be based on the precise date/time 2006-01-02T15:04:05Z07:00), and a string to parse, and returns the equivalent time.Time and nil—or nil and an error. Next, the invoice’s Items field is populated by iterating over the *XMLItems in the XMLInvoice’s Item field and creating equivalent *Items. And at the end, the *Invoice is returned. Just as when writing XML, we don’t have to concern ourselves with unescaping any XML we read—the xml.Decoder.Decode() function takes care of this auto- matically. The xml package supports much more sophisticated tags than we have needed here, including nesting. For example, the tag name `xml:"Books>Author"` would result in the XML content. Also, in addition to `xml:",attr"`, the package supports `xml:",chardata"` to write the field as char- acter data, `xml:",innerxml"` to write the field verbatim, and `xml:",comment"` to write the field as an XML comment. So, by using tagged structs we are able to take full advantage of the convenient encoding and decoding functions, and at the same time exercise considerable control over how the XML is written and read. 8.1.3. Handling Plain Text Files For plain text files we must create our own custom format, ideally one that is easy to parse and extend. Here is the data for a single invoice in a custom plain text format. INVOICE ID=5441 CUSTOMER=960 RAISED=2012-09-06 DUE=2012-10-06 PAID=true ITEM ID=BE9066 PRICE=400.89 QUANTITY=7: Keep out of sunlight ITEM ID=AM7240 PRICE=183.69 QUANTITY=2 ITEM ID=PT9110 PRICE=105.40 QUANTITY=3: Flammable F F ptg7913109 378 Chapter 8. File Handling The format for each invoice is an INVOICE line, then one or more ITEM lines, and finally a form-feed character. Each line (whether for invoices or their items) has the same essential structure: a word identifying the type of line, a sequence of space-separated key=values, and optionally, a colon-space followed by note text. Writing Plain Text Files Writing plain text is straightforward thanks to Go’s powerful and flexible fmt package’s print functions. (These were covered earlier; §3.5, 93 ➤ .) type TxtMarshaler struct{} func (TxtMarshaler) MarshalInvoices(writer io.Writer, invoices []*Invoice) error { bufferedWriter := bufio.NewWriter(writer) defer bufferedWriter.Flush() var write writerFunc = func(format string, args ...interface{}) error { _, err := fmt.Fprintf(bufferedWriter, format, args...) return err } if err := write("%s %d\n", fileType, fileVersion); err != nil { return err } for _, invoice := range invoices { if err := write.writeInvoice(invoice); err != nil { return err } } return nil } This method begins by creating a buffered reader to operate on the file it is passed. It is essential that we defer the flushing of the buffer to guarantee that everything we write really does get written to the file (unless an error occurs). Rather than checking every write with code of the form if _, err := fmt.Fprintf( bufferedWriter, ...); err != nil { return err } we have created a function literal that provides two simplifications. First, the write() function ignores the number of bytes written that the fmt.Fprintf() function reports. Second, the function captures the bufferedWriter so we don’t have to mention it explicitly in our code. We could have simply passed our write() function to helper functions, for example, writeInvoice(write, invoice). But instead we have gone a step fur- ther and have added methods to the writerFunc type. This is done simply by declaring methods (i.e., funcs) that take a writerFunc as their receiver, just ptg7913109 8.1. Custom Data Files 379 as we would do for any other type. This is what allows us to make calls like write.writeInvoice(invoice), that is, to be able to call methods on the write() function itself; and since such methods receive the write() function as their re- ceiver, they can make use of it. Notice that we had to explicitly specify the write() function’s type (writerFunc)— had we not done so,Go would consider its type to be func(string, ...interface{}) error (which it is, of course) and would not have allowed us to call writerFunc methods on it (unless we used a type conversion to type writerFunc). With the convenient write() function (and its methods) now available, we begin by writing the file type and file version (the latter to make it easier to adapt to changing data). Then, we iterate over every invoice, and for each one we call the write() function’s writeInvoice() method. const noteSep = ":" type writerFunc func(string, ...interface{}) error func (write writerFunc) writeInvoice(invoice *Invoice) error { note := "" if invoice.Note != "" { note = noteSep + ""+ invoice.Note } if err := write("INVOICE ID=%d CUSTOMER=%d RAISED=%s DUE=%s "+ "PAID=%t%s\n", invoice.Id, invoice.CustomerId, invoice.Raised.Format(dateFormat), invoice.Due.Format(dateFormat), invoice.Paid, note); err != nil { return err } if err := write.writeItems(invoice.Items); err != nil { return err } return write("\f\n") } This method is used to write each invoice. It accepts the invoice to write and writes using the write() function it is called on. The invoice line is written in one go. If a note is present we write it preceded by colon-space; otherwise we write nothing at all for it. For the date/times (i.e., time.Time values), we use the time.Time.Format() method, the same as we did when writing the data in JSON and XML format (368 ➤ ). And for the Boolean we use the %t format verb (§3.5.1, 97 ➤ ); alternatively we could have used the %v verb or the strconv.FormatBool() function (116 ➤ ). Once the invoice line has been written, we write the items, and at the end, we write a form-feed and a newline to indicate the end of the invoice’s data. ptg7913109 380 Chapter 8. File Handling func (write writerFunc) writeItems(items []*Item) error { for _, item := range items { note := "" if item.Note != "" { note = noteSep + ""+ item.Note } if err := write("ITEM ID=%s PRICE=%.2f QUANTITY=%d%s\n", item.Id, item.Price, item.Quantity, note); err != nil { return err } } return nil } The writeItems() method acceptsthe invoice’s itemsand writes using the write() function it is called on. It iterates over all the items, writing each one in turn, and just as for invoices, writes only nonempty notes. Reading Plain Text Files It is almost as easy to open and read in plain text as it is to write it—but parsing the text to reconstruct the original data can be tricky depending on the complexity of the format. There are four approaches that can be used. The first three approaches involve splitting lines up and then using conversion functions such as strconv.Atoi() and time.Parse() for nonstring fields. These approaches are, first, manual pars- ing (e.g., character by character or word by word)—this can be tedious to imple- ment, fragile, and slow; second, using the fmt.Fields() or fmt.Split() functions to split up each line; and third, using regular expressions. For the invoicedata program we have used the fourth approach: This does not require us to split up lines or to use conversion functions since almost everything we need can be han- dled by the fmt package’s scan functions. func (TxtMarshaler) UnmarshalInvoices(reader io.Reader) ([]*Invoice, error){ bufferedReader := bufio.NewReader(reader) if err := checkTxtVersion(bufferedReader); err != nil { return nil, err } var invoices []*Invoice eof := false for lino := 2;!eof; lino++ { line, err := bufferedReader.ReadString('\n') if err == io.EOF { ptg7913109 8.1. Custom Data Files 381 err = nil // io.EOF isn't really an error eof = true // this will end the loop at the next iteration } else if err != nil { return nil, err // finish immediately for real errors } if invoices, err = parseTxtLine(lino, line, invoices); err != nil { return nil, err } } return invoices, nil } This method creates a buffered reader for the io.Reader it is passed, and passes each line in turn to a parser function. As usual for text files, we handle io. EOF specially, so that the last line is always read whether or not it ends with a newline. (Of course, this is rather liberal for this particular format.) The file is read line by line using 1-based line numbering, as is conventional. The first line is checked to see that it has a valid file type and version, hence the line number (lino) begins at 2 when processing the actual invoice data. Since we are working line by line and each invoice is represented by two or more lines (an INVOICE line and one or more ITEM lines), we need to keep track of the current invoice so that we can add to it as each line is read. This is easily done because invoices are appended to the invoices slice, so the current invoice is always the one at position invoices[len(invoices)-1]. When the parseTxtLine() function parses an INVOICE line it creates a new Invoice value and appends a pointer to this value to the invoices slice. There are two techniques we can use if we want to append to a slice inside a function. The first technique is to pass a pointer to the slice and operate on the pointed-to slice. The second technique is to pass in the slice value and return the (possibly modified) slice back for the caller to assign to the original slice. The parseTxtLine() function uses the second technique. (We saw an example of the first technique earlier; §5.7, 244 ➤ .) func parseTxtLine(lino int, line string, invoices []*Invoice) ([]*Invoice, error){ var err error if strings.HasPrefix(line, "INVOICE"){ var invoice *Invoice invoice, err = parseTxtInvoice(lino, line) invoices = append(invoices, invoice) } else if strings.HasPrefix(line, "ITEM"){ if len(invoices) == 0 { err = fmt.Errorf("item outside of an invoice line %d", lino) ptg7913109 382 Chapter 8. File Handling } else { var item *Item item, err = parseTxtItem(lino, line) items := &invoices[len(invoices)-1].Items ➊ *items = append(*items, item) } } return invoices, err } This function takes a line number (lino; used for error reporting), the line to parse, and the slice of invoices that we want to populate. If the line begins with the text “INVOICE”, we call the parseTxtInvoice() func- tion to parse the line and create an Invoice value—and to return a pointer to it. We then append this *Invoice to the invoices slice, and at the end return the in- voices and nil or an error. Note that at this point the invoice is incomplete—we only have its ID, customer ID, raised and due dates, whether it has been paid, and any note—but we have none of its items. If the line begins with the text “ITEM”, we first check to see that there is a current invoice, (i.e., that the invoices slice isn’t empty). If there is, we call the parseTxtItem() function to parse the line and create an Item value—and to return a pointer to the item. We must then add the item to the current invoice’s items. This is done by taking a pointer to the current invoice’s items (➊) and setting this pointer’s value (i.e., the []*Item it points to) to the result of appending the new *Item to it. Of course, we could have added the *Item directly, using the code invoices[len(invoices)-1].Items = append(invoices[len(invoices)-1].Items, item). Any other lines (e.g., empty and form-feed lines) are ignored. Incidentally, in theory this function would be faster if we made the “ITEM” case the first one since there are usually far more items than invoices or empty lines. func parseTxtInvoice(lino int, line string) (invoice *Invoice, err error){ invoice = &Invoice{} var raised, due string if _, err = fmt.Sscanf(line, "INVOICE ID=%d CUSTOMER=%d "+ "RAISED=%s DUE=%s PAID=%t",&invoice.Id, &invoice.CustomerId, &raised, &due, &invoice.Paid); err != nil { return nil, fmt.Errorf("invalid invoice %v line %d", err, lino) } if invoice.Raised, err = time.Parse(dateFormat, raised); err != nil { return nil, fmt.Errorf("invalid raised %v line %d", err, lino) } ptg7913109 8.1. Custom Data Files 383 if invoice.Due, err = time.Parse(dateFormat, due); err != nil { return nil, fmt.Errorf("invalid due %v line %d", err, lino) } if i := strings.Index(line, noteSep); i > -1 { invoice.Note = strings.TrimSpace(line[i+len(noteSep):]) } return invoice, nil } We begin by creating a zero-valued Invoice value and assigning a pointer to it to the invoice variable (of type *Invoice). The scan functions can handle strings, numbers, and Booleans, but not time.Time values, so we handle the raised and due datesby scanning them in as stringsand then parsing these separately. The scan functions are listed in Table 8.2. Table 8.2 The Fmt Package’s Scan Functions Parameter r is an io.Reader to read from; s is a string to read from; fs is a format string as used by the fmt package’s print functions(see Table 3.4,95 ➤ ); args stands for one or more pointers (i.e., addresses) of values to populate. All the scan functions return the number of items successfully parsed (i.e.,populated) and either nil or an error. Syntax Description fmt.Fscan(r, args) Reads r for successive space- or newline-separated values to populate args fmt.Fscanf(r, fs, args) Reads r for successive space-separated values as speci- fied by the fs format to populate args fmt.Fscanln(r, args) Reads r for successive space-separated values to popu- late args and expects a newline or io.EOF at the end fmt.Scan(args) Reads os.Stdin for successive space-separated values to populate args fmt.Scanf(fs, args) Reads os.Stdin for successive space-separated values as specified by the fs format to populate args fmt.Scanln(args) Reads os.Stdin for successive space-separated values to populate args and expects a newline or io.EOF at the end fmt.Sscan(s, args) Reads s for successive space- or newline-separated values to populate args fmt.Sscanf(s, fs, args) Reads s for successive space-separated values as speci- fied by the fs format to populate args fmt.Sscanln(s, args) Reads s for successive space-separated values to popu- late args and expects a newline or io.EOF at the end ptg7913109 384 Chapter 8. File Handling If the fmt.Sscanf() function wasn’t able to read in as many items as we provided values for, or if some other error occurred (e.g., a read error), it will return a non-nil error. The dates are parsed using the time.Parse() function that was discussed in an earlier subsubsection (§, 375 ➤ ). If the invoice line has a colon it means that there is a note at the end, so we retrieve this with any whites- pace trimmed off. We could have used the expression line[i+1:] rather than line[i+len(noteSep):] since we know that the noteSep’s colon character occupies a single UTF-8 byte, but we prefer to be defensive and use an approach that will work for any character, no matter how many bytes it occupies. func parseTxtItem(lino int, line string) (item *Item, err error){ item = &Item{} if _, err = fmt.Sscanf(line, "ITEM ID=%s PRICE=%f QUANTITY=%d", &item.Id, &item.Price, &item.Quantity); err != nil { return nil, fmt.Errorf("invalid item %v line %d", err, lino) } if i := strings.Index(line, noteSep); i > -1 { item.Note = strings.TrimSpace(line[i+len(noteSep):]) } return item, nil } This function works just like the parseTxtInvoice() function we have just seen, except that all the item values, apart from the note, can be scanned directly. func checkTxtVersion(bufferedReader *bufio.Reader) error { var version int if _, err := fmt.Fscanf(bufferedReader, "INVOICES %d\n",&version); err != nil { return errors.New("cannot read non-invoices text file") } else if version > fileVersion { return fmt.Errorf("version %d is too new to read", version) } return nil } This function is used to read the very first line of the invoices text file. It uses the fmt.Fscanf() function to read the bufio.Reader directly. If the file isn’t an invoices file or if the version is too recent for the program to handle, it reports an error; otherwise it returns nil. Writing text files is easy using the fmt package’s print functions. Parsing text files is fairly challenging but we are well provided for with Go’s regexp package, ptg7913109 8.1. Custom Data Files 385 its strings.Fields() and strings.Split() functions, and the fmt package’s scan functions. 8.1.4. Handling Go Binary Files The Go binary (“gob”) format is a self-describing sequence of binary values. In- ternally, the Go binary format consists of a sequence of zero or more chunks, each of which has a byte count, a sequence of zero or more typeId–typeSpecifi- cation pairs, and a typeId–value pair. The typeId–typeSpecificationpairs may be omitted if the value pair’s typeId is predefined (e.g., bool, int, string, etc.); oth- erwise each of the type pairs is used to describe a custom type (e.g., a custom struct). Type pair typeIds are negated to distinguish between type pairs and value pairs. As we will see, we don’t need to know any of the internals to make use of gob format, since the encoding/gob package takes care of all the low-level details for us, behind the scenes.★ The encoding/gob package provides encoder and decoder functionality in much the same way as the encoding/json package does, and is just as easy to use. In general, gob format is the most convenient Go format to use for data files or for transmitting data over network connections, providing that human readability is not a requirement. Writing Go Binary Files Here is a method for writing an entire data set of []*Invoice items to an open file (or anything else that fulfills the io.Writer interface) in gob format. type GobMarshaler struct{} func (GobMarshaler) MarshalInvoices(writer io.Writer, invoices []*Invoice) error { encoder := gob.NewEncoder(writer) if err := encoder.Encode(magicNumber); err != nil { return err } if err := encoder.Encode(fileVersion); err != nil { return err } return encoder.Encode(invoices) } We begin by creating a gob encoder that wraps the io.Writer and provides a writer to which we can write our data. ★ A more detailed description of the format is given in the documentation, gob/. And Rob Pike has written an interesting blog post on gob format, gobs-of-data.html . ptg7913109 386 Chapter 8. File Handling We write the data using the gob.Encoder.Encode() method. This method copes perfectly with our slice of invoices each of which contains its own slice of items. The method returns an error or nil; if we get an error we immediately return it to the caller. Writing a magic number and a file version isn’t necessary, of course, but as one of the exercises will show, doing this makes it easier to change the file format later on. Notice that the method isn’t really concerned with the types of the data it en- codes, so it is trivial to create similar functions for writing gob data. Further- more, the GobMarshaler.MarshalInvoices() method will not require any changes to write in a new format. Since our Invoice struct’s fields are all Booleans, numbers, strings, time.Times, or structs (i.e., Items) which themselvesonly contain Booleans,numbers, strings, time.Times, or structs, the code shown here works as is. If our structs contained fields that are not gob-encodable we would have to make our structs satisfy the gob.GobEncoder and gob.GobDecoder interfaces. The gob encoder is smart enough to check whether a value it is asked to encode is a gob.GobEncoder, and if it is, the encoder uses the value’s own GobEncode() method rather than using its own built-in encoding code. The same thing applies to decoding with the check being made for a GobDecode() method that satisfies the gob.GobDecoder interface.(The invoicedata example’s source code’s gob.go file includes the—unnecessary, and commented out—code to make an Invoice a gob encoder and decoder, just to show an easy way to do it.) Making a struct satisfy these interfaces can significantly slow down gob writing and reading, as well as producing larger files. Reading Go Binary Files Reading gob data is just as easy as writing it—providing we read it back into top-level variables of the same types as the ones written. The GobMar- shaler.UnmarshalInvoices() method takes an io.Reader (e.g., a file opened for reading) from which to read the gob data. func (GobMarshaler) UnmarshalInvoices(reader io.Reader) ([]*Invoice, error){ decoder := gob.NewDecoder(reader) var magic int if err := decoder.Decode(&magic); err != nil { return nil, err } if magic != magicNumber { return nil, errors.New("cannot read non-invoices gob file") } ptg7913109 8.1. Custom Data Files 387 var version int if err := decoder.Decode(&version); err != nil { return nil, err } if version > fileVersion { return nil, fmt.Errorf("version %d is too new to read", version) } var invoices []*Invoice err := decoder.Decode(&invoices) return invoices, err } We have three items of data to read in: the magic number,the file version,and all the invoices data. The gob.Decoder.Decode() method takes a pointer to the value it must populate with the gob data it decodes,and returnsan error or nil.Weuse the first two variables (magic and version) to check that we have a gob invoices file and that the file version is one we can handle. Then we read the invoices, in the process of which the gob.Decoder.Decode() method will increase the size of the invoices slice to accommodate the invoices it reads and will populate the slice with pointers to Invoices (and their Items) that the function creates on the fly as necessary. At the end, the method returns the invoices and nil or an error if a problem occurred. If the invoices data is changed by the addition of exported fields, this method will continue to work as is for Booleans, numbers, strings, time.Times, and structs containing these types. Of course, for data that contains other types, we must update the methods that satisfy the gob.GobEncoder and gob.GobDecoder interfaces. When dealing with struct types,the gob format is very flexible, coping seamless- ly with some differences. For example, if a struct that has a value is written in gob format, the value can be read back into the same struct—or into many similar struct types, perhaps one which has a pointer to the value, or where the type of the value is different but compatible (int vs. uint or similar). And as the invoicedata example shows, gob format has no problem handling nested data (although, at the time of this writing, it cannot handle recursive values). The gob documentation discusses what differences the format can cope with and explains the format’s internals, none of which need concern us providing we use the same types for writing and for reading—just as we have done in this example. 8.1.5. Handling Custom Binary Files Although Go’s encoding/gob package is very easy to use and requires very little code, we might still need to create our own custom binary formats. A custom binary format is likely to achieve the most compact data representation possible ptg7913109 388 Chapter 8. File Handling and can be very fast to write and read. In practice, we have found that writing and reading in Go Binary format is dramatically faster than using a custom binary format—and creates files that are not much bigger. However, some of this advantage is lost if we must handle data that isn’t gob-encodable by satisfying the gob.GobEncoder and gob.GobDecoder interfaces. Of course, in some situations we may need to interoperate with other software that uses its own custom binary format that we must be able to write and read, so knowing how to work with binary files can be very useful. Figure 8.1 provides a schematic view of how the .inv custom binary format rep- resents a single invoice. Integer values are represented by integers of specific sizes and signedness. Booleans are represented by an int8 with value 1 for true and 0 for false. Strings are represented by a byte count (of type int32) followed by a []byte of their UTF-8-encoded bytes. For dates we have taken a slightly unusual approach by representing them as int32sbased on treating an ISO-8601 format date (without the hyphens) as a number. For example, we represent the date 2006-01-02 as the integer 20060102. Each invoice’s items are represented by a count of how many items there are followed by the items themselves. (Re- call that unlike invoice IDs, item IDs are strings and not ints; 362 ➤ .) Id CustomerId Raised Due Paid Note item count items int32 int32 int32 int32 int8 int32 int32 []byte float64 int16 int32 []byte int32 []byte Id Price Quantity Note Figure 8.1 The .inv custom binary format Writing Custom Binary Files Writing data in binary format is straightforward thanks to the encoding/binary package’s binary.Write() function. type InvMarshaler struct{} var byteOrder = binary.LittleEndian func (InvMarshaler) MarshalInvoices(writer io.Writer, invoices []*Invoice) error { ptg7913109 8.1. Custom Data Files 389 var write invWriterFunc = func(x interface{}) error { return binary.Write(writer, byteOrder, x) } if err := write(uint32(magicNumber)); err != nil { return err } if err := write(uint16(fileVersion)); err != nil { return err } if err := write(int32(len(invoices))); err != nil { return err } for _, invoice := range invoices { if err := write.writeInvoice(invoice); err != nil { return err } } return nil } This method writes all the invoices to the given io.Writer. It begins by creating a convenience write() function which captures both the io.Writer and the byte order we want to use. Just as we did for .txt format, we have set the write() function to be of a specific type (invWriterFunc), and have created some methods for the write() function (e.g., invWriterFunc.writeInvoices()), to make it more convenient to use later on. Note that it is essential that we use the same byte order for writing and reading binary data. (We can’t make our byteOrder a constant, because binary.Little- Endian—or binary.BigEndian for that matter—isn’t a simple value like a string or number.) Writing the data is very similar to what we have seen before for other formats. One important difference is that once we have written the magic number and file version, we write a count of how many invoices are going to be written. (It would also have been possible to skip the count and simply write out the invoices; then, when reading back, we would have to keep reading one invoice at a time until we reached io.EOF.) type invWriterFunc func(interface{}) error func (write invWriterFunc) writeInvoice(invoice *Invoice) error { for _, i := range []int{invoice.Id, invoice.CustomerId} { if err := write(int32(i)); err != nil { return err } ptg7913109 390 Chapter 8. File Handling } for _, date := range []time.Time{invoice.Raised, invoice.Due} { if err := write.writeDate(date); err != nil { return err } } if err := write.writeBool(invoice.Paid); err != nil { return err } if err := write.writeString(invoice.Note); err != nil { return err } if err := write(int32(len(invoice.Items))); err != nil { return err } for _, item := range invoice.Items { if err := write.writeItem(item); err != nil { return err } } return nil } The writeInvoice() method is called for each invoice. It accepts a pointer to the invoice to write and writes using the write() function it is called on. The method begins by writing the invoice’s ID and customer ID as int32s. It is legal to write plain ints, but this is nonportable since the size of an int may vary depending on the underlying machine and the version of Go being used, so it is very important to always write specifically signed and sized integers such as uint32, int32, and so on. Next, we write the raised and due dates using a custom writeDate() method, and then the paid Boolean and the note string, again using custom methods. Finally, we write a count of how many items the invoice has, followed by the items themselves, each written by the custom writeItem() method. const invDateFormat = "20060102" // This date must always be used. func (write invWriterFunc) writeDate(date time.Time) error { i, err := strconv.Atoi(date.Format(invDateFormat)) if err != nil { return err } return write(int32(i)) } ptg7913109 8.1. Custom Data Files 391 We discussed the time.Time.Format() function—and why we must use the specific date 2006-01-02 in format strings—earlier (368 ➤ ). Here we have used an ISO-8601-like format, but with no hyphens so that we get a string of exactly eight digits with leading zeros for single-digit month and day numbers. We then convert this string into an integer—for example, if we have the date 2012-08-05, we convert it to an equivalent number, that is, 20120805, and then write this number as an int32. Incidentally, if we wanted to store date/times rather than just dates, or just wanted a faster computation, we could replace calls to this method with write( int64(date.Unix())) and store the secondssince the Unix epoch. The correspond- ing reader would be something like var d int64; if err := binary.Read(reader, byteOrder,&d); err != nil { return err }; date := time.Unix(d,0). func (write invWriterFunc) writeBool(b bool) error { var v int8 if b{ v = 1 } return write(v) } At the time of this writing, the encoding/binary package has no support for writing or reading bools so we have created this simple method to handle them. Incidentally, we didn’t need to use a conversion (e.g., int8(v)), because variable v is already a signed and sized type. func (write invWriterFunc) writeString(s string) error { if err := write(int32(len(s))); err != nil { return err } return write([]byte(s)) } Strings must be written out as their underlying UTF-8-encoded bytes. Here, we first write a count of how many bytes we will write and then the bytes themselves. (If we had fixed-width data we wouldn’t need the count, of course, providing that when it came to reading, we created an empty []byte of the same size as was written.) func (write invWriterFunc) writeItem(item *Item) error { if err := write.writeString(item.Id); err != nil { return err } if err := write(item.Price); err != nil { return err ptg7913109 392 Chapter 8. File Handling } if err := write(int16(item.Quantity)); err != nil { return err } return write.writeString(item.Note) } This method is called to write every item for every invoice. For the string ID and for the note we use the invWriterFunc.writeString() method,and for the quantity we use an integer of a particular size and signedness. For the price, though, we write it as is since it is already a sized type (float64). There’s nothing difficult about writing binary data, providing we are careful to write counts before variable-width data so that we know how many items to read back in. Using gob format is much more convenient, of course, but using a custom binary format should produce smaller files. Reading Custom Binary Files Reading custom binary data is almost as straightforward as writing it. We don’t have to parse the data as such; we just read each datum, using the same byte ordering as was used for writing, into a value of the same type as was written. func (InvMarshaler) UnmarshalInvoices(reader io.Reader) ([]*Invoice, error){ if err := checkInvVersion(reader); err != nil { return nil, err } count, err := readIntFromInt32(reader) if err != nil { return nil, err } invoices := make([]*Invoice, 0, count) for i:=0; i < count; i++ { invoice, err := readInvInvoice(reader) if err != nil { return nil, err } invoices = append(invoices, invoice) } return invoices, nil } This method begins by checking that we have an invoices file of a version that we can handle. Then it reads in the number of invoices in the file using a custom readIntFromInt32() function. We set the invoices slice to have zero length (i.e., ptg7913109 8.1. Custom Data Files 393 no invoices), but with the exact capacity that we need. Then we read in each invoice in turn and append it to the invoices slice. An alternative approach would have been to replace the make() call with make( []*invoice, count) and the append() line with invoices[i] = invoice. However, we prefer to create slices with the capacity they need, since this is potentially faster than growing a slice on the fly. After all, if we append to a slice whose length is equal to its capacity, a new slice will be created under the hood and the original slice’s data will be copied into it—whereas if the capacity is sufficient in the first place, no copying is required. func checkInvVersion(reader io.Reader) error { var magic uint32 if err := binary.Read(reader, byteOrder, &magic); err != nil { return err } if magic != magicNumber { return errors.New("cannot read non-invoices inv file") } var version uint16 if err := binary.Read(reader, byteOrder, &version); err != nil { return err } if version > fileVersion { return fmt.Errorf("version %d is too new to read", version) } return nil } This function tries to read in the file’s magic number and version. It returns nil if the file is acceptable and an error otherwise. The binary.Read() function is the complement of the binary.Write() function—it takes an io.Reader to read from, the byte order to use, and a pointer to an item of the exact type for it to populate. func readIntFromInt32(reader io.Reader) (int, error){ var i32 int32 err := binary.Read(reader, byteOrder, &i32) return int(i32), err } This custom helper function is used to read an int32 from a binary file and return it as a plain int. ptg7913109 394 Chapter 8. File Handling func readInvInvoice(reader io.Reader) (invoice *Invoice, err error){ invoice = &Invoice{} for _, pId := range []*int{&invoice.Id, &invoice.CustomerId} { if *pId, err = readIntFromInt32(reader); err != nil { return nil, err } } for _, pDate := range []*time.Time{&invoice.Raised, &invoice.Due} { if *pDate, err = readInvDate(reader); err != nil { return nil, err } } if invoice.Paid, err = readBoolFromInt8(reader); err != nil { return nil, err } if invoice.Note, err = readInvString(reader); err != nil { return nil, err } var count int if count, err = readIntFromInt32(reader); err != nil { return nil, err } invoice.Items, err = readInvItems(reader, count) return invoice, err } This function is called to read each invoice in turn. It begins by creating a new zero-valued invoice and storing a pointer to it in the invoice variable. The invoice ID and customer ID are read using the custom readIntFromInt32() function. The code is slightly subtle in that we iterate over pointers to the invoice ID and customer ID and assign the int returned by the function to the value that the pointer (pId) points to. An alternative would be to handle each ID separately—for example, if invoice. Id, err = readIntFromInt32(reader); err != nil { return err }, and so on. Reading the raised and due dates follows exactly the same pattern as reading the IDs, only this time we use a custom readInvDate() function. Just as for the IDs, we could have handled each date separately and more simply—for example, if invoice.Due, err = readInvDate(reader); err != nil { return err }, and so on. The paid Boolean and the note string are read in using helper functions that we will review in a moment. After the invoice’s data has been read we read how ptg7913109 8.1. Custom Data Files 395 many items it has and then read them all in using the readInvItems() function, passing it the io.Reader to read from and the number of items to read. func readInvDate(reader io.Reader) (time.Time, error){ var n int32 if err := binary.Read(reader, byteOrder, &n); err != nil { return time.Time{}, err } return time.Parse(invDateFormat, fmt.Sprint(n)) } This function reads an int32 that represents a date (e.g., 20130501), and then parses the string representation of the number as a date and returns the corresponding time.Time (e.g., 2013-05-01). func readBoolFromInt8(reader io.Reader) (bool, error){ var i8 int8 err := binary.Read(reader, byteOrder, &i8) return i8 == 1, err } This simple helper returns true if the int8 it reads is 1; and false otherwise. func readInvString(reader io.Reader) (string, error){ var length int32 if err := binary.Read(reader, byteOrder, &length); err != nil { return "", nil } raw := make([]byte, length) if err := binary.Read(reader, byteOrder, &raw); err != nil { return "", err } return string(raw), nil } This function reads in a []byte, but the principle it uses applies to slices of any type that have been written preceded by a count of how many items they contain. First, the number of items is read into the length variable. Then a slice of this exact length is created. When the binary.Read() function is passed a pointer to a slice it reads as many items of the slice’s type as there are items in the slice (or fails and returns a non-nil error).Notice that it is the slice’s length that matters, not its capacity (which may equal or exceed the length). ptg7913109 396 Chapter 8. File Handling In this case the []byte holds UTF-8-encoded bytes that we return converted to a string. func readInvItems(reader io.Reader, count int) ([]*Item, error){ items := make([]*Item, 0, count) for i:=0; i < count; i++ { item, err := readInvItem(reader) if err != nil { return nil, err } items = append(items, item) } return items, nil } This function reads in all of an invoice’s items. It knows how many items to read because it is passed the count. func readInvItem(reader io.Reader) (item *Item, err error){ item = &Item{} if item.Id, err = readInvString(reader); err != nil { return nil, err } if err = binary.Read(reader, byteOrder, &item.Price); err != nil { return nil, err } if item.Quantity, err = readIntFromInt16(reader); err != nil { return nil, err } item.Note, err = readInvString(reader) return item, nil } This function reads each individual item. Structurally, it is very similar to the readInvInvoice() function in that it creates a zero-valued Item and stores a point- er to it in the item variable, and then populates the item’s fields. The price can be read directly since that was written as a float64—that is,as a sized type—and the Item.Price field has the same type. (We have omitted the readIntFromInt16() function since it is almost identical to the readIntFromInt32() function we saw earlier; 393 ➤ .) This completes our review of writing and reading custom binary data. Working with binary data isn’t difficult, so long as we are careful to use specifically signed and sized integers and to precede variable-length values (e.g.,slices) with their length. ptg7913109 8.1. Custom Data Files 397 Go’s support for binary files includes random access. In such caseswe must open the file using the os.OpenFile() function (rather than os.Open()), and pass suit- able permission flags and mode (e.g., os.O_RDWR “read–write”) arguments.★ We can then use the os.File.Seek() method to position ourselves in the file for read- ing or writing, or the os.File.ReadAt() and os.File.WriteAt() methods for read- ing or writing from or to the file at specific byte offsets. Other useful methods are also provided, including os.File.Stat() which returns an os.FileInfo that provides details of the file’s size, permissions, and date/times. 8.2. Archive Files Go’s standard library provides support for several compression formats. This includes gzip, so it is easy to make Go programs able to seamlessly write and read files with gzip compression if they end with a .gz suffix and without compression otherwise. In addition, the library has packages which allow us to write and read .zip files and tarballs (.tar and .tar.gz), and to read .bz2 files (typically, .tar.bz2 files). In this section we will review extracts from two programs. The first, pack (in file pack/pack.go), accepts the name of an archive and a list of files to store in the archive on the command line. It determines which file format to use based on the archive filename’s suffix. The second, unpack (in file unpack/unpack.go), also accepts the name of an archive on the command line, and attempts to extract all the archive’s files, recreating the directory structure as it goes if necessary. 8.2.1. Creating Zip Archives To use the zip package for zipping we must first open a file to write to and then create a *zip.Writer to write into it. Then, for each file we want to put in the .zip archive, we must read the file and write its contents into an io.Writer obtained from the *zip.Writer. The pack program uses two functions, createZip() and writeFileToZip(), to create a .zip file using this approach. func createZip(filename string, files []string) error { file, err := os.Create(filename) if err != nil { return err } defer file.Close() zipper := zip.NewWriter(file) ★ File permission flags are traditionally written using octal numbers, as indicated by a leading 0.A value of 0666 makes the file readable and writable by everyone—however, a umask of 0022 (a common setting) modifies this to 0644, thus making a file readable and writable by its creator and readable but not writable by everyone else. ptg7913109 398 Chapter 8. File Handling defer zipper.Close() for _, name := range files { if err := writeFileToZip(zipper, name); err != nil { return err } } return nil } This function creates an empty .zip file, then creates a *zip.Writer (zipper) to write to the file, and then iterates over all of the files, writing each one in turn to the .zip file. The createZip() and writeFileToZip() functions are both very short and so it might be tempting to incorporate them into a single function. This would be unwise since it would mean that in the for loop we would open file after file (i.e., all those in the files slice), and might exceed the operating system’s capacity for open files—something we discussed briefly in the previous chapter (352 ➤ ). Of course, instead of deferring the os.File.Close() calls we could do them at each iteration; but this would take some care to ensure that the file is closed whether or not an error occurs. So, the simplest and cleanest solution it to always create a separate function for handling each individual file as we have done here. func writeFileToZip(zipper *zip.Writer, filename string) error { file, err := os.Open(filename) if err != nil { return err } defer file.Close() info, err := file.Stat() if err != nil { return err } header, err := zip.FileInfoHeader(info) if err != nil { return err } header.Name = sanitizedName(filename) writer, err := zipper.CreateHeader(header) if err != nil { return err } _, err = io.Copy(writer, file) return err } ptg7913109 8.2. Archive Files 399 We begin by opening the file to be zipped ready for reading, and defer closing it, in the familiar way. Next, we call the os.File.Stat() method to retrieve the file’s timestamp and permission values as an os.FileInfo value. We then feed this value into the zip.FileInfoHeader() function which returns a zip.FileHeader value that is populated with the timestamp, permissions, and filename. We are not obliged to use the same filename in the archive as the original filename, and here we have chosen to overwrite the filename (in the zip.FileHeader.Name field) with a sanitized version. With the header set up we call the zip.CreateHeader() function, passing it the header as argument. This creates an entry in the .zip archive with the header’s timestamp,permissions,and filename, and returns an io.Writer that we can use to write the contents of the file to be archived. For this purpose, we have used the io.Copy() function—this returns the number of bytes copied (which we have discarded), and either nil or an error. If an error occurs at any point we return it immediately for the caller to handle. And if there is no error, at the end, the .zip archive contains the given file. func sanitizedName(filename string) string { if len(filename) > 1 && filename[1]==':' && runtime.GOOS == "windows" { filename = filename[2:] } filename = filepath.ToSlash(filename) filename = strings.TrimLeft(filename, "/.") return strings.Replace(filename, "../","",-1) } If an archive contains files with absolute paths or paths that include .. path components, when we unpack the archive we could end up overwriting impor- tant files by accident. To reduce this risk we sanitize the name of every file we store in the archive. The sanitizedName() function gets rid of a leading drive letter and colon (if present), then strips away any leading directory separators and periods, and any .. path components. It also forces file separators to be slashes. 8.2.2. Creating Optionally Compressed Tarballs Creating tarballs is fairly similar to creating .zip archives, with the key differ- ences being that we write all the data to the same writer,and that we must write a full header before each file’s data, not just a filename. Our implementation for the pack program uses the createTar() and writeFileToTar() functions. ptg7913109 400 Chapter 8. File Handling func createTar(filename string, files []string) error { file, err := os.Create(filename) if err != nil { return err } defer file.Close() var fileWriter io.WriteCloser = file if strings.HasSuffix(filename, ".gz"){ fileWriter = gzip.NewWriter(file) defer fileWriter.Close() } writer := tar.NewWriter(fileWriter) defer writer.Close() for _, name := range files { if err := writeFileToTar(writer, name); err != nil { return err } } return nil } This function creates the given tarball file and adds gzip filtering if the file- name’s suffix indicates that the tarball should be compressed. The gzip. NewWriter() function returns a *gzip.Writer which fulfills the io.WriteCloser in- terface (just as the opened *os.File does). Once the file is ready for writing to, we create a *tar.Writer to write to it. Then we iterate over all the files and attempt to write each one in turn. func writeFileToTar(writer *tar.Writer, filename string) error { file, err := os.Open(filename) if err != nil { return err } defer file.Close() stat, err := file.Stat() if err != nil { return err } header := &tar.Header{ Name: sanitizedName(filename), Mode: int64(stat.Mode()), Uid: os.Getuid(), Gid: os.Getgid(), Size: stat.Size(), ptg7913109 8.2. Archive Files 401 ModTime: stat.ModTime(), } if err = writer.WriteHeader(header); err != nil { return err } _, err = io.Copy(writer, file) return err } This function begins by opening the given file for reading and deferring closing the file. It then does a stat call on the file to retrieve the file’s mode, size, and modification date/time—this data is used to populate the *tar.Header that must be created for each file that is written to the tarball. (In addition we set the header’s user and group IDs; these are used on Unix-like systems.) We should always at the very least set the header’s filename (the Name field), and we must set its Size field to the size of the file, or the tarball will be invalid. Once the *.tar.Header has been created and populated, we write the header into the tarball. Then, finally, we copy the file’s contents into the tarball. 8.2.3. Unpacking Zip Archives Unzipping a .zip file is just as straightforward as zipping one, only we must re-create the directory structure if the archive contains filenames that include paths. func unpackZip(filename string) error { reader, err := zip.OpenReader(filename) if err != nil { return err } defer reader.Close() for _, zipFile := range reader.Reader.File { name := sanitizedName(zipFile.Name) mode := zipFile.Mode() if mode.IsDir() { if err = os.MkdirAll(name, 0755); err != nil { return err } } else { if err = unpackZippedFile(name, zipFile); err != nil { return err } } } ptg7913109 402 Chapter 8. File Handling return nil } This function opens the given .zip file for reading. Instead of us having to call os.Open() to open the file and then calling zip.NewReader(), the zip package provides the zip.OpenReader() function which conveniently does both and re- turns a *zip.ReadCloser for us to work with. The most important aspect of the zip.ReadCloser is that it contains an exported zip.Reader struct field that con- tains a []*zip.File—a slice of pointers to zip.File structs, each one represent- ing a file inside the .zip file. We iterate over each of the reader’s zip.File structs and create a sanitized file or directory name (using the same sanitizedName() function as we used in the pack program; 399 ➤ ), to reduce the risk of overwriting important files. If we have a directory (as reported by the *zip.File’s os.FileMode’s IsDir() method),we try to create the directory. The os.MkdirAll() function hasthe useful properties that it will create any intermediate directories that are necessary to create the specified directory, and that it safely does nothing and returns nil if the directory already exists.★ If we have a file, we pass on the work of unzipping it to a custom unpackZippedFile() function. func unpackZippedFile(filename string, zipFile *zip.File) error { writer, err := os.Create(filename) if err != nil { return err } defer writer.Close() reader, err := zipFile.Open() if err != nil { return err } defer reader.Close() if _, err = io.Copy(writer, reader); err != nil { return err } if filename == zipFile.Name { fmt.Println(filename) } else { fmt.Printf("%s [%s]\n", filename, zipFile.Name) } return nil } ★ As noted earlier,file permission flags are traditionally written using octal numbers,with 0666 being a sensible choice for files. For directories, a sensible choice is 0755. ptg7913109 8.2. Archive Files 403 This function extracts a single file from the .zip archive file and writes it to the file system. It begins in the usual way by creating the file to be written. Then it opens the given .zip file’s file using the zip.File.Open() function and writes the archived file’s data to the newly created file. At the end, providing no error has occurred, the function prints the name of the file it created to the console, with the original filename in brackets if the sanitized filename is different. Incidentally, the *zip.File type has other useful methods, such as zip.File. Mode() (used earlier in the unpackZip() function), zip.File.ModTime() (which returns the file’s modification time as a time.Time), and zip.FileInfo() which returns the file’s os.FileInfo value. 8.2.4. Unpacking Optionally Compressed Tarballs Unpacking tar files is slightly easier than packing them. However, just as when we unzip a .zip file, we must re-create the directory structure if the archive contains filenames that include paths. func unpackTar(filename string) error { file, err := os.Open(filename) if err != nil { return err } defer file.Close() var fileReader io.ReadCloser = file if strings.HasSuffix(filename, ".gz"){ if fileReader, err = gzip.NewReader(file); err != nil { return err } defer fileReader.Close() } reader := tar.NewReader(fileReader) return unpackTarFiles(reader) } This function opens the tarball in Go’s conventional way and defers closing the file. If the file is gzip-compressed we create a gzip decompression filter and defer closing it. The gzip.NewReader() function returns a *gzip.Reader which fulfills the io.ReadCloser interface, just as the file (of type *os.File) does. With the file reader set up, we create a *tar.Reader to read from it and pass on the rest of the work to a helper function. ptg7913109 404 Chapter 8. File Handling func unpackTarFiles(reader *tar.Reader) error { for { header, err := reader.Next() if err != nil { if err == io.EOF { return nil // OK } return err } filename := sanitizedName(header.Name) switch header.Typeflag { case tar.TypeDir: if err = os.MkdirAll(filename, 0755); err != nil { return err } case tar.TypeReg: if err = unpackTarFile(filename, header.Name, reader); err != nil { return err } } } return nil } This function has an infinite loop that iterates over each tarball entry until io.EOF is reached (or until an error occurs). The tar.Next() method returns the first—or next—tarball entry’s*tar.Header struct,orreports an error. If the error is io.EOF it just means that we have finished so we return a nil error value. If a *tar.Header is successfully obtained we create a sanitized filename based on the header’s Name field. Then we switch depending on the entry’s type flag. For this simple example program we only consider directories and regular files, but in fact, tarballs can contain various other entries (e.g., symbolic links). If the entry is for a directory we create the directory just the same as we did for directories in .zip files (402 ➤ ). And if the entry is for a file we pass on the work to a helper function. func unpackTarFile(filename, tarFilename string, reader *tar.Reader) error { writer, err := os.Create(filename) if err != nil { return err } defer writer.Close() ptg7913109 8.2. Archive Files 405 if _, err = io.Copy(writer, reader); err != nil { return err } if filename == tarFilename { fmt.Println(filename) } else { fmt.Printf("%s [%s]\n", filename, tarFilename) } return nil } This function creates a new file for the tarball’s next entry, and defers closing it. Then it copies the tarball’s next entry’s data to the file. And just like we did for the unpackZippedFile() function, we print name of the file that was created to the console, with the original filename in brackets if the sanitized filename is different. This concludesour coverage of compressed and archive files,and of file handling in general. Go’s approach to file handling using the io.Reader, io.ReadCloser, io.Writer, and io.WriteCloser interfaces makes it easy to read and write files or other streams (such as network connections or even strings) using the same consistent coding patterns. 8.3. Exercises There are three exercises for this chapter. The first one involves a small but slightly subtle modification to one of the programs presented in this chapter. The second requires the writing of a short—but tricky—new program from scratch. The third involves substantial changes to another of this chapter’s ex- amples. 1. Copy the unpack directory to, say, my_unpack, and modify the unpack.go pro- gram so that it can additionally unpack .tar.bz2 (bzip2-compressed) files. This requires small changes to a couple of functions, and the addition of about ten lines to the unpackTar() function. The change is a tiny bit tricky because the bzip2.NewReader() function doesn’t return an io.ReadCloser.A solution is provided in the file unpack_ans/unpack.go which is about ten lines longer than the original example. 2. Windows text files (.txt) often use the UTF-16-LE (UTF-16 little-endian) encoding. UTF-16-encoded files must always begin with a byte order mark, [0xFF, 0xFE] for little-endian or [0xFE, 0xFF] for big-endian. Write a program that reads a UTF-16-encoded file named on the command line, and writes out the same text using the UTF-8 encoding either to os.Stdout or to a file named on the command line. Be sure to read both little- and big-endian UTF-16 encodings correctly. The book’s examples come with a couple of ptg7913109 406 Chapter 8. File Handling tiny test files:utf16-to-utf8/utf-16-be.txt and utf16-to-utf8/utf-16-le.txt. The binary package’s Read() function can read uint16 values (which is what UTF-16 charactersare) using a specified endianness. And the unicode/utf16 package’s Decode() function can convert a slice of uint16 values to a slice of code points (i.e., to a []rune); so wrapping the result of a utf16.Decode() call in string() is sufficient to produce a UTF-8-encoded string. A solution is in the file utf16-to-utf8/utf16-to-utf8.go, and is around 50 lines excluding imports. 3. Copy the invoicedata directory to, say, my_invoicedata, and modify the invoicedata program in a few distinct ways. First, change the Invoice and Item structs to those shown below. type Invoice struct {// fileVersion Id int // 100 CustomerId int // 100 DepartmentId string // 101 Raised time.Time // 100 Due time.Time // 100 Paid bool // 100 Note string // 100 Items []*Item // 100 } type Item struct {// fileVersion Id string // 100 Price float64 // 100 Quantity int // 100 TaxBand int // 101 Note string // 100 } Now modify the program so that it always writes invoices in the new format (i.e., one that handles the new structs), and can read in invoices in the original format or in the new format. When the program reads in data in the original format, the zero values for the extra fields are not acceptable, so populate these fields with values according to these rules: Set an invoice’s department ID to "GEN" if the invoice ID is less than 3000,to "MKT" if the ID is less than 4 000, to "COM" if the ID is less than 5000,to "EXP" if the ID is less than 6000,to "INP" if the ID is less than 7000, to "TZZ" if the ID is less than 8000, to "V20" if the ID is less than 9000, and to "X15" otherwise. Set each item’s tax band to the integer value of the item ID’s third character. For example, if the ID is "JU4661", the tax band should be 4. A solution is provided in the invoicedata_ans directory. The solution adds three functions to invoicedata.go: one to update all the invoices in an []*In- voice (i.e., to provide acceptable values for the new fields), one to update an individual invoice, and one to update an individual item. The solution needed changes to all the .go files, with the jsn.go, xml.go, and txt.go files most affected. In all, the changes amount to around 150 lines of addition- al code. ptg7913109 9 Packages §9.1. Custom Packages ➤ 408 §9.1.1. Creating Custom Packages ➤ 408 §9.1.2. Importing Packages ➤ 416 §9.2. Third-Party Packages ➤ 417 §9.3. A Brief Survey of Go’s Commands ➤ 418 §9.4. A Brief Survey of the Go Standard Library ➤ 419 §9.4.1. Archive and Compression Packages ➤ 419 §9.4.2. Bytes and String-Related Packages ➤ 419 §9.4.3. Collection Packages ➤ 421 §9.4.4. File, Operating System, and Related Packages ➤ 423 §9.4.5. Graphics-Related Packages ➤ 425 §9.4.6. Mathematics Packages ➤ 425 §9.4.7. Miscellaneous Packages ➤ 425 §9.4.8. Networking Packages ➤ 427 §9.4.9. The Reflect Package ➤ 427 The Go standard library has a large number of packages, which provide a wide range of functionality out of the box. In addition, many third-party packages are available from the Go Dashboard at Go also allows us to create our own custom packages. These packages can be installed into our copy of Go’s standard library, or kept in our own Go tree (i.e., in the, or one of the, GOPATH paths). In this chapter we will look at how to create and import packages, including our own custom packages and third-party packages. Then we will very briefly re- view some of the commands(programs) supplied with the gccompiler. And final- ly, we will briefly review Go’s standard library so that we can avoid reinventing the wheel. 407 ptg7913109 408 Chapter 9. Packages 9.1. Custom Packages Up to now, almost all of the examples we have reviewed have been in a single package: main. For any given package, Go allows us to split the package’s code over as many files as we like, providing only that they are all in the same di- rectory. For example, Chapter 8’s invoicedata example uses a single package (main), even though it consists of six separate files (invoicedata.go, gob.go, inv.go, jsn.go, txt.go,and xml.go).This is achieved simply by making the first statement in each file (excluding comments), package main. For larger applications we might want to create application-specific packages to help partition an application’s functionality into logical units. Also, we might want to create packages containing functionality that we want a family of ap- plications to be able to use. Go doesn’t make any distinction between a pack- age intended for use by a single application and a package to be shared across our applications; however, we can create an implied distinction by putting our application-specific packages in subdirectories of our application and shared packages in subdirectories directly under a GOPATH source directory.A GOPATH source directory isa directory called src;every directory in the GOPATH should con- tain an src directory,since this is what Go’s tools (commands)expect. The source code for our programs and packages should be kept in subdirectories under the (or a, if there is more than one) GOPATH src directory. It is also possible to install our own packages directly into the Go tree (i.e.,under GOROOT), but there is no advantage to doing this and it could be inconvenient on systems where Go is installed using a package management system or an installer or even if it is built by hand. 9.1.1. Creating Custom Packages It is best to create custom packages in the GOPATH src directory (or one of the GOPATH src directories). Application-specific packages can be created within the application directory, but packages we want to share ought to be created directly under a GOPATH src directory, ideally under a unique directory to avoid name conflicts. By convention, the source code for a package is put in a directory with the same name as the package. The source code may be split across as many files as we like and the files can have arbitrary names (so long as they end with .go). In this book we have adopted the convention of giving the .go file (or one of them, if there is more than one) the same name as the package. The stacker example from Chapter 1 (§1.5, 21 ➤ ) consists of a program (in file stacker.go) and an application-specific package (stack in file stack.go), and uses the following directory layout: aGoPath/src/stacker/stacker.go ptg7913109 9.1. Custom Packages 409 aGoPath/src/stacker/stack/stack.go Here, aGoPath is the GOPATH directory (if there is just one), or one of the colon- (semicolon on Windows) separated paths in the GOPATH environment variable. If we are in the stacker directory and execute the command go build, we will get an executable called stacker (stacker.exe on Windows). However, if we want the executable to be in a GOPATH bin directory,or if we want other programs to be able to use the stacker/stack package, we must use go install. When go install builds the stacker program it creates two directories (if they don’t exist): aGoPath/bin containing the stacker executable, and aGoPath/pkg/ linux_amd64/stacker, which contains the static stack package’s binary library file. (Naturally, the operating system/architecture directory will match those of the machine being used, e.g., windows_386 for 32-bit Windows.) The stack package can be imported by the stacker program with the statement import "stacker/stack", that is, giving its full (Unix-style) path, but excluding the aGoPath/src part. In fact, any program or package in the GOPATH can use exactly this import since Go doesn’t distinguish between application-specific and shared packages. The ordered map from Chapter 6 (§6.5.3, 302 ➤ ; in file omap.go) is in package omap. It is intended to be used by multiple applications. To avoid name conflicts, we have created a directory for packages we want to share under our GOPATH directory (or one of our GOPATH directories), and given it what should be a unique name (in this case a domain name). Here is the directory structure: aGoPath/src/ Any of our programs (so long as they are in a GOPATH path) can access the or- dered map package using import "". If we had other shared pack- ages we would put them under aGoPath/src/, alongside the ordered map package. When go install builds the omap package it creates the aGoPath/pkg/linux_amd64/ directory (if it doesn’t exist) containing the omap package’s binary li- brary file, with the operating system/architecture subdirectory varying depend- ing on the machine. If we want to create packages inside other packages, we can do so without formality. First we create a package directory, say, aGoPath/src/my_package. And then we create one subdirectory per package underneath that, for exam- ple, aGoPath/src/my_package/pkg1 and aGoPath/src/my_package/pkg2, along with their corresponding files aGoPath/src/my_package/pkg1/pkg1.go and aGoPath/src/ my_package/pkg2/pkg2.go. Then, to import, say, pkg2, we would write import "my_package/pkg2". The Go source code’s archive package is an example of this approach. It is also possible to have code in the package itself (in this example, ptg7913109 410 Chapter 9. Packages by creating the file aGoPath/src/my_package/my_package.go). See the source code’s image package for an example of this. Go packages are imported from under GOROOT (specifically, $GOROOT/pkg/${GOOS}_ ${GOARCH}, e.g., /opt/go/pkg/linux_amd64),and from under the directory or directo- ries in the GOPATH environment variable. This means that name conflicts are pos- sible. The easiest way to avoid name conflicts is to ensure that GOPATH paths have a unique directory, such as a domain name as we used for the omap package. The go program works seamlessly with packages from the standard library and from GOPATH paths, as well as understanding the platform-specific source files discussed in the following subsubsection. Platform-Specific Code In some situations we need to have code that differs between platforms. For example, on Unix-like systems the shell does wildcard expansion (called glob- bing), so *.txt on the command line might be received by the program as, say, ["README.txt", "INSTALL.txt"] in the os.Args[1:] slice. But on Windows the pro- gram would just get ["*.txt"].We can get the program to do such globbing using the filepath.Glob() function, but of course, we need only do this on Windows. One solution is to decide whether to use filepath.Glob() at runtime by using the test if runtime.GOOS == "windows" {…}, and this is what most of the book’s examples do (e.g., cgrep1/cgrep.go). Another solution is to put platform-specific code into its own .go file or files. For example, the cgrep3 program consists of three files, cgrep.go, util_linux.go, and util_windows.go. Inside util_linux.go there is a single function defined. func commandLineFiles(files []string) []string { return files } Clearly, this function doesn’t do any file globbing since there is no need to on Linux. The util_windows.go file defines a different function that has the same name. func commandLineFiles(files []string) []string { args := make([]string, 0, len(files)) for _, name := range files { if matches, err := filepath.Glob(name); err != nil { args = append(args, name) // Invalid pattern } else if matches != nil {// At least one match args = append(args, matches...) } } return args } ptg7913109 9.1. Custom Packages 411 Figure 9.1 The omap package’s documentation When we build cgrep3 using go build, on Linux machines the util_linux.go file will be compiled and the util_windows.go file will be ignored—and vice versa on Windows machines. This ensures that only one of the commandLineFiles() functions is compiled for any particular build. On Mac OS X and FreeBSD systems neither util_linux.go nor util_windows.go will be compiled, so the build will fail. Since the shells on both these plat- forms do globbing, we can either soft (symbolic) link or copy util_linux.go to util_darwin.go and to util_freebsd.go (and similarly for any other platforms that Go supports and that are required). With these links or copies in place, the program will build on Mac OS X and FreeBSD platforms. Documenting Packages Packages, particularly those that are intended to be shared, need decent docu- mentation. Go provides a documentation tool, godoc, that can be used to show the documentation for packages and functions on the console, or can be used as a web server to serve the documentation as web pages, as illustrated in Fig- ure 9.1.★ If the package is in a GOPATH, godoc will automatically find it and provide a link to it at the left of the “Packages” link. If the package is not in a GOPATH, run godoc with the -path option (in addition to the -http option), giving it the path to ★ The documentation screenshots show godoc’s HTML rendering at the time of this writing; it may have changed since. ptg7913109 412 Chapter 9. Packages the package,and again godoc will provide a link next to the “Packages” link. (We discussed godoc in the sidebar “The Go Documentation”, 8 ➤ .) What constitutes good documentation is a contentious matter, so in this subsub- section we will concern ourselves purely with the mechanics of documenting Go packages. By default only exported types, classes, constants, and variables are shown by godoc, so all of these should be documented. Documentation is written directly in source code files. // Package omap implements an efficient key-ordered map. // // Keys and values may be of any type, but all keys must be comparable // using the less than function that is passed in to the omap.New() // function, or the less than function provided by the omap.New*() // construction functions. package omap For a package, the comment immediately preceding the package statement is used as the package’s description, with the first line (up to the first period if there isone, or to the newline)serving asa one-line summary. Thisisan example taken from the omap package (in file—this package was covered in Chapter 6, §6.5.3, 302 ➤ ). // Map is a key-ordered map. // The zero value is an invalid map! Use one of the construction functions // (e.g., New()), to create a map for a specific key type. type Map struct { The documentation for an exported type must be written immediately before the type statement and should always indicate whether the type’s zero value is valid. // New returns an empty Map that uses the given less than function to // compare keys. For example: // type Point { X, Y int } // pointMap := omap.New(func(a, b interface{}) bool { // α, β := a.(Point), b.(Point) // if α.X != β.X { // return α.X < β.X // } // return α.Y < β.Y // }) func New(less func(interface{}, interface{}) bool) *Map { ptg7913109 9.1. Custom Packages 413 Figure 9.2 The omap package’s New() function’s documentation The documentation for functions and methods must immediately precede their first line. This is the documentation for the omap package’s generic New() construction function. Figure 9.2 shows what the function’s documentation looks like when godoc is used to serve it as a web page. This figure also illustrates that indented text in the documentation is rendered as “code” in HTML. However, at the time of this writing, godoc has no support for any kind of markup (e.g., bold, italic, links, etc.). // NewCaseFoldedKeyed returns an empty Map that accepts case-insensitive // string keys. func NewCaseFoldedKeyed() *Map { The above snippet showsthe documentation for one of the convenience construc- tion functions that provides a predefined less than function. // Insert inserts a new key-value into the Map and returns true; or // replaces an existing key-value pair's value if the keys are equal and // returns false. For example: // inserted := myMap.Insert(key, value). func (m *Map) Insert(key, value interface{}) (inserted bool) { And here is the documentation for the Insert() method. Notice that it is conven- tional in Go to start a function or method’s documentation with the function or method’sname,and (unconventionally)not to use parentheseswhen referring to functions or methods. ptg7913109 414 Chapter 9. Packages Unit Testing and Benchmarking Packages The Go standard library includes good support for unit testing with the testing packages. Setting up unit testing for a package is a simple matter of creating a test file in the same directory as the package we want to test. This file’s name should begin with the package name and end with _test.go. For example, the omap package’s test file is called omap_test.go. In the book’s examples, test files are put in their own unique package (e.g., omap_test), and they import the package they are testing and the testing pack- age, plus any other packages that are needed. This constrains us to use black box testing. However, some Go programmers prefer white box testing. This can easily be done by putting test files in the same package as they are testing (e.g., omap), in which case there is no need to import the package being tested. This latter approach means that nonexported types can be tested, and methods can be added to nonexported types specifically to support testing. The test file is unusual in that it has no main() function. Instead, it has one or more exported functions whose name begins with Test and which take a single argument of type *testing.T and that return nothing. We can add any supporting functions we need, of course, providing their names don’t start with Test. func TestStringKeyOMapInsertion(t *testing.T) { wordForWord := omap.NewCaseFoldedKeyed() for _, word := range []string{"one","Two","THREE","four","Five"}{ wordForWord.Insert(word, word) } var words []string wordForWord.Do(func(_, value interface{}) { words = append(words, value.(string)) }) actual, expected := strings.Join(words, ""), "FivefouroneTHREETwo" if actual != expected { t.Errorf("%q != %q", actual, expected) } } Here is one of the tests in the omap_test.go file. It begins by creating an empty omap.Map, then it inserts some string keys (which are treated case-insensitively), and string values. We then iterate over all the key–value pairs in the map using the Map.Do() method, and append each value to a slice of strings. Finally, we join the strings into a single string and see if this matches what we expect. If the match fails we call the testing.T.Errorf() method to report the failure and provide some explanation. If no error or failure function is called then the test is assumed to have passed. ptg7913109 9.1. Custom Packages 415 Here is an example test run where the tests pass. $ go test ok PASS And here is the same test run, but this time with verbose output switched on using the -test.v option. $ go test -test.v ok === RUN TestStringKeyOMapInsertion-4 --- PASS: TestStringKeyOMapInsertion-4 (0.00 seconds) === RUN TestIntKeyOMapFind-4 --- PASS: TestIntKeyOMapFind-4 (0.00 seconds) === RUN TestIntKeyOMapDelete-4 --- PASS: TestIntKeyOMapDelete-4 (0.00 seconds) === RUN TestPassing-4 --- PASS: TestPassing-4 (0.00 seconds) PASS If we change the constant string to force the test to fail and run the tests again (in the default nonverbose mode), here is the output we will get. $ go test FAIL --- FAIL: TestStringKeyOMapInsertion-4 (0.01 seconds) omap_test.go:35: "FivefouroneTHREETwo" != "FivefouroneTHREEToo" FAIL In addition to the Errorf() method used in the example, the testing package’s *testing.T type has various other methods such as testing.T.Fail(), testing. T.Fatal(), and so on. All these methods provide us with a good level of control over how we respond to test failures. In addition, the testing package has support for benchmarking. Benchmark functions can be added to package_test.go files in the same way as test functions, only in this case the functions’ names must begin with Benchmark and they receive a single *testing.B argument and return nothing. func BenchmarkOMapFindSuccess(b *testing.B) { b.StopTimer() // Don't time creation and population intMap := omap.NewIntKeyed() for i:=0;i<1e6; i++ { intMap.Insert(i, i) ptg7913109 416 Chapter 9. Packages } b.StartTimer() // Time the Find() method succeeding for i:=0; i < b.N; i++ { intMap.Find(i % 1e6) } } This function begins by stopping the timer since we don’t want to time the creation and population of the omap.Map. Then we create an empty omap.Map and populate it with a million key–value pairs. By default go test does not run any benchmarks, so if we want them to be run we must explicitly say so by using the -test.bench option and providing it with a regular expression that matches the names of the benchmarkswe want to run. A regexp of .* matches anything, that is, all the benchmark functions in the test file, but plain . also works. $ go test -test.bench=. PASS PASS BenchmarkOMapFindSuccess-4 1000000 1380 ns/op BenchmarkOMapFindFailure-4 1000000 1350 ns/op This output shows that two benchmarks were run with 1000000 loop iterations each, and with the given numbers of nanoseconds per operation. The number of iterations (i.e., the value of b.N) is chosen by go test, but we can use the -test.benchtime option to set the approximate number of seconds we want each benchmark to run if we prefer. In addition to the omap package, a few of the book’s other examples have pack- age_test.go files. 9.1.2. Importing Packages Go allows us to alias package names. This feature can be convenient and useful—for example, making it easy to switch between two implementations of a package. For instance,we could import a package like this:import bio "bio_v1", so that in our code the bio_v1 package is accessed as bio instead of bio_v1. Later, when a more mature implementation is available,we could switch to it by chang- ing the import to import bio "bio_v2". This will work if both bio_v1 and bio_v2 provide the same APIs (or if bio_v2’s API is a superset of bio_v1’s), and means that the rest of the code can be left unchanged. On the other hand, aliasing standard library package names is probably best avoided, since it could cause confusion or irritation to maintainers later on. ptg7913109 9.1. Custom Packages 417 As we mentioned in Chapter 5 (§5.6.2, 224 ➤ ), when a package is imported its init() functions are executed (if it has any). In some situations we don’t want to make explicit use of a package, but do want its init() functions to be executed. For example, if we are processing imageswe might want to register all the image formats that Go supports, but not actually use any functions from the packages that provide the formats. Here is the import statement for the imagetag1 pro- gram’s imagetag1.go file (from Chapter 7’s exercises). import ( "fmt" "image" "os" "path/filepath" "runtime" _ "image/gif" _ "image/jpeg" _ "image/png" ) Here we import the image/gif, image/jpeg, and image/png packages purely to execute their init() functions(which registerstheir image format with the image package).Each of these packages is aliased to the blank identifier, so Go will not complain about us not making explicit use of any of them in our code. 9.2. Third-Party Packages The go tool that we have used throughout the book to build programs and pack- ages (e.g., the omap package) can also be used to download, build, and install third-party packages. (Of course, this assumes that our computer is connect- ed to the Internet.) A list of third-party packages is maintained at godash- alternative approach is to download the source code—often obtained as a copy directly from a distributed version control system —and build the package ourselves locally.) To install one of the Go Dashboard’s packages, first click its link to go to the package’s home page. Somewhere on the package’s web site there should be a go get command that shows how to download and install the package. For example, if we were to click the Go Dashboard’s hg/freetype link, it would take us to the page which shows the installation command up front (at the time of this writing): go get When it comes to third-party packages, go get must, of course, install them somewhere. By default it will use the first path listed in the GOPATH environment ptg7913109 418 Chapter 9. Packages variable if that is set, and failing that it will put the package under the GOROOT directory. If we want to force go get to use the GOROOT directory, we can simply unset the GOPATH environment variable before running go get. If we execute the go get command it will silently download,build, and install the package. We can view the newly installed package’s documentation by running godoc as a web server (e.g., godoc -http=:8000), and navigating to the package. To avoid name conflicts, third-party packages usually use domain names to ensure uniqueness. For example, to use the FreeType package we would use an import statement of import "". Of course, when it comes to using functions from the package, as always we only need to use the last component of the name—for example, font, err := freetype.ParseFont(fontdata). And in the unlikely event of a name collision in the last component, we can always use aliasing, for instance, import ftype "", and then in our code font, err := ftype.ParseFont(fontdata). Third party packages are normally available for Go 1, but some might require a later Go version, or have multiple downloads available—for example, ones that work with the cutting edge development versions of Go. In general, it is best to always use a stable version of Go (e.g., Go 1), and to use packages that are compatible with it. 9.3. A Brief Survey of Go’s Commands A Go installation for the gc compiler naturally includes compilers and linkers (6g, 6l, etc.), but also a number of other tools. The most useful of these is go, which can be used as a build tool for our own programs and packages, as a tool for downloading and installing third-party programs and packages, and as a tool for executing unit tests and for benchmarking, as we saw earlier (§, 414 ➤ ). Execute go help for a full list of the commands available and go help command for help on the specified command. There is also the godoc tool for showing documentation (“The Go Documentation”, 8 ➤ ). In addition to the tools we have used in the book, there are several other tools and go tool commands, a few of which we will mention here. One is go vet which does some simple error checking on Go programs, specifically on the fmt package’s print functions. Another command is go fix. Sometimes a new Go release incorporates changes to the language—or more often, to library APIs—that invalidate existing code. By running go fix over our code base, all of our code can be automatically updated. We strongly recommend that .go files are registered with a version control system, and that they are all checked in, or are at least backed up, before running go fix.This will allow us to easily see what changes were applied and be able to roll back any or all of them if the fixes break the code. We can also use ptg7913109 9.3. A Brief Survey of Go’s Commands 419 go fix’s -diff option in the first place, since this shows the changes go fix would apply without actually applying them. The last command we will mention is gofmt. This command formats Go code in a standardized way and its use is highly recommended by the Go developers. The advantages of gofmt are that it eliminates arguments over the best way to lay out code, and that it ensures that all Go code has a uniform look. All the book’s code was formatted using gofmt (but lines longer than 75 characters were then manually wrapped to fit on the book’s pages). 9.4. A Brief Survey of the Go Standard Library The Go standard library includes a large number of packages that between them provide a wide range of functionality. The overview provided here is highly selective and very brief. This is because the contents of the library are likely to continue to grow after this book is published, so it is best to look at the library APIs online ( or locally using godoc, both to see the most up-to-date information and to get a comprehensive overview of what is available in each package. The exp “experimental” package is where packages that might potentially be added to the standard library begin life, so these packages should not be used unless we specifically want to participate in their development (e.g., testing, commenting, or submitting patches). The exp package is normally available when pulling Go from Google’s Go source tree, and but it may not be included in prebuilt packages. All the other packages are okay to use, although at the time of this writing, some were incomplete. 9.4.1. Archive and Compression Packages Go can read and write tarballs and .zip files. The relevant packages are archive/tar and archive/zip, and for compressed tarballs, compress/gzip and compress/bzip2. The book’s pack and unpack examples illustrate the use of these (§8.2, 397 ➤ ). Other compression formats are also supported; for example, Lempel-Ziv-Welch (compress/lzw) which is used for .tiff images and .pdf files. 9.4.2. Bytes and String-Related Packages The bytes and strings packages have many functions in common, only the former operateson []byte values and the latter on string values. For strings,the strings package provides all the most useful utilities to find substrings, replace substrings, split strings, trim strings, and change case (see §3.6.1, 107 ➤ ). The strconv package provides conversionsfrom numbers and Booleans to stringsand vice versa (see §3.6.2, 113 ➤ ). ptg7913109 420 Chapter 9. Packages The fmt package provides a variety of extremely useful print and scan functions. The print functions were covered in Chapter 3 (§3.5, 93 ➤ ) with examples of use shown throughout the book, and the scan functions are shown in Table 8.2 (383 ➤ ), with some examples of use nearby (§, 380 ➤ ). The unicode package provides functions for determining character properties, such as whether a character is a printable character, or whether it is a digit (see §3.6.4, 118 ➤ ). The unicode/utf8 and unicode/utf16 packages provide functions for decoding and encoding runes (i.e., Unicode code points/characters). For the unicode/utf8 package, see §3.6.3, 117 ➤ . Some use of the unicode/utf16 package is shown in Chapter 8’s utf16-to-utf8 exercise solution. The text/template and html/template packages can be used to create tem- plates which can then be used to generate textual output (e.g., HTML), based on data that is fed into them. Here is a tiny and very simple example of the text/template package in use. type GiniIndex struct { Country string Index float64 } gini := []GiniIndex{{"Japan", 54.7}, {"China", 55.0}, {"U.S.A.", 80.1}} giniTable := template.New("giniTable") giniTable.Parse( `` + `{{range .}}` + `{{printf "" .Country .Index}}`+ `{{end}}` + `
`) err := giniTable.Execute(os.Stdout, gini)
The template.New() function creates a new *template.Template with the given name. Template names are useful to identify templates that are in effect nested inside other templates. The template.Template.Parse() function parses a tem- plate (typically from an .html file), ready for use. The template.Template.Exe- cute() function executes the template sending the resultant output to the given io.Writer, and reading the data that should be used to populate the template from its second argument. In this example, we have output to os.Stdout and passed the gini slice of GiniIndex structs as the data. (We have split the output over several lines to make it clearer.) ptg7913109 9.4. A Brief Survey of the Go Standard Library 421 Inside a template, actions are enclosed in double braces ({{ and }}). The {{range}} …{{end}} action can be used to iterate over every item in a slice; here we have set each GiniIndex in the slice to the dot (.), that is, to be the current item. We can access a struct’s exported fields using their names, preceded, of course,with the dot to signify the current item. The {{printf}} action works just like the fmt.Printf() function, but with spaces replacing the parentheses and argument-separating commas. The text/template and html/template packages support a sophisticated templat- ing language in their own right, with many actions, including iteration and con- ditional branching, support for variables and method calls, and much else be- sides. In addition, the html/template package is safe against code injection. 9.4.3. Collection Packages Slices are the most efficient collection type provided by Go, but sometimes it is useful or necessary to use a more specialized collection type. For many situations the built-in map type is sufficient, but the Go standard library also provides the container package which contains various collection packages. The container/heap package provides functions for manipulating a heap, where the heap must be a value of a custom type that satisfies the heap.Interface defined in the heap package. A heap (strictly speaking, a min-heap) maintains its values in an order such that the first element is always the smallest (or largest for a max-heap)—this is known as the heap property. The heap.Interface embeds the sort.Interface and adds Push() and Pop() methods. (We discussed the sort.Interface in §4.2.4, 160 ➤ and §5.7, 244 ➤ .) It is easy to create a simple custom heap type that satisfies the heap.Interface. Here is an example of such a heap in use. ints := &IntHeap{5, 1, 6, 7, 9, 8, 2, 4} heap.Init(ints) // Heapify ints.Push(9)// IntHeap.Push() doesn't preserve the heap property ints.Push(7) ints.Push(3) heap.Init(ints) // Must reheapify after heap-breaking changes for ints.Len() > 0 { fmt.Printf("%v ", heap.Pop(ints)) } fmt.Println() // prints: 12345677899 Here is the complete custom heap implementation. type IntHeap []int func (ints *IntHeap) Less(i, j int) bool { ptg7913109 422 Chapter 9. Packages return (*ints)[i] < (*ints)[j] } func (ints *IntHeap) Swap(i, j int) { (*ints)[i], (*ints)[j] = (*ints)[j], (*ints)[i] } func (ints *IntHeap) Len() int { return len(*ints) } func (ints *IntHeap) Pop() interface{} { x := (*ints)[ints.Len()-1] *ints = (*ints)[:ints.Len()-1] return x } func (ints *IntHeap) Push(x interface{}) { *ints = append(*ints, x.(int)) } This implementation is sufficient for many situations. We could make the code slightly nicer to read by specifying the type as type IntHeap struct { ints []int }, since then we could refer to ints.ints rather than *ints inside the methods. The container/list package provides a doubly linked list. Items added to the list are added as interface{} values. Items retrieved from the list have type list.Element, with the original value accessible as list.Element.Value. items := list.New() for _, x := range strings.Split("ABCDEFGH",""){ items.PushFront(x) } items.PushBack(9) for element := items.Front(); element != nil; element = element.Next() { switch value := element.Value.(type){ case string: fmt.Printf("%s ", value) case int: fmt.Printf("%d ", value) } } fmt.Println() // prints: HGFEDBA9 In this example we push eight single-letter strings onto the front of a new list and then push an int onto the end. Then we iterate over the list’s elements and print each element’s value. We didn’t really need the type switch since we ptg7913109 9.4. A Brief Survey of the Go Standard Library 423 could have printed using fmt.Printf("%v ", element.Value), but if we weren’t merely printing we would need the type switch if the list contained elements of different types. Of course, if all the elements had the same type we could use a type assertion—for example, element.Value.(string) for string elements. (Type switches were covered in §, 197 ➤ and type assertions in §5.1.2, 191 ➤ .) In addition to the methods shown in the snippet above, the list.List type provides many other methods, including Back(), Init() (to clear the list), In- sertAfter(), InsertBefore(), Len(), MoveToBack(), MoveToFront(), PushBackList() (to push one list onto the end of another), and Remove(). The standard library also provides the container/ring package which imple- ments a circular list.★ While all the collection types hold their data in memory, Go also has a database/ sql package that provides a generic interface for SQL databases. To work with actual databases, separate database-specific driver packages must be installed. These, along with many other collection packages, are available from the Go Dashboard ( And as we saw earlier, the book’s source code includes the ordered map omap.Map type which is based on a left-leaning red-black tree (§6.5.3, 302 ➤ ). 9.4.4. File, Operating System, and Related Packages The standard library provides many packages to support file and directory han- dling and interaction with the operating system. In many cases these packages provide operating-system-neutral abstractions that make it straightforward to create cross-platform Go applications. The os (“operating system”) package provides functions for operating-system interactions, such as changing the current working directory,changing file mode and ownership, getting and setting environment variables, and creating and removing files and directories. In addition, this package provides functions for creating and opening files (os.Create() and os.Open()), and for retrieving file attributes (e.g., via the os.FileInfo type), all of which we have seen used in earlier chapters. (See, for example, §7.2.5, 349 ➤ , and Chapter 8.) Once a file is opened, especially in the case of text files, it is very common to want to access it via a buffer (e.g., to read lines as strings rather than as byte slices). The functionality we need is provided by the bufio package, and again, we have seen many examples of its use in earlier chapters. In addition to using bufio.Readers and bufio.Writers for reading and writing strings, we can also read (and unread) runes, read (and unread) single bytes, read multiple bytes, as well as write runes and single or multiple bytes. ★ Older versions of Go may have the container/vector package. This package is deprecated—use slices and the built-in append() function instead (§4.2, 148 ➤ ). ptg7913109 424 Chapter 9. Packages The io (“input/output”) package provides a large number of functions for work- ing with io.Readers and io.Writers. (Both of these interfaces are satisfied by *os.File values.) For example, we have used the io.Copy() function to copy data from a reader to a writer (§8.2.1, 397 ➤ ). This package also contains functions for creating synchronous in-memory pipes. The io/ioutil package provides a few high-level convenience functions. Among others, the package provides the ioutil.ReadAll() function that reads all of an io.Reader’s data and returns it as a []byte; the ioutil.ReadFile() function that does the same but accepts a string argument (the filename) rather than an io.Reader; the ioutil.TempFile() function which returns a temporary file (an *os.File); and the ioutil.WriteFile() function which writes a []byte to a file whose name it is given. The path package has functions for manipulating Unix-style paths such as Linux and Mac OS X paths, URL paths, git “references”, FTP files, and so on. The path/filepath package provides the same functions as path—and many others—and is designed to provide platform-neutral path handling. This pack- age also provides the filepath.Walk() function for recursively iterating over all the files and directories in a given path, as we saw in an earlier chapter (§7.2.5, 349 ➤ ). The runtime package contains many functions and types that give access to Go’s runtime system. Most of these are advanced and should not be needed when cre- ating standard maintainable Go programs. However, a couple of the package’s constants can be useful—for example, runtime.GOOS which holds a string (e.g., "darwin","freebsd","linux", or "windows"), and runtime.GOARCH which also holds a string (e.g., "386","amd64",or"arm"). The runtime.GOROOT() function returns the GOROOT environment variable’s value (or the Go build’s root if the environment variable isn’t set), and the runtime.Version() function returns the Go version (as a string).We saw how to use the runtime.GOMAXPROCS() and runtime.NumCPU() func- tions to ensure that Go uses all the machine’s processors, in Chapter 7 (327 ➤ ). File Format-Related Packages Go’s excellent support for file handling applies both to text files (using the 7-bit ASCII encoding or the UTF-8 and UTF-16 Unicode encodings), and to binary files. Go provides specific packages for handling JSON and XML files, as well as its own very fast, compact, and convenient Go binary format. (All of these formats, plus custom binary formats, were covered in Chapter 8.) In addition, Go has a csv package for reading .csv (“comma-separated values”) files. This package treats such files as records (one per line) each of which con- sists of (comma-separated) fields. The package is quite versatile—for example, it is possible to change the delimiter (e.g., from a comma to a tab or other char- acter), as well as other aspects of how it reads and writes records and fields. ptg7913109 9.4. A Brief Survey of the Go Standard Library 425 The encoding package contains several packages, one of which, encoding/binary, we have already used for reading and writing binary data (§8.1.5, 387 ➤ ). The other packages provide encoding and decoding for various other formats—for example, the encoding/base64 package can be used to encode and decode URLs which often use this format. 9.4.5. Graphics-Related Packages Go’s image package provides some high-level functions and types for creating and holding image data. It also has a number of packages that provide encoders and decoders for various standard graphics file formats, such as image/jpeg and image/png. We discussed some of these earlier in this chapter (§9.1.2, 416 ➤ ), and in one of Chapter 7’s exercises. The image/draw package provides some basic drawing functionality as we saw in Chapter 6 (§6.5.2,289 ➤ ).The third-party freetype package adds more functions for drawing. The freetype package itself can draw text using any specified TrueType font, and the freetype/raster package can draw lines and cubic and quadratic curves. (We discussed obtaining and installing the freetype package earlier; §9.2, 417 ➤ .) 9.4.6. Mathematics Packages The math/big package provides unlimited (except by memory) size integers (big.Int) and rationals (big.Rat); these were discussed earlier (§2.3, 57 ➤ ). The pi_by_digits example shows the use of big.Ints (§, 61 ➤ ). The math/big package also provides a big.ProbablyPrime() function. The math package provides all the standard mathematical functions (based on float64s) and several standard constants. See Tables 2.8, 2.9, and 2.10 (65–67 ➤ ). The math/cmplx package provides some standard functions for complex numbers (based on complex128s). See Table 2.11 (71 ➤ ). 9.4.7. Miscellaneous Packages In addition to the packages that can be roughly grouped together, the standard library contains a number of packages that stand more or less alone. The crypto package can provide hashes using the MD5, SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512 algorithms. (Support for each algorithm is supplied by a package, e.g., crypto/sha512.) In addition, the crypto package has packages that provide encryption and decryption using a variety of algorithms, such as AES, DES, and so on, each in packages with corresponding names (e.g., crypto/aes, crypto/des). ptg7913109 426 Chapter 9. Packages The exec package is used to run external programs. This can also be done using the os.StartProcess() function, but the exec.Cmd type is much more convenient to use. The flag package provides a command-line parser. It accepts X11-style options (e.g., -width, not GNU-style -w and --width). The package produces a very basic usage message and does not provide any validation beyond a value’s type. (So, the package can be used to specify an int option, but not what values are ac- ceptable.) Several alternatives are available from the Go Dashboard (godash- The log package provides functions for logging information (by default to os.Stdout), and for terminating the program or panicking with a log mes- sage. The log package’s output destination can be changed to any io.Writer using the log.SetOutput() function. Log messages are output in the form of a timestamp and then the message; the timestamp can be eliminated by calling log.SetFlags(0) before the first log function call. It is also possible to create cus- tom loggers using the log.New() function. The math/rand package provides many useful pseudo-random number generat- ing functions including rand.Int() which returns a random int and rand.Intn(n) which returns a random int in the range [0, n). The crypto/rand package has a function for producing cryptographically strong pseudo-random numbers. The regexp package provides a very fast and powerful regular expression engine that supports the RE2 engine’s syntax. We have used this package in several of the book’s examples, although we have deliberately used simple regexeps and not used the package’s full power so as not to stray off-topic. The package was introduced earlier (§3.6.5, 120 ➤ ). The sort package provides convenience functions for sorting slices of ints, float64s, and strings, and for performing fast (binary chop) searches on such sorted slices. It also provides generic sort.Sort() and sort.Search() functions that can be used for custom data. (See, for example, §4.2.4, 160 ➤ ; Table 4.2, 161 ➤ ; and §5.6.7, 238 ➤ .) The time package has functions for measuring time and for parsing and for- mating date, date/time, and time values. The time.After() function can be used to send the current time on the channel it returns after a specified num- ber of nanoseconds have passed, as we saw in an earlier example (332 ➤ ). The time.Tick() and time.NewTicker() functions can be used to provide a channel to which a “tick” is sent repeatedly at a specified interval. The time.Time struct has methods for providing the current time, for formatting a date/time as a string, and for parsing date/times. (We saw time.Time examples in Chapter 8, e.g., 375 ➤ ). ptg7913109 9.4. A Brief Survey of the Go Standard Library 427 9.4.8. Networking Packages The Go standard library has many packages that support networking and relat- ed programming. The net package provides functions and types for communi- cating using Unix domain and network sockets, TCP/IP, and UDP. The package also provides functions for domain name resolution. The net/http package makes use of the net package and has functionality for parsing HTTP requests and replies, and provides a basic HTTP client. The net/http package also includes an easy-to-extend HTTP server, as we saw in Chapter 2 (§2.4, 72 ➤ ) and Chapter 3’s exercises. The net/url package provides URL parsing and query escaping. Some other high-level networking packagesare included in the standard library. One is the net/rpc (“Remote Procedure Call”) package which allows a server to provide objects whose exported methods can be called by clients. Another is the net/smtp (“Simple Mail Transport Protocol”) package which can be used to send email. 9.4.9. The Reflect Package The reflect package provides runtime reflection (also called introspection), that is, the ability to access and interact with values of arbitrary types at runtime. The package also provides some useful utility functions such as reflect.Deep- Equal() which can compare any two values—for example, slices, which aren’t comparable using the == and != operators. Every value in Go has two attributes:its actual value and its type. The reflect. TypeOf() function can tell us the type of any value. x:=8.6 y := float32(2.5) fmt.Printf("var x %v = %v\n", reflect.TypeOf(x), x) fmt.Printf("var y %v = %v\n", reflect.TypeOf(y), y) var x float64 = 8.6 var y float32 = 2.5 Here we have output two floating-point variables and their types as Go var declarations using reflection. When the reflect.ValueOf() function is called on a value it returns a reflect. Value which holds the value but isn’t the value itself. If we want to access the held value we must use one of the reflect.Value methods. word := "Chameleon" value := reflect.ValueOf(word) ptg7913109 428 Chapter 9. Packages text := value.String() fmt.Println(text) Chameleon The reflect.Value type has many methods for extracting the underlying type including reflect.Value.Bool(), reflect.Value.Complex(), reflect.Value.Float(), reflect.Value.Int(), and reflect.Value.String(). The reflect package can also work with collection types such as slices and maps, as well as with structs; it can even access structs’ tag text. (This ability is used by the json and xml encoders and decoders, as we saw in Chapter 8.) type Contact struct { Name string "check:len(3,40)" Id int "check:range(1,999999)" } person := Contact{"Bjork", 0xDEEDED} personType := reflect.TypeOf(person) if nameField, ok := personType.FieldByName("Name"); ok { fmt.Printf("%q %q %q\n", nameField.Type, nameField.Name, nameField.Tag) } "string" "Name" "check:len(3,40)" The underlying value held by a reflect.Value can be changed if it is “settable”. The setability can be checked by calling reflect.Value.CanSet(), which returns a bool. presidents := []string{"Obama","Bushy","Clinton"} sliceValue := reflect.ValueOf(presidents) value = sliceValue.Index(1) value.SetString("Bush") fmt.Println(presidents) [Obama Bush Clinton] Although Go stringsare immutable, any given item in a []string can be replaced by another string, and this is what we have done here. (Naturally, in this particular example, the easiest way to perform the change would be to write presidents[1] = "Bush", and not use introspection at all.) It is not possible to change immutable values themselves, but we can replace an immutable value with another value if we have the original value’s address. count := 1 if value = reflect.ValueOf(count); value.CanSet() { ptg7913109 9.4. A Brief Survey of the Go Standard Library 429 value.SetInt(2)// Would panic! Can't set an int. } fmt.Print(count, "") value = reflect.ValueOf(&count) // Can't call SetInt() on value since value is a *int not an int pointee := value.Elem() pointee.SetInt(3)// OK. Can replace a pointed-to value. fmt.Println(count) 13 This snippet’s output shows that the if condition’s conditional evaluates to false, so its body isn’t executed. Although we cannot set immutable values such as ints, float64s, or strings, we can use the reflect.Value.Elem() method to retrieve a reflect.Value through which we can set a pointed-to value, and this is what we do at the end of the snippet. It is also possible to use reflection to call arbitrary functions and methods. Here is an example that calls a custom TitleCase() function (not shown) twice, once conventionally and once using reflection. caption := "greg egan's dark integers" title := TitleCase(caption) fmt.Println(title) titleFuncValue := reflect.ValueOf(TitleCase) values := titleFuncValue.Call([]reflect.Value{reflect.ValueOf(caption)}) title = values[0].String() fmt.Println(title) Greg Egan's Dark Integers Greg Egan's Dark Integers The reflect.Value.Call() method takes and returns a slice of type []reflect. Value. In this case we pass in a single value (i.e., as a slice of length 1), and retrieve a single result value. We can call methods similarly—and in fact,we can even query to see if a method exists and call it only if it does. a := list.New() // a.Len() == 0 b := list.New() b.PushFront(1)// b.Len() == 1 c := stack.Stack{} c.Push(0.5) c.Push(1.5)// c.Len() == 2 d:=map[string]int{"A": 1,"B": 2,"C": 3}// len(d) == 3 ptg7913109 430 Chapter 9. Packages e:="Four" // len(e) == 4 f := []int{5, 0, 4, 1, 3}// len(f) == 5 fmt.Println(Len(a), Len(b), Len(c), Len(d), Len(e), Len(f)) 012345 Here we create two lists (using the container/list package),one of which we add an item to. We also create a stack (using the custom stacker/stack package we created in Chapter 1;§1.5,21 ➤ ),and add two items to it. And we create a map,a string, and a slice of ints, all of different lengths. We then use a generic custom Len() function to get their lengths. func Len(x interface{}) int { value := reflect.ValueOf(x) switch reflect.TypeOf(x).Kind() { case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String: return value.Len() default: if method := value.MethodByName("Len"); method.IsValid() { values := method.Call(nil) return int(values[0].Int()) } } panic(fmt.Sprintf("'%v' does not have a length", x)) } This function returns the length of the value it is passed or panics if the value’s type isn’t one that supports the notion of length. We begin by getting the value as a reflect.Value since we will need this further on. Then we switch depending on the value’s reflect.Kind. If the value’s kind is one of the built-in types that supports the built-in len() function, we can call the reflect.Value.Len() function directly on the value. Otherwise, we have either a type that doesn’t support the notion of length, or a type that has a Len() method. We use the reflect.Value.MethodByName() method to retrieve the method—or to retrieve an invalid reflect.Value. If the method is valid we call it. There are no arguments to pass in this case because conventional Len() methods take no arguments. When we retrieve a method using the reflect.Value.MethodByName() method, the returned reflect.Value holds both the method and the value. So, when we call reflect.Value.Call(), the value is passed as the receiver. The reflect.Value.Int() method returns an int64; we have converted this to a plain int to match the generic Len() function’s return value’s type. ptg7913109 9.4. A Brief Survey of the Go Standard Library 431 If a value is passed in that doesn’t support the built-in len() function and doesn’t have a Len() method, the generic Len() function panics. We could have handled this error case in other ways—for example, by returning -1 to signify “no length available”, or by returning an int and an error. Go’s reflect package is incredibly flexible and allows us to do things at runtime that depend on the program’s dynamic state. However, to quote Rob Pike, reflection is “a powerful tool that should be used with care and avoided unless strictly necessary”.★ 9.5. Exercises This chapter has three inter-related exercises. The first exercise involves the creation of a small custom package. The second exercise involves the creation of a test for the package. And the third exercise is to write a program that makes use of the package. The exercises increase in difficulty with the third one being rather challenging. 1. Create a package, called, say, my_linkutil (in file my_linkutil/my_linkutil. go). The package should provide two functions. The first function is LinksFromURL(string) ([]string, error) which given a URL string (e.g., ""), returns a slice of all the web page’s unique anchor links (i.e., tags’ href attribute values), and nil (or nil and an error). The second function is LinksFromReader(io.Reader) ([]string, error) which does the same thing only it reads from an io.Reader (e.g., an open file or an http.Response.Body). The LinksFromURL() function should use the LinksFromReader() function internally. A solution is given in linkcheck/linkutil/linkutil.go. The solution’s first function is about 11 lines and makes use of the net/http package’s http.Get() function. The second function is around 16 lines and makes use of the regexp.Regexp.FindAllSubmatch() function. 2. Go’s standard library provides support for HTTP testing (e.g., the net/http/ httptest package), but for this exercise we will be content with testing the my_linkutil.LinksFromReader() function developed in the previous exer- cise. To this end, create a test file (e.g., my_linkutil/my_linkutil_test.go) containing a single test, TestLinksFromReader(*testing.T). The test should read in an HTML file from the file system and a links file which lists the file’s unique anchor links, and it should then compare the links found in the HTML file by the my_linkutil.LinksFromReader() function with the links in the links file. ★ Rob Pike has written an interesting and useful blog entry on Go reflection, 09/laws-of-reflection.html . ptg7913109 432 Chapter 9. Packages It may be convenient to copy the linkcheck/linkutil/index.html and link- check/linkutil/index.links files into the my_linkutil directory for use by the test program. A solution is given in the file linkcheck/linkutil/linkutil_test.go. The solu- tion’s test function is around 40 lines and makes use of the sort.Strings() function to order the found and expected links and the reflect.DeepEqual() function to do the comparison. To help testers the test function lists the first nonmatching links if the test fails. 3. Write a program called, say, my_linkcheck which accepts a single URL on the command line (with or without the http:// prefix), and that checks that every link is valid. The program should work recursively, checking every linked-to page—but excluding non-HTTP links, non-HTML files, and links to external sites. Every page that is checked should be checked in a separate goroutine—thiswill result in many concurrent network accesses,which will be much faster than doing each access sequentially. Naturally, some of the same links will be present on different pages and such links should only be checked once. The program should, of course, use the my_linkutil package developed in the first exercise. A solution is given in linkcheck/linkcheck.go and is around 150 lines. To avoid checking duplicate links the solution keeps a map of the URLs it has seen. The map is kept inside a separate goroutine and three channels are used to communicate with it—one to add a URL, one to query whether a URL has been seen, and one to respond to the query. (An alternative would be to use the safemap from Chapter 7.) An extract from the solution’s output for the command line linkcheck is shown below (with many lines elided either completely or partially). + read ... + read + read ... + read + read ... + links on + checked + checked - can't check non-http link: + checked + read + links on + checked ptg7913109 9.5. Exercises 433 + links on + checked + checked + checked + checked + checked + checked - can't check non-http link: + checked + checked + checked ... ptg7913109 This page intentionally left blank ptg7913109 A Epilogue The Go developers took a long hard look at some of the most widely used pro- gramming languages and tried to discern which features were really useful and productive to have, and which were redundant or even counterproductive. They also drew on the many decadesof programming experiencethat they collectively have had. As a result they produced the Go programming language. In the tradition of Objective-C and C++, Go is an object-oriented “better C”. Like Java, Go has its own syntax,so it doesn’t have to maintain C compatibility in the way that Objective-C and C++ do. But unlike Java, Go compiles to native code and isn’t limited to the speed of a virtual machine. In addition to Go’s novel approach to object orientation with its emphasis on ab- stract interfaces and concrete types with smart embedding and aggregation, Go also supports advanced features such as function literals and closures. And Go’s built-in map and slice types between them serve almost every data structure need. Go’s Unicode-based string type uses the world’s de facto standard encod- ing (UTF-8), and the standard library providesexcellent support at both the byte and character level. Go’s support for concurrency is outstanding. Its lightweight goroutines and its type-safe and high-level channels make it much easier to create concurrent programs compared with many other languages (e.g., C, C++, or Java). And Go’s lightning-fast compilation times are a breath of fresh air, especially to anyone used to building large C++ programs and libraries. Go is already being used by a variety of commercial and noncommercial organizations. And Go is used internally by Google, as well as being available alongside Java and Python, as a language for developing web applications with the Google App Engine ( The language is still evolving quite quickly,yet thanks to the go fix tool,it is easy to update code to work with the latest release. Furthermore, the Go developers intend to keep all Go 1.x versions backward compatible with Go 1 to ensure that Go users have a language that is both stable and being improved at the same time. Go’s standard library is very wide ranging, but even in those cases where it doesn’t have the functionality we need, we can always see if what we want is available from the Go Dashboard (, or in some 435 ptg7913109 436 Appendix A. Epilogue cases we can use external libraries written in other languages. The best place to go for the latest information on Go is; this web site has the current release’s documentation, the (very readable) language specification, the Go Dashboard, blogs, videos, and numerous other supporting documents. Most people learning Go will have knowledge of some other programming language (e.g., C++, Java, Python), and will come to Go with experience of inheritance-based object orientation. Go deliberately doesn’t support inheri- tance, so while it is relatively easy to convert code between, say, C++ and Java, when it comes to Go it is best to go back to the fundamentals of what the code is designed to do—rather than how it does it—and rewrite from scratch in Go. Perhaps the most important distinction is that inheritance-based languages allow code and data to be mixed, whereas Go forces them to be kept separate. This separation gives great flexibility, and also makes it much easier to create concurrent programs, but it can take time and practice for programmers used to inheritance-based languages to adapt to the Go approach. Russ Cox, one of Go’s core developers, says: “It’s unfortunatethat every timesomeoneasks for inheritancethe answer is ‘well, there’s embedding’. Embedding is useful and a kind of inheri- tance,but whenpeoplecome looking for inheritance,embedding isnot the answer. The answer is: you’re thinking in C++, or in Python or Java or Eiffel or whatever. Don’t do that. Think in Go.” Go is a fascinating language to learn and use, as well as being a pleasure to pro- gram with. Go programmers may well find it worth their while to join the Go mailing list—this has many excellent posters and is an ideal place for discus- sion and for questions ( And since Go is developed in the open, it is possible to become a Go developer to help maintain, improve, and extend the language itself ( ptg7913109 B The Dangers of Software Patents Patents are a curious anomaly in capitalist economies, since they are the grant of a private monopoly by the State. Adam Smith roundly condemns monopolies in The Wealth of Nations. In modern times patents enjoy widespread support from a broad range of businesses—fromsmall vacuumcleaner manufacturersto giant pharmaceutical companies. But when it comes to software patents, it is difficult to find anyone who positively supports them except for patent trolls (companies that buy and lease out patent rights but which create nothing themselves),and their lawyers. Back in 1991 Bill Gates said, “If people had understood how patents would be granted when most of today’s ideas were invented, and had taken out patents, the industry would be at a complete standstilltoday.” Of course,hisview appears to be somewhat more nuanced today. Software patents affect every business that produces software—whether for sale or for in-house use. Even nonsoftware giants such as Kraft Foods and Ford Motor Co. are having to spend large sums of money defending against software patent lawsuits. But every programmer is exposed to risk. For example, linked lists have been patented—but not by their inventors Allen Newell, Cliff Shaw, and Herbert Simon when they came up with the idea back in 1955–6, but by someone else, 50 years later ( linked+list). The same thing has happened with skip lists, invented by William Pugh in 1990 and patented over a decade later by someone else. Sadly, there are tens of thousands of other software patents that could be cited as examples, al- though we will mention just one more, “A system and method causes a computer to detect and perform actions on structuresidentified in computer data” granted to Apple in 1999 and which covers all software that manipulates data structures (,946,647). It is easy to imagine that overly broad, obvious, or otherwise meritless patents would be easy to invalidate, but in practice, even giants like Google have found themselves paying out millions of dollars in legal fees to defend themselves. How then can startups and small and medium-sized enterprises (SMEs) hope to bring innovative software to market without being shaken down—again and again—by patent troll “businesses” which feed like parasites on the work of others? 437 ptg7913109 438 Appendix B. The Dangers of Software Patents Here is how the system works in the U.S.—and anywhere else that has similar patent systems. First of all, keep in mind that patents are enforceable even if the “violator” doesn’t know about the patent. Also, fines for patent violation can be huge. Now imagine that a software developer working on some closed-source software completely independently develops a smart algorithm for performing some operation. A patent troll hears on the grapevine that the developer’s com- pany has a new innovation that might make some money. So the troll takes out an injunction against the developer claiming a violation of one of their re- ally general patents (such as the Apple one mentioned above). Now the source code must be submitted for independent analysis. Naturally, the analysis will cover not just the cited patent, but all of the patents held by the troll. The troll now gets to see the developer’s smart algorithm and might even try to patent it—after all they have deep pockets for legal fees and SMEs usually don’t. Of course, trolls don’t want to go to court since their “business model” is based on extortion: They want the developer to sell their products and to pay license fees for the general patents that the troll says have been violated. Of course, most if not all of the patentsare unenforceable,in that they have no merit—but fighting in the courts would bankrupt most SMEs, so they end up paying license fees. And once they are paying this gives added legitimacy to the troll who can then cite their list of licensees to their next victim. Big companies can afford to acquire patents and defend themselves from patent trolls, and don’t necessarily have much sympathy for SMEs which might be, or grow to be, potential rivals—so most of them don’t seem to be concerned. Some companies, including open source company Red Hat, are filing software patents as a defense that can be used to do cross-licensing deals and minimize legal costs. How effective such action will be when giants like Apple, Google, and Microsoft have built up or acquired patent portfolios at a cost of billions of dollars, remains to be seen. Companies that have made such huge investments are unlikely to want to end the patent regime—however destructive it is—as this would incur massive write-offs that would be hard to justify to shareholders or to CEOs whose compensation depends on their company’s share price. SMEs and individual innovators usually don’t have the financial muscle to de- fend themselves from patent troll shakedowns. A very few might try their luck by moving overseas, but most will end up paying serious money for defense (or going out of business in the process),or paying license fees on meritless patents. Software patents are already having a chilling effect on individual and SME software innovation in the U.S., making it harder and more expensive for these businesses—and thereby reducing their capacity to expand and create more jobs for programmers. Of course,many lawyers do benefit from software patents—to the tune of US$11.2 billion in 2008 alone★—while even prosoftware patent economists don’t seem to be able to count a single cent in economic benefits de- riving from software patents. ★ See ptg7913109 Appendix B. The Dangers of Software Patents 439 Nor do software developers outside the U.S. have cause for joy either; some have already had to withdraw their software from the U.S. market to avoid the patent troll protection racket. This means that some innovative software is no longer available in the U.S., potentially giving a competitive advantage to non-U.S. businesses. Furthermore, the Anti-Counterfeiting Trade Agreement (ACTA) specifically covers patents of all kinds, and is being adopted by many countries throughout the world—including the European Union—but not, at the time of this writing,Brazil,China, or Russia. And in addition,the European so called “unitary patent” ( is likely to bring U.S.-style patents to the entire EU. Software is a form of intellectual property that is perfectly well protected by copyright. (For example, Bill Gates was, for a time, the world’s richest person—purely on the basis of software copyright, and prior to the pernicious idea of patenting software.) Despite the success of applying copyright to soft- ware, the U.S. and many other countries have chosen—or been forced by inter- national trade agreements—to incorporate software into their patent regimes. Let us imagine for a moment that all the meritless patents (e.g., those that are overly broad,obvious,or for which there is prior art) were somehow to disappear. This would greatly reduce the shakedowns and would be a boon for innovation. But it still leaves open the key question: Should software be patentable at all? In most countries—including the U.S.—it is not possible to patent mathematical formulas, no matter how new or original they are. Yet mathematical formulas are ideas (which is what patents are designed to “protect”). And we know from the Church-Turing thesis that the logic of any software can be reduced to a mathematical formula, so really, software is mathematics written in a very particular form. This is exactly the argument Donald Knuth makes against the patenting of software. (See Professor Knuth’s short and fascinating letter on this topic: The problem is solvable—but will require legislation to outlaw software patents (or to designate software as mathematics, which is what it is), and to reign in patent offices (whose income is often proportional to the number of patents they grant, irrespective of merit). This is difficult because gaining the attention of politicians is expensive, and, of course, those who can afford to acquire patents can certainly afford to lobby. Also, the subject is so dry and industry-specific that it will hardly make any politician’scareer. But there are people lobbying for change, and doing so in a bipartisan manner. Probably the best starting point to learn more about why software patents are so disastrous and how to combat them is (and in Europe, ptg7913109 This page intentionally left blank ptg7913109 C Selected Bibliography Advanced Programming in the UNIX® Environment,Second Edition W. Richard Stevens and Stephen A. Rago (Addison-Wesley, 2005, ISBN-13: 978-0-201-43307-4) A thorough in-depth introduction to Unix programming using the Unix system call APIs and the standard C library. (The book’s examples are in C.) The Art of Multiprocessor Programming Maurice Herlihy and Nir Shavit (Morgan Kaufmann, 2008, ISBN-13: 978-0-12-370591-4) This book providesa thorough introduction to mostly low-level multithread- ed programming, including small but complete working examples (in Java) that demonstrate all the key techniques. Clean Code: A Handbook of Agile Software Craftsmanship Robert C. Martin (Prentice Hall, 2009, ISBN-13: 978-0-13-235088-4) This book addresses many of the “tactical” issues in programming: good naming, function design, refactoring, and similar. The book has many interesting and useful ideas that should help any programmer improve their coding style and make their programsmore maintainable. (The book’s examples are in Java.) Code Complete: A Practical Handbook of Software Construction,Second Edition Steve McConnell (Microsoft Press, 2004, ISBN-13: 978-0-7356-1967-8) This book shows how to build solid software, going beyond the language specifics into the realms of ideas, principles, and practices. The book is packed with ideas that will make any programmer think more deeply about their programming. Design Patterns: Elements of Reusable Object-Oriented Software Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Addison- Wesley, 1995, ISBN-13: 978-0-201-63361-0) One of the most influential programming books of modern times, even if it isn’t always easy to read. The design patterns are fascinating and of great practical use in everyday programming. 441 ptg7913109 442 Appendix C. Selected Bibliography Domain-Driven Design: Tackling Complexity in the Heart of Software Eric Evans (Addison-Wesley, 2004, ISBN-13: 978-0-321-12521-7) A very interesting book on software design, particularly useful for large multipersonprojects. At heart it isabout creating and refining domain mod- els that represent what the system is designed to do, and about creating a ubiquitous language through which all those involved with the system—not just software engineers—can communicate their ideas. Don’t Make Me Think!: A Common Sense Approach to Web Usability,Second Edition Steve Krug (New Riders, 2006, ISBN-13: 978-0-321-34475-5) A short, interesting, and very practical book on web usability backed up by considerable research and experience. Applying the easy-to-understand ideas in this book will improve any web site of any size. Linux Programming by Example: The Fundamentals Arnold Robbins (Prentice Hall, 2004, ISBN-13: 978-0-13-142964-2) A useful and accessible introduction to Linux programming using the Linux system call APIs. (The book’s examples are in C.) Mastering Regular Expressions,Third Edition Jeffrey E.F. Friedl (O’Reilly, 2006, ISBN-13: 978-0-596-52812-6) This is the standard text on regular expressions—a very interesting and useful book. ptg7913109 Index Symbols & Numbers ! logical NOT operator, 57 != inequality operator, 56–57, 68–69, 70, 164 ""double quotes, 83 #! shebang scripts, 10 $ replacements in regular expres- sions, 120, 126, 129 % modulus operator and formatting placeholder, 47, 60, 69; see also format specifier %= augmented modulus operator, 60 & address of and bitwise AND opera- tor, 45, 46, 55, 60, 142, 143, 144, 167, 246, 247, 248, 267, 269, 284, 382, 383, 384, 387, 393, 394, 395 && logical AND operator, 56, 57 &= augmented bitwise AND operator, 60 &^ bitwise clear operator, 60 &^= augmented bitwise clear opera- tor, 60 * multiplication, dereference, point- er declaration operator and for- matting placeholder, 26, 59, 69, 96, 100, 142, 143, 144, 178, 247, 248, 249, 259, 284, 305, 370, 382, 394 *= augmented multiplication opera- tor, 59, 147 + addition, concatenation, and unary plus operator, 20, 59, 84, 85, 226 ++ increment operator, 20, 59, 186, 188 += augmented addition and string append operator, 20, 59, 84, 85, 88, 140; see also append() - subtraction and unary minus oper- ator, 59 -- decrement operator, 20, 59, 186, 188 -= augmented subtraction operator, 59 . selector operator, 148, 275 ... ellipsis operator, 149, 156, 158, 160, 176, 219, 221, 222, 233, 242, 268, 287, 378 / division operator, 59 /**/ multiline comments, 14, 51 // single-line comments, 14, 51 /= augmented division operator, 59 := short variable declaration opera- tor, 15, 18, 36, 53, 188, 189, 198, 203 ; semicolon, 15, 186 < less than comparison operator, 56–57 <- send/receive communication oper- ator, 44, 45, 207, 210, 318–357 << bitwise shift left operator, 55, 60 <<= augmented bitwise shift left op- erator, 60 <= less than or equal comparison op- erator, 56–57 = assignment operator, 16, 18, 36, 188, 212 == equality operator, 56–57, 68–69, 70, 164 > greater than comparison operator, 56–57 >= greater than or equal comparison operator, 56–57 >> bitwise right shift operator, 60 >>= augmented bitwise right shift operator, 60 [] index and slice operator,16,28,85, 91, 203, 242, 339, 355, 357, 393 443 ptg7913109 444 Index \(backslash), 84 \a (alert or bell), 84 \b (backspace), 84 \f (form feed), 84 \n (newline), 51, 84 \r (carriage return), 84 \t (tab), 84 \Uhhhhhhhh (rune literal), 84 \uhhhh (rune literal), 84 \v (vertical tab), 84 \xhh (rune literal), 84 ^ bitwise XOR and complement opera- tor, 60 ^= augmented bitwise XOR operator, 60 _ blank identifier, 36, 52–53, 154, 170, 188, 291, 358, 417; see also identifiers ``backticks, 75, 78, 96 {}braces, 15, 186 | bitwise OR operator, 55, 60 |= augmented bitwise OR operator, 60 || logical OR operator, 56, 57, 178 5g, 6g, 8g (tool), 9 5l, 6l, 8l (tool), 9 7-bit ASCII encoding, 82 A Abs() cmplx package, 71 math package, 65, 68 abstract vs. concrete types, 22 abstraction and abstract types; see interfaces access operator; see [] index oper- ator access, serialized, 318–319, 335, 341 accessing maps, 39, 168–169, 231 accuracy, floating-point, 64 Acos() cmplx package, 71 math package, 65 Acosh() cmplx package, 71 math package, 65 Add() Int type, 63 WaitGroup type, 350, 351, 352, 354 address; see pointers After() (time package), 332, 333, 426 aggregation, 254–256, 275–282; see also embedding alert or bell (\a), 84 aliasing, package names, 409, 418 americanise (example), 29–40 and, logical; see && operator anonymous fields, struct keyword anonymous functions, 36, 37, 110, 112, 206, 208, 212, 216, 218, 225, 226, 239, 240, 243, 290 anonymous struct, 275 apachereport (example), 341–349 API (Application Programming In- terface); see interfaces App Engine, Google, 2, 435 append() (built-in),25, 27, 55, 77, 129, 132, 150,151,156–157,158,159, 160, 170, 176, 178, 179, 187, 232, 240, 247, 249, 272, 355, 374, 382, 392, 410; see also += AppendBool() (strconv package), 114 AppendFloat() (strconv package), 114 AppendInt() (strconv package), 114 AppendQuote() (strconv package), 114 AppendQuoteRune() (strconv package), 114 AppendQuoteRuneToASCII() (strconv package), 114 AppendUInt() (strconv package), 114 archive files; see .tar files and .zip files archive (package) tar (package); see top-level entry zip (package); see top-level entry archive_file_list (exercise), 250 ptg7913109 Index 445 Args slice (os package), 14, 15, 17, 19, 131–132, 232 arguments, command line, 16, 17,19, 232; see also flag package and commandLineFiles() arguments, function and method; see parameters arrays, 140, 148–150; see also slices iterating, 203 multidimensional, 148 mutability, 149 ASCII encoding, 82 Asin() cmplx package, 71 math package, 65 Asinh() cmplx package, 71 math package, 65 assertions, type; see type assertions assertions, zero-width in regular ex- pressions, 122 assignment operators; see = and := operators assignments, multiple, 31, 188 associative array; see map type asynchronous, channel, 207; see also channels Atan() cmplx package, 71 math package, 65 Atan2() (math package), 65 Atanh() cmplx package, 71 math package, 65 Atoi() (strconv package), 116, 134, 390 atomic updates, 338 audio format, Vorbis, 130 B backslash (\), 84 backreferences, in regular expres- sions, 126 backspace (\b), 84 backticks (``), 75, 96 backtracking,in regular expressions, 120 balanced binary tree; see omap exam- ple bare returns, 34, 189, 219 Base() (filepath package), 19, 131–132, 194, 327 benchmarking, 415–416 big (package; math package) Int (type); see top-level entry NewInt(),63 ProbablyPrime(), 425 Rat (type); see top-level entry big-O notation, 89 bigdigits (example), 16–21, 48 bigdigits (exercise), 48 BigEndian (variable; binary package), 389 binary files, 387–397; see also .gob files binary number, formatting as, 98 binary (package; encoding package), 388, 391 BigEndian (variable), 389 LittleEndian (variable), 388, 389 Read(), 391, 393, 395 Write(), 388 binary tree; see omap example binary vs. linear search, 162–163 bisectLeft() (example), 314 blank identifier (_), 36, 52–53, 154, 170, 188, 291, 358, 417; see also identifiers blocking, channel, 207–208, 209; see also channels bool (type; built-in), 53, 56–57, 195, 204, 318 formatting, 97 Bool() (Value type), 428 Boolean expressions, 193, 204 braces ({}), 15, 186 ptg7913109 446 Index branching, 192–202 break (statement), 24, 177, 186, 204, 205, 331 Buffer (type; bytes package), 111, 200, 201, 243 ReadRune(), 113 String(), 88, 200, 243 WriteRune(), 111 WriteString(), 88, 111, 200, 243 buffers; see bufio package and File type bufio (package), 30, 34, 38 NewReader(), 35, 176, 333, 380 NewWriter(), 35, 378 Reader (type); see top-level entry Writer (type); see top-level entry building Go programs, 11–13 built-in functions append(), 25, 27, 55, 77, 129, 132, 150, 151, 156–157, 158, 159, 160, 170, 176, 178, 179, 187, 232, 240, 247, 249, 272, 355, 374, 382, 392, 410; see also += cap(), 24, 149, 151, 152, 157, 187, 324 close(), 44, 187, 211, 320, 321, 324, 325, 329, 330, 340, 343 complex(), 58, 187; see also cmplx package copy(), 157–158, 159, 187, 268 delete(), 165, 169, 187, 339 imag(), 70, 101, 187 len(), 15, 20, 24, 27, 69, 85, 90, 148, 149, 151, 152, 157, 159, 165, 169, 187, 340 make(), 26, 38, 39, 43, 44, 127, 129, 150, 151, 152, 157, 159, 165, 172, 176, 178, 179, 187, 207, 208, 209, 211, 240, 242, 246, 247, 323, 324, 328, 337, 339, 341, 346, 348, 355, 374, 392, 393, 395, 410 new(), 145, 146, 152, 187, 346 panic(), 32, 69, 70, 113, 187, 191, 196, 213–218, 219, 220, 243 real(), 70, 101, 187 recover(), 32, 187, 213–218 see also functions and special functions built-in types; see bool, byte, error, int, map, rune, string, uint; see also standard library’s types byte ordering, 83, 389 byte (type; built-in), 20, 59, 60, 82, 104, 132, 190 conversion from string, 89–90, 164, 373, 391 conversion of []byte to string, 38, 85, 164, 334, 395 formatting, 102 see also rune and string types bytes (package), 419 Buffer (type); see top-level entry TrimRight(), 333, 334 C C code, external, 9 Call() (Value type), 429, 430 Caller() (runtime package), 291 calling functions, 220–221; see also functions CanBackquote() (strconv package), 114 CanSet() (Value type), 428 cap() (built-in), 24, 149, 151, 152, 157, 187, 324 carriage return (\r), 84 case (keyword); see select and switch statements Cbrt() (math package), 65 Ceil() (math package), 65 cgo (tool), 9 cgrep (example), 326–334 chan (keyword), 43, 44, 208, 209, 210, 318–357; see also channels ptg7913109 Index 447 channels, 41, 43–44, 206–212, 318–357 infinite, 208 iterating, 203 character; see rune type character classes in regular expres- sions, 122 character encoding,fixed vs.variable width, 83 character literal, 20; see also rune type checked type assertion; see type as- sertions class, 254; see also type keyword close() (built-in), 44, 187, 211, 320, 321, 324, 325, 329, 330, 340, 343 Close() (File type), 31, 176, 213, 293, 333, 343, 353, 398, 400 closures, 40, 163, 225–227, 239, 240, 243, 244, 352 cmplx (package; math package), 70 Abs(),71 Acos(),71 Acosh(),71 Asin(),71 Asinh(),71 Atan(),71 Atanh(),71 Conj(),71 Cos(),71 Cosh(),71 Cot(),71 Exp(),71 Inf(),71 IsInf(),71 IsNaN(),71 Log(),71 Log10(),71 NaN(),71 Phase(),71 Polar(),71 Pow(),71 Rect(),71 Sin(),71 Sinh(),71 Sqrt(),71 Tan(),71 Tanh(),71 code point, Unicode; see rune type collection packages, 421–423 collections, slices, map type, and omap example color (package) RGBA (type); see top-level entry command-line arguments, 16, 17, 19, 232; see also flag package commandLineFiles() (example), 176, 410 commas() (example), 357 comments, Go, 14, 51 CommonPathPrefix() (exercise), 250 CommonPrefix() (exercise), 250 Communicating Sequential Process- es (CSP), 3, 315 communication, between goroutines; seechannels,goroutines,and the chan and go keywords comparisons, 56–57, 70, 84, 86–87; see also <, <=, ==,!=, >=, and > op- erators compilation speed, 2 Compile() (regexp package), 37, 121, 214, 327 CompilePOSIX() (regexp package), 121 complex() (built-in), 58, 70, 187; see also cmplx package Complex() (Value type), 428 complex64 (type; built-in), 64, 70 complex128 (type; built-in), 64, 70, 101, 187 comparisons, 70 conversion to complex64,70 formatting, 101 literals, 53, 70 see also cmplx package and imag() and real() ptg7913109 448 Index composing functionality, 35 composite literals, 18, 45, 150, 152, 153, 166, 167 compositing, image, 290 composition;seeaggregation and em- bedding compress (package) gzip (package), see top-level en- try concatenation, fast string,88 concatenation,string;see+ and += op- erators concrete vs. abstract types, 22 concurrency; see channels, gorou- tines, and the chan and go key- words conditional branching, 192–202 Conj() (cmplx package), 9 console; see Stderr, Stdin, and Std- out streams const (keyword), 45, 53, 54, 58, 133, 336, 364, 379, 390 constant expressions, 58 constants,numeric;seeunder literals and specific types construction functions, 27, 263, 264, 306 constructors; see construction func- tions container (package), 421–423 heap (package), 421–422 list (package), 422–423 ring (package), 423 containers; see slices, map type, and omap example Contains() (strings package), 108 contents of; see pointers and * deref- erence operator continue (statement), 132, 133, 186, 204, 205, 324 conversions,42,57,61,162,190–191, 288 []byte to string, 38, 85, 164, 334, 395 []rune to string, 91, 272 complex128 to complex64,70 downsizing, 58, 61 float64 to int,69 int to float64, 61, 73 int to int64,63 rune to string, 87–88, 246 string to []byte, 85, 164, 373, 391 string to []rune,85 string to float64,77 see also the strconv package copy() (built-in), 157–158, 159, 187, 268 Copy() (io package), 353, 354, 399, 401, 402, 405 copy on write, of strings, 140 Copysign() (math package), 65 coroutines; see channels, goroutines, and the chan and go keywords Cos() cmplx package, 71 math package, 45, 65 Cosh() cmplx package, 71 math package, 65 Cot() (cmplx package), 9 Count() (strings package), 108, 172, 173 Cox, Russ, 2, 120, 436 Create() (File type), 31, 293, 397, 400 CreateHeader() (Writer type), 398, 399 cross-platform code, 410–411 crypto (package), 425 rand (package), see top-level en- try sha1 (package), see top-level en- try .csv files, 424 csv (package; encoding package), 424 currying; see closures ptg7913109 Index 449 custom packages, 24–29, 408–417; see also packages custom types, 55, 103, 255–282 named vs. unnamed, 22 see also type D Dashboard, Go, 2 data structures, slices, map type, and struct database (package) sql (package), 423 deadlock, 317–318, 340 debugging, 55, 103–106 decimal number, formatting as, 99; see also int type declarations, order of, 21 Decode() gob package Decoder type, 387 json package Decoder type, 369, 370 xml package Decoder type, 375, 377 DecodeConfig() (image package), 358 DecodeLastRune() (utf8 package), 118 DecodeLastRuneInString() (utf8 pack- age), 91, 118, 229, 230 Decoder (type) gob package Decode(), 387 json package), 369 Decode(), 369, 370 xml package Decode(), 375, 377 DecodeRune() (utf8 package), 118 DecodeRuneInString() (utf8 package), 91, 92, 93, 118, 203, 229, 230 DeepEqual() (reflect package), 57, 236, 427 default (keyword); see select and switch statements defer (statement), 31–32, 35, 36, 44, 97, 176, 211–213, 216, 218, 333, 343, 353, 378, 398, 400 defining methods, 25 definitions, order of, 21 delegation; see embedding delete() (built-in), 165, 169, 187, 339 dereferencing pointers; see pointers dictionary; see map type Dim() (math package), 65 Div() (Int type), 63 division by zero, 68 documentation, Go, 8 documentation, package, 411–413 domain name resolution; see net package Done() (WaitGroup type), 350, 352, 354 double quotes (" "), 83 doubly linked list; see list package downloading Go, 9 downsizing; see conversions Draw() (draw package), 290 draw (package; image package) Draw(), 290 Image (interface), 290, 293, 319 duck typing, 21, 32, 254–255, 268 Duration (type; time package), 332, 333 E E(constant; math package), 65, 104, 105 Elem() (Value type), 429 else (keyword); see if statement embedding, 254–256, 261, 270–274, 275–282, 294, 300; see also ag- gregation empty interface; see interface{} empty struct, 328 ptg7913109 450 Index Encode() gob package Encoder type, 385, 386 json package Encoder type, 367, 370 xml package Encoder type, 373, 375 Encoder (type) gob package Encode(), 385, 386 json package, 367 Encode(), 367, 370 xml package, 373 Encode(), 373, 375 EncodeRune() (utf8 package), 118 encoding ASCII (7-bit, US-), 82 of characters, fixed vs. variable width, 83 UTF-8; see Unicode encoding (package) binary (package), see top-level en- try csv (package); see top-level entry gob (package); see top-level entry json (package), see top-level en- try xml (package); see top-level entry end of file; see EOF endianness, 83, 389 (web site), 439 entry point, 14, 224–225 enumerations, 54–56; see also const and iota environment variables GOPATH, 8, 13, 23, 408, 409, 410, 411, 417, 418 GOROOT, 8, 10, 11, 23, 408, 410, 418, 424 PATH,10 see also variables EOF (io package), 34, 35, 37, 113, 177, 268, 333, 343, 381, 404 EqualFloat() (example), 68–69 EqualFloatPrec() (example), 69 EqualFold() (strings package), 108, 163 equality comparisons (==,!=), 56–57, 68–69, 70, 164 Erf() (math package), 65 Erfc() (math package), 65 error handling, 24, 32, 145, 213 Error() (method), 31 error (type; built-in), 24, 27, 34, 58, 93, 134, 145, 213, 214, 216, 284, 285 Errorf() (fmt package), 33, 58, 94, 97, 216, 285, 293, 365, 382, 384 errors (package), 24 New(), 27, 33, 194, 384 escapes, 84, 102, 375, 377; see also regexp package’s escapes EscapeString() (html package), 78 examples, 10 americanise, 29–40 apachereport, 341–349 bigdigits, 16–21, 48 bisectLeft(), 314 cgrep, 326–334 commandLineFiles(), 176, 410 commas(), 357 EqualFloat(), 68–69 EqualFloatPrec(),69 filter, 322–326 Filter(), 240–241 findduplicates, 349–357 FuzzyBool, 282–288 guess_separator, 171–174 hello, 14–16 Humanize(), 100 indent_sort, 244–249 InsertStringSlice(), 158 InsertStringSliceCopy(), 157–158 invoicedata, 362–397 logPanics(), 218 ptg7913109 Index 451 m3u2pls, 130–135 Memoize(), 242–244 omap, 302–310, 409, 412–413, 414–416 pack, 397–405 Pad(),99 palindrome, 232 pi_by_digits, 62–64 polar2cartesian, 40–47 RemoveStringSlice(), 160 RemoveStringSliceCopy(), 159–160 RomanForDecimal(), 243–244 safemap, 334–340 shaper, 289–301 SimplifyWhitespace(), 111 SliceIndex(), 238–239 stacker, 21–29, 408–409 statistics, 72–78 statistics_nonstop, 216–218 unpack, 397–405 wordfrequencies, 174–180 exceptions; see panic() and recov- er() exec (package; os package), 426 exercises archive_file_list, 250 bigdigits,48 CommonPathPrefix(), 250 CommonPrefix(), 250 Flatten(), 181 font, 311 imagetag, 358 .ini file to map, 181 invoicedata, 406 IsPalindrome(), 250 linkcheck, 432 linkutil, 431–432 Make2D(), 181 map to .ini file, 182 oslice, 313–314 playlists, 135–136 quadratic,79 safeslice, 357–358 shaper, 311–313 sizeimages, 359 soundex, 136–137 statistics,79 UniqueInts(), 180 unpack, 405 utf16-to-utf8, 405 Exit() (os package), 19, 20, 32, 131–132, 327 exiting; see termination and Exit() Exp() cmplx package, 71 Int type, 63 math package, 65 Exp2() (math package), 65 Expand() (Regexp type), 124 ExpandString() (Regexp type), 124 Expm1() (math package), 65 exponential notation, 64, 101 exported identifiers, 52, 202, 264 expression switches, 195–197 Ext() (filepath package), 293, 324, 325 extension, file, 194 external C code, 9 F factory function, 226, 291, 298 fallthrough (statement), 186, 195, 196–197 false (built-in); see bool type fast compilation, 2 fast string concatenation, 88 Fatal() (log package), 19, 20, 32, 131–132, 342 Fatalf() (log package), 20, 327 FieldByName() (Value type), 428 fields; see struct keyword Fields() (strings package), 38, 39, 76, 77, 107–110, 108, 111 FieldsFunc() (strings package), 108, 178 ptg7913109 452 Index file globbing, 176, 410–411 file suffix, 194 File (type; os package), 31, 32, 176 Close(), 31, 176, 213, 293, 333, 343, 353, 398, 400 Create(), 31, 293, 397, 400 Open(), 31, 176, 212, 333, 342, 353, 398, 400 OpenFile(), 31, 397 ReadAt(), 397 Readdir(), 361 Readdirnames(), 361 Seek(), 397 Stat(), 397, 398, 399, 400 WriteAt(), 397 file types .csv, 424 .go, 84, 408, 410 .gob, 385–387 .ini, 131, 181–182 .jpg and .jpeg, 293 .m3u, 130–135 .pls, 130–135 .png, 293 .tar, 399–401, 403–405 .txt, 377–384 .zip, 397–399, 401–403 FileInfo (interface; os package), 351, 361, 397, 399 Mode(), 351, 401 ModTime(), 401 Size(), 351, 353, 401 FileInfoHeader() (zip package), 398, 399 filepath (package; path package), 17, 19, 424 Base(), 19, 131–132, 194, 327 Ext(), 293, 324, 325 FromSlash(), 135 Glob(), 176, 410 Separator (constant), 134, 135 ToSlash(), 399 Walk(), 349, 352 filter (example), 322–326 Filter() (example), 240–241 Find() (Regexp type), 124 FindAll() (Regexp type), 124 FindAllIndex() (Regexp type), 124 FindAllString() (Regexp type), 124, 127 FindAllStringIndex() (Regexp type), 124 FindAllStringSubmatch() (Regexp type), 124, 127 FindAllStringSubmatchIndex() (Regexp type), 124, 128 FindAllSubmatch() (Regexp type), 124 FindAllSubmatchIndex() (Regexp type), 124 findduplicates (example), 349–357 FindIndex() (Regexp type), 124 FindReaderIndex() (Regexp type), 124 FindReaderSubmatchIndex() (Regexp type), 124 FindString() (Regexp type), 124 FindStringIndex() (Regexp type), 124 FindStringSubmatch() (Regexp type), 124, 127, 343, 344, 348 FindStringSubmatchIndex() (Regexp type), 124 FindSubmatch() (Regexp type), 125 FindSubmatchIndex() (Regexp type), 125 fixed vs. variable-width character encoding, 83 flag (package), 426 flags, regular expression, 123 Flatten() (exercise), 181 Float() (Value type), 428 float32 (type; built-in), 61, 64, 70, 283, 285, 427 Float32bits() (math package), 65 Float32frombits() (math package), 65 float64 (type;built-in), 61, 62, 64–70, 73, 100, 187, 221, 304, 318 accuracy, 64 ptg7913109 Index 453 comparisons, 57, 68–69 conversion from int, 61, 69, 73 conversion from string, see Parse- Float() formatting, 100–101 literals, 53, 58 see also math package Float64bits() (math package), 65 Float64frombits() (math package), 65 Float64s() (sort package), 73, 161 Float64sAreSorted() (sort package), 161 Floor() (math package), 66 Flush() (Writer), 35, 36, 378 fmt (package), 55, 93–106, 192 Errorf(), 33, 58, 94, 97, 216, 285, 293, 365, 382, 384 format specifier, 96, 97; see also % symbol Fprint(), 76, 94, 96 Fprintf(), 76, 94, 97, 200, 201, 378 Fprintln(), 45, 94, 96 Fscan(), 383 Fscanf(), 383, 384 Fscanln(), 383 Print(), 94, 96 Printf(), 19, 47, 94–106, 113, 178, 192 Println(), 19, 24, 45, 53, 72, 94–106 Scan(), 383 Scanf(), 383 Scanln(), 383 Sprint(), 94, 99, 178, 242, 357 Sprintf(), 43, 55, 69, 78, 85, 94, 97, 100, 101, 242, 286, 355 Sprintln(),94 Sscan(), 383 Sscanf(), 45, 46, 382, 383 Sscanln(), 383 Stringer (interface), 265, 266–267, 286 font (exercise), 311 for loop, 19, 23, 24, 38, 39, 74, 89, 110, 132, 147, 154, 155, 168, 170, 172, 177,186,200,203–205,320, 321, 324, 325, 330, 331, 339, 343, 355, 390 form feed (\f), 84 Form (field; Request type), 76 format specifier, fmt package, 96, 97; see also % symbol Format() (Time type), 368, 379, 390 FormatBool() (strconv package), 114, 116 FormatFloat() (strconv package), 114 FormatInt() (strconv package), 114, 117 formatting bools, 97 complex numbers, 101 floating-point numbers, 100–101 for debugging, 103–106 integers, 98–99 maps, 106 pointers, 96, 104 runes, 99 slices, 101–103 strings, 101–103 FormatUInt() (strconv package), 114 Fprint() (fmt package), 76, 94, 96 Fprintf() (fmt package), 76, 94, 97, 200, 201, 378 Fprintln() (fmt package), 45, 94, 96 Frexp() (math package), 66 FromSlash() (filepath package), 135 Fscan() (fmt package), 383 Fscanf() (fmt package), 383, 384 Fscanln() (fmt package), 383 FullRune() (utf8 package), 118 FullRuneInString() (utf8 package), 118 ptg7913109 454 Index func (keyword), 14, 15, 25, 35, 45, 55, 208, 216, 218, 219, 223, 226, 232, 238, 240, 241, 243, 246, 291, 303, 305, 323, 324, 343, 378, 379, 388, 389, 413 FuncForPC() (runtime package), 291, 292 functionality, composing, 35 functions, 219–244 anonymous, 36, 37, 110, 112, 206, 208, 212, 216, 218, 225, 226, 239, 240, 243, 290 calling, 220–221 closures, 40, 163, 225–227, 239, 240, 243, 244 construction, 27, 306 factory, 226, 291, 298 generic, 232–238; see also higher order functions higher order, 37, 38, 238–244, 257 literal; see closures optional parameters, 222–223 parameters, 220–223, 254–255 pure, 241; see also memoizing recursive, 227–230, 247, 307 references to, 92, 110, 112, 140, 148, 223, 226, 230–231, 242, 310 variadic, 198, 219, 221–222 wrapper, 218, 226 see also built-in functions and special functions FuzzyBool (example), 282–288 G Gamma() (math package), 66 garbage collector, 3, 32, 40, 139, 141 gc (tool), 9 gccgo (tool), 9 generic functions, 232–238; see also higher order functions Gerrand, Andrew, 2, 207 Getgid() (os package), 401 getters, 264–265 Getuid() (os package), 401 GID; see Getgid() Glob() (filepath package), 176, 410 globbing, file, 176, 410–411 Go building programs, 11–13 comments, 14, 51 Dashboard, 2 documentation, 8 downloading, 9 history of, 1 identifiers, 9, 42, 52–53, 58; see also blank identifier installing, 9, 10–11 shebang (#!) scripts, 10 source code encoding, 9 specification, 69 go build (tool), 11–12, 23, 409, 411 .go files, 84, 408, 410 go fix (tool), 418 go get (tool), 417–418 go install (tool), 1, 13, 409 go (statement), 45, 206, 208, 209, 211, 224, 320–357; see also gor- outines go test (tool), 415–416 go version (tool), 11 go vet (tool), 418 GOARCH (constant; runtime package), 410, 424 .gob files, 385–387 gob (package; encoding package), 385 GobDecoder (interface), 386 GobEncoder (interface), 386 NewDecoder(), 386 NewEncoder(), 385 (web site), 407, 417, 423, 426 godoc (tool), 8, 411–413, 419 gofmt (tool), 186, 188, 419 (web site), 8, 436 ptg7913109 Index 455 GOMAXPROCS() (runtime package), 327 gonow (third-party tool), 10 Google, 1–2 App Engine, 2, 435 GOOS (constant; runtime package), 43, 176, 399, 410, 424 GOPATH (environment variable), 8, 13, 23, 408, 409, 410, 411, 417, 418 GOROOT (environment variable), 8, 10, 11, 23, 408, 410, 418, 424 GOROOT() (runtime package), 424 goroutines, 3, 41, 45, 206–212, 318–357 gorun (third-party tool), 10 goto (statement), 205 greedy matching in regular expres- sions, 123, 127 Griesemer, Robert, 1 grouping constants, imports, and variables, 54 grouping in regular expressions, 123 guard, type switch, 198, 199 guess_separator (example), 171–174 gzip (package; compress package), 400 NewReader(), 403 NewWriter(), 400 Reader (type), 403 Writer (type), 400 H HandleFunc() (http package), 75, 218 handling errors, 24, 32, 213 hash table; see map type HasPrefix() (strings package), 108, 132, 194, 246, 260, 382 HasSuffix() (strings package), 108, 131–132, 226, 400, 403 Header (constant; xml package), 373 Header (type; tar package), 401, 404 heap (package; container package), 421–422 hello (example), 14–16 hexadecimal number, formatting as, 98, 102 higher order functions, 37, 38, 238–244, 257 history, of Go, 1 Hoare, C. A. R, 3 html (package) EscapeString(),78 template (package), see top-level entry HTMLEscape() (template package), 78 http (package; net package) HandleFunc(), 75, 218 ListenAndServe(),75 Request (type); see top-level entry ResponseWriter (interface), 76 Humanize() (example), 100 Hyphen (constant; unicode package), 272 Hypot() (math package), 66, 304 I identifiers, Go, 9, 42, 52–53, 58; see also blank identifier IEEE-754 floating-point format, 64 if (statement), 15, 189, 192–194, 220; see also switch statement Ilogb() (math package), 66 imag() (built-in), 70, 101, 187 Image (interface) draw package, 290, 293, 319 image package, 289, 293, 319 image (package), 289, 425 DecodeConfig(), 358 draw (package), 290 Image (interface), 289, 293, 319 jpeg (package), 293 NewRGBA(), 290, 319 png (package), 293 RGBA (type), 290 Uniform(), 290 ptg7913109 456 Index image (package) (continued) ZP (zero Point), 290 imagetag (exercise), 358 imaginary numbers; see complex128 type and imag() immutability, of strings, 84 import paths, 23 import (statement), 14, 15, 358, 409, 416–417, 418 indent_sort (example), 244–249 Index() reflect package, 235, 236, 428 strings package, 92, 103, 108, 133, 134, 383 index operator; see [] index and slice operator IndexAny() (strings package), 108, 133, 134 IndexFunc() (strings package), 92, 93, 108 indexing slices, 153–154 indexing strings, 20, 90–93 IndexRune() (strings package), 108 indirection; see pointers and * deref- erence operator Inf() cmplx package, 71 math package, 66 inferred type, 53, 70 infinite channel, 208 infinite loop, 24, 203, 208; see also for loop inheritance, 240, 294, 300, 436 .ini file to map (exercise), 181 .ini files, 131, 181–182 init() (special function), 43, 215, 224–225, 231–232, 241, 242, 243, 290, 417 initializing, 27 slices, 17 variables, 15, 74 input/output (I/O); see File type and fmt package InsertStringSlice() (example), 158 InsertStringSliceCopy() (example), 157–158 installing Go, 9, 10–11 instances; see values Int (type; big package), 57, 61–64 Add(),63 Div(),63 Exp(),63 Mul(),63 Sub(),63 int (type; built-in), 55, 57, 58, 59–61, 69, 116, 117, 188, 208, 215, 237, 318, 390, 394 comparisons, 57 conversion from int64,63 conversion to float64, 61, 69 conversion to string,85 formatting, 98–99 literals, 53, 58 see also strconv package Int() (Value type), 428, 430 int8 (type; built-in), 60, 391, 395 int16 (type; built-in), 60, 392 int32 (type;built-in),59,60, 388,390, 395; see also rune type int64 (type;built-in),60,61, 116,117, 215, 241, 356, 391, 401, 430 conversion from int,63 integer literals, 53, 58 integers; see int and similar types and Int and Rat types Interface (interface; sort package), 161, 162, 246, 249, 421 interface (keyword), 220, 237, 265–274, 294, 295, 335, 364 interface{}, 24, 27, 150, 165, 191, 192, 197, 198, 199, 220, 234, 235, 237, 241, 242, 243, 255, 265, 284, 303, 337, 378, 388, 389, 421–423 Interface() (reflect package), 235 ptg7913109 Index 457 interfaces, 22, 202, 255, 265–274, 301, 319 see also Image, Interface, Reader, ReaderWriter, ResponseWriter, Stringer, and Writer inter- faces interpreted string literals, 83 Intn() (rand package), 209, 426 introspection; see reflect package Ints() (sort package), 161, 180 IntsAreSorted() (sort package), 161 inversion, map, 170–171, 179 invoicedata (example), 362–397 invoicedata (exercise), 406 io (package), 30, 34, 424 Copy(), 353, 354, 399, 401, 402, 405 EOF, 34, 35, 37, 113, 177, 268, 333, 343, 381, 404 Pipe(), 322 ReadCloser (interface), 403 Reader (interface), 32, 34, 35, 255, 268, 269–270, 364, 365, 369, 375, 380, 384, 386, 392, 393 ReaderWriter (interface), 32 WriteCloser (interface), 400 Writer (interface), 32, 34, 35, 93, 255, 354, 364, 366, 373, 378, 385, 388, 399 iota (keyword), 54, 336 ioutil (package; io package), 30, 424 ReadAll(), 424 ReadFile(), 38, 131–132, 424 TempFile(), 424 WriteFile(), 424 Is() (unicode package), 118, 119, 258, 272 IsControl() (unicode package), 119 IsDigit() (unicode package), 119 IsGraphic() (unicode package), 119 IsInf() cmplx package, 71 math package, 66 IsLetter() (unicode package), 119, 178 IsLower() (unicode package), 119 IsMark() (unicode package), 119 IsNaN() cmplx package, 71 math package, 66 IsOneOf() (unicode package), 119 IsPalindrome() (exercise), 250 IsPrint() (strconv package), 114 IsPrint() (unicode package), 119 IsPunct() (unicode package), 119 IsSorted() (sort package), 161 IsSpace() (unicode package), 92, 111, 119, 272 IsSymbol() (unicode package), 119 IsTitle() (unicode package), 119 IsUpper() (unicode package), 119 IsValid() (Value type), 430 iterating; see for loop and range iterating arrays, 203 iterating channels, 203 iterating maps, 170, 203 iterating slices, 154–156, 203 iterating strings, 88–90, 203 Itoa() (strconv package), 85, 114, 117 J J0() (math package), 66 J1() (math package), 66 JavaScript Object Notation; see JSON Jn() (math package), 66 Join() (strings package), 14, 16, 55, 108, 111, 180, 414 .jpeg and .jpg files, 293 jpeg (package; image package), 293 JSON (JavaScript Object Notation), 199–202, 363, 365–371 json (package; encoding package), 202, 366 ptg7913109 458 Index json (package; encoding package) (continued) Decoder (type); see top-level entry Encoder (type); see top-level entry Marshal(), 368, 370 NewDecoder(), 369 NewEncoder(), 367 Unmarshal(), 199, 201, 202, 369, 370 justification, of output, 96, 98 K keywords, 52 case; see select and switch state- ments chan, 43, 44, 208, 209, 210, 318–357; see also channels const, 45, 53, 58, 133, 336, 364, 379, 390 default; see select and switch statements else; see if statement func, 14, 15, 25, 35, 45, 55, 208, 216, 218, 219, 223, 226, 232, 238, 240, 241, 243, 246, 291, 303, 305, 323, 324, 343, 378, 379, 388, 389, 413 interface, 220, 237, 265–274,294, 295, 335, 364 iota, 54, 336 nil, 27, 216, 257, 305 range, 19, 38, 39, 74, 89, 110, 147, 154, 155, 168, 170, 172, 200, 203–205, 324, 325, 330, 331, 339, 343, 355, 390 struct, 42, 73, 96, 104, 132, 167, 199, 202, 222, 223, 233, 237, 241, 245, 255, 259–260, 261–262, 275–282, 284, 285, 305, 308, 326, 328, 330, 335, 337, 343, 350, 354, 362, 366, 372, 387 type, 24, 42, 55, 73, 132, 198–199, 202, 223, 245, 246, 254, 255, 256–257, 265, 284, 294, 295, 305, 335, 366, 379, 389, 412 var, 23, 53, 188, 192, 201, 241, 257, 272, 378, 382, 388 see also statements Kind (type; reflect package), 430 Kind() (Value type), 235 L label, 205, 331 LastIndex() (strings package), 92, 108, 194 LastIndexAny() (strings package), 108 LastIndexFunc() (strings package), 92, 108 Ldexp() (math package), 66 left-justification, of output, 96, 98 left-leaning red-black tree; see omap example len() (built-in), 15, 20, 24, 27, 69, 85, 90, 148, 149, 151, 152, 157, 159, 165, 169, 187, 340 Len() (reflect package), 235, 430 Lgamma() (math package), 66 library types; see standard library’s types linear vs. binary search, 162–163 linefeed; see newline linkcheck (exercise), 432 linkutil (exercise), 431–432 list (package; container package), 422–423 ListenAndServe() (http package), 75 LiteralPrefix() (Regexp type), 125 literals, 58 character, 20 complex, 53, 70 composite, 18, 45, 150, 152, 153, 166, 167 ptg7913109 Index 459 floating point, 53; see also float64 type function; see closures integer, 53; see also int and simi- lar types string, 75, 83 LittleEndian (variable; binary pack- age), 388 Ln2 (constant; math package), 66 Ln10 (constant; math package), 66 local variables, 40, 45, 141 Lock() (RWMutex type), 346 Log() cmplx package, 71 math package, 66 log (package), 426 Fatal(), 19, 20, 32, 131–132, 342 Fatalf(), 20, 327 Printf(), 134, 217, 291 Println(), 176, 177, 353 SetFlags(), 426 SetOutput(), 426 Log2E (constant; math package), 66 Log10() cmplx package, 71 math package, 66 Log10E (constant; math package), 66 Log1p() (math package), 66 Log2() (math package), 66 Logb() (math package), 66 logic, short circuit, 56 logical operators; see !,&&, ^, and || operators logPanics() (example), 218 lookups, map, 39, 168–169, 231 looping; see for loop M .m3u files, 130–135 m3u2pls (example), 130–135 main (package), 14, 15, 206, 224 main() (special function), 14, 15, 206, 214, 215, 224–225, 327 make() (built-in), 26, 38, 39, 43, 44, 127, 129, 150, 151, 152, 157, 159, 165, 172, 176, 178, 179, 187, 207, 208, 209, 211, 240, 242, 246, 247, 323, 324, 328, 337, 339, 341, 346, 348, 355, 374, 392, 393, 395, 410 Make2D() (exercise), 181 Map() (strings package), 108, 111–112, 132, 133, 258 map to .ini file (exercise), 182 map (type; built-in), 38, 39, 77, 127, 146, 164–171, 175–180, 199, 242, 243, 298, 318, 335, 339, 345, 346, 348, 355, 356 accessing, 39, 168–169, 231 formatting, 106 inversion, 170–171, 179 iterating, 170, 203 modifying, 169 multivalued, 175 operations, 165 see also omap example Marshal() (json package), 368, 370 Match() regexp package, 121 Regexp type, 125, 333, 334 MatchReader() regexp package, 121 Regexp type, 125 MatchString() regexp package, 121 Regexp type, 125 math (package), 69 Abs(), 65, 68 Acos(),65 Acosh(),65 Asin(),65 Asinh(),65 Atan(),65 Atan2(),65 Atanh(),65 ptg7913109 460 Index math (package) (continued) big (package); see top-level entry Cbrt(),65 Ceil(),65 cmplx (package), see top-level en- try Copysign(),65 Cos(), 45, 65 Cosh(),65 Dim(),65 E(constant), 65, 104, 105 Erf(),65 Erfc(),65 Exp(),65 Exp2(),65 Expm1(),65 Float32bits(),65 Float32frombits(),65 Float64bits(),65 Float64frombits(),65 Floor(),66 Frexp(),66 Gamma(),66 Hypot(), 66, 304 Ilogb(),66 Inf(),66 IsInf(),66 IsNaN(),66 J0(),66 J1(),66 Jn(),66 Ldexp(),66 Lgamma(),66 Ln2 (constant),66 Ln10 (constant),66 Log(),66 Log2E (constant),66 Log10(),66 Log10E (constant),66 Log1p(),66 Log2(),66 Logb(),66 Max(),66 MaxInt32 (constant), 69, 215, 239 MaxUint8 (constant), 58 Min(), 66, 68 MinInt32 (constant), 69, 215 Mod(), 66, 68 Modf(), 67, 68, 69, 70, 100 NaN(), 67, 68 Nextafter(), 67, 68 Phi (constant), 67, 105 Pi (constant), 45, 67, 105 Pow(),67 Pow10(),67 rand (package), see top-level en- try Remainder(),67 Signbit(),67 Sin(), 45, 67 SinCos(),67 Sinh(),67 SmallestNonzeroFloat64 (con- stant), 68 Sqrt(), 67, 221 Sqrt2 (constant), 67 SqrtE (constant), 67 SqrtPhi (constant), 67 SqrtPi (constant), 67 Tan(),67 Tanh(),67 Trunc(),67 Y0(),67 Y1(),67 Yn(),67 Max() (math package), 66 maximum characters to output, 96, 103 MaxInt32 (constant;math package),69, 215, 239 MaxRune (constant; unicode package), 82 MaxUint8 (constant; math package), 58 Memoize() (example), 242–244 memoizing, 241–244 ptg7913109 Index 461 memory management; see garbage collection method expressions, 263 method sets, 22, 191, 260 MethodByName() (Value type), 430 methods, 29, 255, 258–265, 277–278 defining, 25 Error(),31 overriding, 261–262 String(), 31, 55, 96, 103, 155, 166, 260, 265, 266–267, 286 Min() (math package), 66, 68 minimum field width, 96, 103 MinInt32 (constant; math package), 69, 215 MkdirAll() (os package), 401, 402, 404 Mod() (math package), 66 Mode() (FileInfo interface), 351, 401 ModeType (constant; os package), 351, 352 Modf() (math package), 67, 69, 70, 100 modifying maps, 169 modifying slices, 147, 156–160 ModTime() (FileInfo interface), 401 Mul() (Int type), 63 multidimensional arrays, 148 multidimensional slices, 17–18, 150, 204–205 multiple assignments, 31, 188 multivalued maps, 175 MustCompile() (regexp package), 35, 37, 121, 126, 214, 343, 348 MustCompilePOSIX() (regexp package), 121 mutability, of arrays, 149 mutability, of slices, 140 mutual recursion, 227, 228–229 N Name (type; xml package), 372, 374 named fields; see struct keyword named replacements in regular ex- pressions, 126 named return values, 36, 189, 212, 219, 221, 309 named vs. unnamed custom types, 22 NaN() cmplx package, 71 math package, 67 net (package), 427 http (package), see top-level en- try rpc (package), 427 smtp (package), 427 url (package), 427 New() errors package, 27, 33, 194, 384 sha1 package, 353, 354 new() (built-in), 145, 146, 152, 187, 346 NewDecoder() gob package, 386 json package, 369 xml package, 375 NewEncoder() gob package, 385 json package, 367 xml package, 373 NewInt() (big package), 63 newline (\n), 51, 84 NewReader() bufio package, 35, 45, 176, 333, 380 gzip package, 403 strings package, 108 tar package, 403 NewReplacer() (strings package), 108 NewRGBA() (image package), 290, 319 NewTicker() (time package), 426 NewWriter() bufio package, 35, 378 gzip package, 400 tar package, 400 ptg7913109 462 Index NewWriter() (continued) zip package, 398 Nextafter() (math package), 67 nil (keyword), 27, 216, 257, 305 nonblocking, channel, 207, 209; see also channels nongreedy matching in regular ex- pressions, 123, 127 normalization, of whitespace, 111 normalization, Unicode, 86 not, logical; see ! operator null; see nil number formatting, 98–101 numbers; see float64, int, and other specific numeric types NumCPU() (runtime package), 327, 328 NumGoroutine() (runtime package), 351, 353 NumSubexp() (Regexp type), 125 O O_RDWR (constant; os package), 397 objects; see values octal number, formatting as, 98 Ogg container, 130 omap (example), 302–310, 409, 412–413, 414–416 Open() (File type), 31, 176, 212, 333, 342, 353, 398, 400 OpenFile() (File type), 31, 397 OpenReader() (zip package), 401, 402 operations on maps, 165 operations on slices, 151 operators ! logical NOT,57 != inequality, 56–57, 68–69, 70, 164 % modulus and formatting place- holder, 47, 60, 69; see also for- mat specifier %= augmented modulus, 60 & address of and bitwise AND, 45, 46, 55, 60, 142, 143, 144, 167, 246, 247, 248, 267, 269, 284, 382, 383, 384, 387, 393, 394, 395 && logical AND, 56, 57 &= augmented bitwise AND,60 &^ bitwise clear, 60 &^= augmented bitwise clear, 60 * multiplication, dereference, pointer declaration and for- matting placeholder, 26, 59, 69, 96, 100, 142, 143, 144, 178, 247, 248, 249, 259, 284, 305, 370, 382, 394 *= augmented multiplication, 59, 147 + addition, concatenation, and unary plus, 20, 59, 85, 226 ++ increment, 20, 59, 186, 188 += augmented addition and string append, 20, 59, 84, 85, 88, 140 - subtraction and unary minus, 59 -- decrement, 20, 59, 186, 188 -= augmented subtraction, 59 . selector, 148, 275 ... ellipsis, 149, 156, 158, 160, 176, 219, 221, 222, 233, 242, 268, 287, 378 / division, 59 /= augmented division, 59 := short variable declaration, 15, 18, 36, 53, 140, 188, 189, 198, 203 < less than comparison, 56–57 <- send/receive communication, 44, 45, 207, 210, 318–357 << bitwise shift left, 55, 60 <<= augmented bitwise shift left, 60 <= less than or equal comparison, 56–57 ptg7913109 Index 463 = assignment, 16, 18, 36, 188, 212 == equality, 56–57, 68–69, 70, 164 > greater than comparison, 56–57 >= greater than or equal compari- son, 56–57 >> bitwise right shift, 60 >>= augmented bitwise right shift, 60 [] index and slice, 15, 28, 85, 91, 203, 242, 339, 355, 357, 393 ^ bitwise XOR and complement, 60 ^= augmented bitwise XOR,60 | bitwise OR, 55, 60 |= augmented bitwise OR,60 || logical OR, 56, 57, 178 overloading, 61 optional parameters, 222–223 optional statement, 193, 195, 198, 203 or, logical; see || operator order of declarationsand definitions, 21 ordered comparisons (<, <=, >=, >), 56–57 ordered map; see omap example os (package), 423 Args (slice), 14, 15, 17, 19, 131–132, 232 exec (package), see top-level en- try Exit(), 19, 20, 32, 131–132, 327 File (type); see top-level entry FileInfo (interface), see top-level entry Getgid(), 401 Getuid(), 401 MkdirAll(), 401, 402, 404 ModeType (constant), 351, 352 O_RDWR (constant), 397 Stderr (stream), 20, 32, 46 Stdin (stream), 31, 32 Stdout (stream), 31, 32, 46, 94, 131 oslice (exercise), 313–314 overloading, 258 overloading, operator, 61 overriding methods, 261–262 P pack (example), 397–405 package, 14, 215–216, 407–431 aliasing names, 409, 418 collection, 421–423; see also con- tainer package custom, 24–29, 408–417 documenting, 411–413 main, 14, 15 third-party, 417–418 variables, 18 see also bufio, bytes, cmplx, con- tainer, crypto, draw, encoding, errors, filepath, fmt, http, im- age, io, ioutil, json, math, net, os, rand, regexp, reflect, run- time, sha1, sort, strings, sync, and time packages package (statement), 408, 412 Pad() (example), 99 padding, of output, 96, 98 palindrome (example), 232 panic() (built-in), 32, 69, 70, 113, 187, 191,196,213–218,219,220, 243 parameters, 22, 141–142, 220–223, 254–255 Parse() (time package), 370, 376, 377, 383, 395 ParseBool() (strconv package), 98, 115, 116 ParseFloat() (strconv package), 77, 115, 116 ParseForm() (Request type), 76 ptg7913109 464 Index ParseInt() (strconv package), 115, 116 ParseUInt() (strconv package), 115, 116 patents, software, 437–439 PATH (environment variable), 10 path, import, 23 path (package), 424 path/filepath package; see filepath package Phase() (cmplx package), 9 Phi (constant; math package), 67, 105 Pi (constant; math package), 45, 67, 105 pi_by_digits (example), 62–64 Pike, Rob, 1, 385, 431 Pipe() (io package), 322 placeholder (%,*); see %, Printf(), and Sprintf(); see also blank identi- fier plain text files; see .txt files platform-specific code, 410–411 playlists (excercise), 135–136 .pls files, 130–135 .png files, 293 png (package; image package), 293 pointers,26, 27, 28, 29, 139, 141–148, 152, 167,247–248,260,267,285, 318, 362, 369 formatting, 96, 104 Polar() (cmplx package), 9 polar2cartesian (example), 40–47 polymorphism, 198 Porter-Duff image compositing, 290 Pow() cmplx pack