Lisp 入门教程(刘鑫最新翻译) Common LISP Hints Geoffrey J. Gordon Friday, February 5, 1993 Modified by Bruno Haible 简体中文版翻译: 刘鑫 Note: This tutorial introduction to Common Lisp was written for the CMU environment, so some of the details of running lisp toward the end may differ from site to site. 注意:这份 Common Lisp 入门教程是针对 CMU 环境编写,所以在其它环境运行 LISP 时可能会有细节上的区别。 Further Information 附: The best LISP textbook I know of is Guy L. Steele Jr. _Common LISP: the Language_. Digital Press. 1984. 据我所知最好的 Lisp 书籍是: Guy L. Steele Jr. _Common LISP: the Language_. Digital Press. 1984. The first edition is easier to read; the second describes a more recent standard. (The differences between the two standards shouldn't affect casual programmers.) 第一版很容易阅读,第二版介绍了更新的标准。(两个标准的差异很 小,对于粗 心的程序员没有什么区别。) A book by Dave Touretsky has also been recommended to me, although I haven't read it, so I can't say anything about it. 我还记得 Dave Touretsky 写了一本,不过我从来没读过,所以不能对那本书 发表评论。 Symbols 符号 A symbol is just a string of characters. There are restrictions on what you can include in a symbol and what the first character can be, but as long as you stick to letters, digits, and hyphens, you'll be safe. (Except that if you use only digits and possibly an initial hyphen, LISP will think you typed an integer rather than a symbol.) Some examples of symbols: 符号仅仅是字符串。你可以在符号中包含字母、数字、连接符等等,唯一的限制 就是要以字母开头。(如果你只输入数字,最多再以一个连接符开头的话,LISP 会认为你输入了一个整数而不是符号。)例如: a b c1 foo bar baaz-quux-garply Some things you can do with symbols follow. (Things after a ">" prompt are what you type to the LISP interpreter, while other things are what the LISP interpreter prints back to you. The ";" is LISP's comment character: everything from a ";" to the end of line is ignored.) 接下来我们可以做些事情。(">"标记表示你向 LISP 输入的东西,其它的是LISP 打印返回给你的。";"是 LISP 的注释符:";"后面的整行都会被忽略。) > (setq a 5) ;store a number as the value of a symbol 5 > a ;take the value of a symbol 5 > (let ((a 6)) a) ;bind the value of a symbol temporarily to 6 6 > a ;the value returns to 5 once the let is finished 5 > (+ a 6) ;use the value of a symbol as an argument to a function 11 > b ;try to take the value of a symbol which has no value Error: Attempt to take the value of the unbound symbol B There are two special symbols, t and nil. The value of t is defined always to be t, and the value of nil is defined always to be nil. LISP uses t and nil to represent true and false. An example of this use is in the if statement, described more fully later: 有两个特殊的符号, t 和 nil 。 t 的值总是定义为 t,nil 的值总是定义为 nil 。LISP用 t 和 nil 代表 true 和 false。以下是使用这个功能的 if 语句, 后面再做详细说明: > (if t 5 6) 5 > (if nil 5 6) 6 > (if 4 5 6) 5 The last example is odd but correct: nil means false, and anything else means true. (Unless we have a reason to do otherwise, we use t to mean true, just for the sake of clarity.) 最后一个例子看起来很怪,但是没有错:nil 代表 false ,其它任意值代表 true。 (为了代码清晰,在没有什么特别原因的情况下,我们用 t 代表 true。) Symbols like t and nil are called self-evaluating symbols, because they evaluate to themselves. There is a whole class of self-evaluating symbols called keywords; any symbol whose name starts with a colon is a keyword. (See below for some uses for keywords.) Some examples: t 和 nil 这样的符号被称为自解析符号,因为他们解析为自身。自解析符号称为 关键字;任一以冒号开头的符号都是关键字。(下面是一些关键字的应用)如下 所示: > :this-is-a-keyword :THIS-IS-A-KEYWORD > :so-is-this :SO-IS-THIS > :me-too :ME-TOO Numbers 数值 An integer is a string of digits optionally preceded by + or -. A real number looks like an integer, except that it has a decimal point and optionally can be written in scientific notation. A rational looks like two integers with a / between them. LISP supports complex numbers, which are written #c(r i) (where r is the real part and i is the imaginary part). A number is any of the above. Here are some numbers: 数值类型是数字文本,可能会以 + 或 - 开头。实数和整数很相像,但是它带有 小数点,还可能写成科学计数法。有理数就像是两个整数之间带有一个/。LISP 支 持复数,写为#c(r i)(r 表示实部,i 表示虚部)。以上统称为数值。下面是一 些数值: 5 17 -34 +6 3.1415 1.722e-15 #c(1.722e-15 0.75) The standard arithmetic functions are all available: +, -, *, /, floor, ceiling, mod, sin, cos, tan, sqrt, exp, expt, and so forth. All of them accept any kind of number as an argument. +, -, *, and / return a number according to type contagion: an integer plus a rational is a rational, a rational plus a real is a real, and a real plus a complex is a complex. Here are some examples: 标准的计算函数包括: +, -, *, /, floor, ceiling, mod, sin, cos, tan, sqrt, exp, expt 等等。所有这些函数都可以接受任意数值类型参数。+、-、* 和 /返回尽可能大的类型:一个整数加一个有理数返回有理数,一个有理数加一个 实数 是一个实数,一个实数加一个复数是一个复数。如下所示: > (+ 3 3/4) ;type contagion 15/4 > (exp 1) ;e 2.7182817 > (exp 3) ;e*e*e 20.085537 > (expt 3 4.2) ;exponent with a base other than e 100.90418 > (+ 5 6 7 (* 8 9 10)) ;the fns +-*/ all accept multiple arguments There is no limit to the absolute value of an integer except the memory size of your computer. Be warned that computations with bignums (as large integers are called) can be slow. (So can computations with rationals, especially compared to the corresponding computations with small integers or floats.) 对于整数来说,唯一的大小限制就是机器的内存。当然大数值运算(这 会调用大整数)可能会很慢。(因此我们可以计算有理数,尤其是小整数和浮点 数 的比较运算) Conses A cons is just a two-field record. The fields are called "car" and "cdr", for historical reasons. (On the first machine where LISP was implemented, there were two instructions CAR and CDR which stood for "contents of address register" and "contents of decrement register". Conses were implemented using these two registers.) cons 就是一个包含两个字段的记录。出于历史原因,两个字段分别被称为 "car"和"cdr"。(在第一台实现 LISP 的机器上,用 CAR 和 CDR 代表"地址寄 存器的内容"和"指令寄存器的内容"。Conses 的实现主要依靠这两个寄存器。) Conses are easy to use: Conses 很容易使用: > (cons 4 5) ;Allocate a cons. Set the car to 4 and the cdr to 5. (4 . 5) > (cons (cons 4 5) 6) ((4 . 5) . 6) > (car (cons 4 5)) 4 > (cdr (cons 4 5)) 5 Lists You can build many structures out of conses. Perhaps the simplest is a linked list: the car of each cons points to one of the elements of the list, and the cdr points either to another cons or to nil. You can create such a linked list with the list fuction: 你可以构造 conses 之外的结构。可能最简单的是链表:每一个cons 的 car 指向 链表 的一个元素,cdr指向另一个 cons 或者 nil。我们可以使用list 函数构造链表。 > (list 4 5 6) (4 5 6) Notice that LISP prints linked lists a special way: it omits some of the periods and parentheses. The rule is: if the cdr of a cons is nil, LISP doesn't bother to print the period or the nil; and if the cdr of cons A is cons B, then LISP doesn't bother to print the period for cons A or the parentheses for cons B. So: 需要注意的是 LISP 用一种特殊的方式打印链表:它忽略掉某些分隔和括号。 规则如下:如果某个 cons 的 cdr 是 nil ,LISP 不打印 nil 和段标记,如果 cons A 的 cdr 是 cons B,LISP 不打印 cons B 的括号和 cons A 的分隔符。 如下: > (cons 4 nil) (4) > (cons 4 (cons 5 6)) (4 5 . 6) > (cons 4 (cons 5 (cons 6 nil))) (4 5 6) The last example is exactly equivalent to the call (list 4 5 6). Note that nil now means the list with no elements: the cdr of (a b), a list with 2 elements, is (b), a list with 1 element; and the cdr of (b), a list with 1 element, is nil, which therefore must be a list with no elements. 最后一个例子相当于调用(list 4 5 6)。要注意的是这里 nil 表示没有元素的空 链表:包含两个元素的链表(a b)中,cdr 是(b),一个含有单个元素的链表;包含 一个元素的链表(b),cdr 是 nil,故此这里必然是一个没有元素的链表。 The car and cdr of nil are defined to be nil. nil 的 car 和 cdr 定义为 nil。 If you store your list in a variable, you can make it act like a stack: 如果你把链表存储在变量中,可以将它当作堆栈来使用: > (setq a nil) NIL > (push 4 a) (4) > (push 5 a) (5 4) > (pop a) 5 > a (4) > (pop a) 4 > (pop a) NIL > a NIL Functions You saw one example of a function above. Here are some more: 前面我们讨论过一些函数的例子,这里还有更多: > (+ 3 4 5 6) ;this function takes any number of arguments 18 > (+ (+ 3 4) (+ (+ 4 5) 6)) ;isn't prefix notation fun? 22 > (defun foo (x y) (+ x y 5)) ;defining a function FOO > (foo 5 0) ;calling a function 10 > (defun fact (x) ;a recursive function (if (> x 0) (* x (fact (- x 1))) 1)) FACT > (fact 5) 120 > (defun a (x) (if (= x 0) t (b (- x)))) ;mutually recursive functions A > (defun b (x) (if (> x 0) (a (- x 1)) (a (+ x 1)))) B > (a 5) T > (defun bar (x) ;a function with multiple statements in (setq x (* x 3)) ;its body -- it will return the value (setq x (/ x 2)) ;returned by its final statement (+ x 4)) BAR > (bar 6) 13 When we defined foo, we gave it two arguments, x and y. Now when we call foo, we are required to provide exactly two arguments: the first will become the value of x for the duration of the call to foo, and the second will become the value of y for the duration of the call. In LISP, most variables are lexically scoped; that is, if foo calls bar and bar tries to reference x, bar will not get foo's value for x. 当我们定义函数的时候,设定了两个参数,x 和 y。现在当我们调用 foo,需要 给出两个参数:第一个在 foo 函数调用时成为 x 的值,第二个成为 y 的值。 在 LISP 中,大部分的变量都是局部的,如果 foo 调用了 bar ,bar 中虽然使用了 名字为 x 的引用,但 bar 得不到 foo 中的 x 。 The process of assigning a symbol a value for the duration of some lexical scope is called binding. 在调用过程中给一个符号赋值的操作被称为绑定。 You can specify optional arguments for your functions. Any argument after the symbol &optional is optional: 我们可以给函数指定可选参数,在符号&optional 之后的参数是可选参数: > (defun bar (x &optional y) (if y x 0)) BAR > (defun baaz (&optional (x 3) (z 10)) (+ x z)) BAAZ > (bar 5) 0 > (bar 5 t) 5 > (baaz 5) 15 > (baaz 5 6) 11 > (baaz) 13 It is legal to call the function bar with either one or two arguments. If it is called with one argument, x will be bound to the value of that argument and y will be bound to nil; if it is called with two arguments, x and y will be bound to the values of the first and second argument, respectively. bar 函数的调用规则是要给出一个或两个参数。如果它用一个参数调用,x 将会绑 定到这个参数值上,而 y 就是 nil;如果用两个参数调用它,x 和 y 会分别绑定 到第一和第二个值上。 The function baaz has two optional arguments. It specifies a default value for each of them: if the caller specifies only one argument, z will be bound to 10 instead of to nil, and if the caller specifies no arguments, x will be bound to 3 and z to 10. baaz 函数有两个可选参数。它为它们分别提供了默认值:如果调用者只给出了一 个参数,z 会绑定为 10 而不是 nil,如果调用者没有给出参数,x 会绑定为 3, 而z绑 定为 10。 You can make your function accept any number of arguments by ending its argument list with an &rest parameter. LISP will collect all arguments not otherwise accounted for into a list and bind the &rest parameter to that list. So: 在参数列表的最后设置一个 &rest 参数,可以使我们的函数接受任意数目的参数。 LISP 把所有的附加参数都放进一个链表并绑定到 &rest 参数。如下: > (defun foo (x &rest y) y) FOO > (foo 3) NIL > (foo 4 5 6) (5 6) Finally, you can give your function another kind of optional argument called a keyword argument. The caller can give these arguments in any order, because they're labelled with keywords. 最后,我们可以为函数指定一种被称为关键字参数的可选参数。调用者可以用 任意顺序调用这些参数,因为他们已经通过关键字标示出来。 > (defun foo (&key x y) (cons x y)) FOO > (foo :x 5 :y 3) (5 . 3) > (foo :y 3 :x 5) (5 . 3) > (foo :y 3) (NIL . 3) > (foo) (NIL) An &key parameter can have a default value too: 关键字参数也可以有默认值: > (defun foo (&key (x 5)) x) FOO > (foo :x 7) 7 > (foo) 5 Printing Some functions can cause output. The simplest one is print, which prints its argument and then returns it. 某些函数可以用来输出。最简单的一个是 print,它可以打印参数并且返回 它们。 > (print 3) 3 3 The first 3 above was printed, the second was returned. 首先打印 3,然后返回它。 If you want more complicated output, you will need to use format. Here's an example: 如果你需要更复杂的输出,可能会用到 format,这里有个例子: > (format t "An atom: ~S~%and a list: ~S~%and an integer: ~D~%" nil (list 5) 6) An atom: NIL and a list: (5) and an integer: 6 The first argument to format is either t, nil, or a stream. T specifies output to the terminal. Nil means not to print anything but to return a string containing the output instead. Streams are general places for output to go: they can specify a file, or the terminal, or another program. This handout will not describe streams in any further detail. 第一个参数可以是 t,nil 或者一个流。t 意味着输出到终端;nil 意味着不打印 任何东西,而是把它返回。流是用于输出的通用方式:它可以是一个指定的文件, 或者一个终端,或者另一个程序。这里不再详细描述流的更多细节。 The second argument is a formatting template, which is a string optionally containing formatting directives. 第二个参数是个格式化模版,即一个包含格式化设定的字符串。 All remaining arguments may be referred to by the formatting directives. LISP will replace the directives with some appropriate characters based on the arguments to which they refer and then print the resulting string. 所有其它的参数由格式化设定引用。LISP 会根据标示所引用的参数,将其替换 为合适的字符,并返回结果字符串。 Format always returns nil unless its first argument is nil, in which case it prints nothing and returns a string. 如果 format 的第一个参数是 nil ,它返回一个字符串,什么也不打印,否则 它总是返回 nil 。 There are three different directives in the above example: ~S, ~D, and ~%. The first one accepts any LISP object and is replaced by a printed representation of that object (the same representation which is produced by print). The second one accepts only integers. The third one doesn't refer to an argument; it is always replaced by a carriage return. 前面的例子中有三种不同的标示:~S,~D 和~%。第一个接受任意 LISP 对象并且 将 其替换为这个对象的打印描述(与使用 print 打印出的描述信息相同)。第二个 Another useful directive is ~~, which is replaced by a single ~. 另一个常用的标示是~~,它替换为单个~。 Refer to a LISP manual for (many, many) additional formatting directives. LISP 手册中介绍了其它(很多,很多)的格式化标示。 Forms and the Top-Level Loop The things which you type to the LISP interpreter are called forms; the LISP interpreter repeatedly reads a form, evaluates it, and prints the result. This procedure is called the read-eval-print loop. 我们输入到 LISP 解释器的东西被称为语句;LISP 解释器逐条循环读取每条语句, 进行解析,将结果打印出来。这个过程被称为读取-解析-打印循环。 Some forms will cause errors. After an error, LISP will put you into the debugger so you can try to figure out what caused the error. LISP debuggers are all different; but most will respond to the command "help" or ":help" by giving some form of help. 某些语句会发生错误,LISP 会引领我们进入调试器,以便我们找出错误原因。 LISP 的各种调试器有很多差异,不过使用"help"或":help"命令就会给出一 些语句帮助。 In general, a form is either an atom (for example, a symbol, an integer, or a string) or a list. If the form is an atom, LISP evaluates it immediately. Symbols evaluate to their value; integers and strings evaluate to themselves. If the form is a list, LISP treats its first element as the name of a function; it evaluates the remaining elements recursively, and then calls the function with the values of the remaining elements as arguments. 通常,一个语句是一个原子(例如,一个符号或者整数,或者字符串)或者一个 列表, 如果换某个语句是原子,LISP 立即解析它。符号解析为它们的值;整数和字符 串解析 为它们自身。如果语句是一个列表,LISP 视它的第一个元素为函数名;它递归 的解析 其余的元素,然后将它们的值作为参数来调用这个函数。 For example, if LISP sees the form (+ 3 4), it treats + as the name of a function. It then evaluates 3 to get 3 and 4 to get 4; finally it calls + with 3 and 4 as the arguments. The + function returns 7, which LISP prints. 例如,如果 LISP 遇到语句 (+ 3 4),它尝试将 + 作为函数名。然后将 3 解析为 3,4 解 析为 4;最后用3和4作为参数调用+。LISP 打印出 + 函数的返回值 7。 The top-level loop provides some other conveniences; one particularly convenient convenience is the ability to talk about the results of previously typed forms. LISP always saves its most recent three results; it stores them as the values of the symbols *, **, and ***. For example: 顶级循环还提供了一些其它的便利;一个特别方便的地方就是获取以前输入的 语句的结果。LISP 总会保存最近三个结果;它将它们保存在*,**和***三个符 号的值中,例如: > 3 3 > 4 4 > 5 5 > *** 3 > *** 4 > *** 5 > ** 4 > * 4 Special forms There are a number of special forms which look like function calls but aren't. These include control constructs such as if statements and do loops; assignments like setq, setf, push, and pop; definitions such as defun and defstruct; and binding constructs such as let. (Not all of these special forms have been mentioned yet. See below.) 有几个特殊语句看起来像函数调用,但其实不是。这里面包括流程控制语句,例 如 if 语句和 do loops;赋值语句,例如 setq, setf,push 和 pop;定义语句 例如 defun 和 defstruct;以及绑定构造,如 let。(这里没有提及所有的特殊 语句。我们继续。) One useful special form is the quote form: quote prevents its argument from being evaluated. For example: 一个很有用的特殊语句是 quote :quote 取消其参数的绑定状态。例如: > (setq a 3) 3 > a 3 > (quote a) A > 'a ;'a is an abbreviation for (quote a) A Another similar special form is the function form: function causes its argument to be interpreted as a function rather than being evaluated. For example: 另一个类似的语句是 fuction:function 使得解释器将其参数视为一个函数而不是 解析值,例如: > (setq + 3) 3 > + 3 > '+ + > (function +) # > #'+ ;#'+ is an abbreviation for (function +) # The function special form is useful when you want to pass a function as an argument to another function. See below for some examples of functions which take functions as arguments. 当我们需要将一个函数作为参数传递给另一个函数时会用到 function 语句。后面 有些示例函数将函数作为参数。 Binding Binding is lexically scoped assignment. It happens to the variables in a function's parameter list whenever the function is called: the formal parameters are bound to the actual parameters for the duration of the function call. You can bind variables anywhere in a program with the let special form, which looks like this: 绑定是词汇作用域赋值(汗,怎么读怎么别扭--译者)。每当函数调用时,它就 发生于函数的参数列变量中:形式参数被取代为调用函数时的实际参数。你可以 在 程序中随处绑定变量,就像下面这样: (let ((var1 val1) (var2 val2) ...) body) Let binds var1 to val1, var2 to val2, and so forth; then it executes the statements in its body. The body of a let follows exactly the same rules that a function body does. Some examples: let 把 val1 绑定到 var1,把 val2 绑定到 var2,依次类推;然后在它的程序体 中执 行语句。let 的程序体与函数体的执行规则完全相同。例如: > (let ((a 3)) (+ a 1)) 4 > (let ((a 2) (b 3) (c 0)) (setq c (+ a b)) c) 5 > (setq c 4) 4 > (let ((c 5)) c) 5 > c 4 Instead of (let ((a nil) (b nil)) ...), you can write (let (a b) ...). 你可以用 (let (a b) ...) 代替 (let ((a nil) (b nil)) ...) 。 The val1, val2, etc. inside a let cannot reference the variables var1, var2, etc. that the let is binding. For example, val1,val2 等等。在 let 内部不能引用 var1,var2 等等 let 正在绑定的成员。 例如(简而言之,在参数表中,形式参数之间不能互相引用--译者): > (let ((x 1) (y (+ x 1))) y) Error: Attempt to take the value of the unbound symbol X If the symbol x already has a global value, stranger happenings will result: 如果符号 x 已经有了一个全局值,会产生一些奇怪的结果: > (setq x 7) 7 > (let ((x 1) (y (+ x 1))) y) 8 The let* special form is just like let except that it allows values to reference variables defined earlier in the let*. For example, let* 语句类似于 let,但它允许引用之前在 let* 中定义的变量的值。例如: > (setq x 7) 7 > (let* ((x 1) (y (+ x 1))) y) 2 The form 语句 (let* ((x a) (y b)) ...) is equivalent to 等价于 (let ((x a)) (let ((y b)) ...)) Dynamic Scoping The let and let* forms provide lexical scoping, which is what you expect if you're used to programming in C or Pascal. Dynamic scoping is what you get in BASIC: if you assign a value to a dynamically scoped variable, every mention of that variable returns that value until you assign another value to the same variable. 与我们在 C 或 Pascal 中编写程序不同,let 和 let* 语句提供了词汇作用域。动态 作用域是我们在 BASIC 里用的那种:如果我们给一个动态作用域变量赋了值,那 么 所有对这个变量的访问都会取得这个值,直到给同一个变量赋了另一个值为止。 In LISP, dynamically scoped variables are called special variables. You can declare a special variable with the defvar special form. Here are some examples of lexically and dynamically scoped variables. 在 LISP 中,动态作用域变量被称为特化变量 。你可以用 special 语句 defvar 定义一个特化变量。这里有一些词汇化和动态作用域变量的示例。 In this example, the function check-regular references a regular (ie, lexically scoped) variable. Since check-regular is lexically outside of the let which binds regular, check-regular returns the variable's global value. 在以下示例中,check-regular 函数引用一个 regular (也就是一个词汇作用域) 变量。因为 check-regular 在绑定 regular 的 let 外部词汇化,check-regular 返回变量的全局值。 > (setq regular 5) 5 > (defun check-regular () regular) CHECK-REGULAR > (check-regular) 5 > (let ((regular 6)) (check-regular)) 5 In this example, the function check-special references a special (ie, dynamically scoped) variable. Since the call to check-special is temporally inside of the let which binds special, check-special returns the variable's local value. 在这个例子中,函数 check-special 引用了一个 特化(动态作用域)变量。 因此 check-special 调用临时发生于特化绑定的 let 内部,check-special 返回变量的局部值。 > (defvar *special* 5) *SPECIAL* > (defun check-special () *special*) CHECK-SPECIAL > (check-special) 5 > (let ((*special* 6)) (check-special)) 6 By convention, the name of a special variable begins and ends with a *. Special variables are chiefly used as global variables, since programmers usually expect lexical scoping for local variables and dynamic scoping for global variables. 方便起见,特化变量以一个 * 开始和结束。特化变量主要用于全局变量,因为 程序员通常期望局部变量使用词汇作用域,全局变量使用动态作用域。 For more information on the difference between lexical and dynamic scoping, see _Common LISP: the Language_. 词汇和动态作用域的更多差异参见《Common LISP: the Language》。 Arrays The function make-array makes an array. The aref function accesses its elements. All elements of an array are initially set to nil. For example: make-array 函数定义一个数组。aref 函数访问它的元素。所有元素初始化为 nil。 例如: > (make-array '(3 3)) #2a((NIL NIL NIL) (NIL NIL NIL) (NIL NIL NIL)) > (aref * 1 1) NIL > (make-array 4) ;1D arrays don't need the extra parens #(NIL NIL NIL NIL) Array indices always start at 0. 数组索引从 0 开始。 See below for how to set the elements of an array. 以后讨论如何设置一个数组的元素。 Strings A string is a sequence of characters between double quotes. LISP represents a string as a variable-length array of characters. You can write a string which contains a double quote by preceding the quote with a backslash; a double backslash stands for a single backslash. For example: 字符串是包含在双引号之间的字符串。LISP 用字符串代表一个变长字符数组。 我们可以用一个反斜杠加一个双引号来表示字符串中的双引号,两个反斜杠表示 一个单独的反斜杠。例如: "abcd" has 4 characters "\"" has 1 character "\\" has 1 character Here are some functions for dealing with strings: 有一些函数可以用于操作字符串: > (concatenate 'string "abcd" "efg") "abcdefg" > (char "abc" 1) #\b ;LISP writes characters preceded by #\ > (aref "abc" 1) #\b ;remember, strings are really arrays The concatenate function can actually work with any type of sequence: concatenate 函数可以用于连接任何类型的序列: > (concatenate 'string '(#\a #\b) '(#\c)) "abc" > (concatenate 'list "abc" "de") (#\a #\b #\c #\d #\e) > (concatenate 'vector '#(3 3 3) '#(3 3 3)) #(3 3 3 3 3 3) Structures LISP structures are analogous to C structs or Pascal records. Here is an example: LISP 结构类似于 C 结构或 Pasacal 记录。如下所示: > (defstruct foo bar baaz quux) FOO This example defines a data type called foo which is a structure containing 3 fields. It also defines 4 functions which operate on this data type: make-foo, foo-bar, foo-baaz, and foo-quux. The first one makes a new object of type foo; the others access the fields of an object of type foo. Here is how to use these functions: 这个示例定义了一个名为 foo 的数据类型,它是一个带有三个字段的结构。 它同时定义了 4 个函数用于操作这个数据类型:make-foo,foo-ba,foo-baaz 和 foo-quux。第一个函数构造了一个 foo 类型的新对象,另外三个用于访问一个 foo 类型的对象。以下是这些函数的用法: > (make-foo) #s(FOO :BAR NIL :BAAZ NIL :QUUX NIL) > (make-foo :baaz 3) #s(FOO :BAR NIL :BAAZ 3 :QUUX NIL) > (foo-bar *) NIL > (foo-baaz **) 3 The make-foo function can take a keyword argument for each of the fields a structure of type foo can have. The field access functions each take one argument, a structure of type foo, and return the appropriate field. meke-foo 函数可以使用 foo 结构类型的字段作为关键字参数。字段访问函数以 foo 类型的结构为参数,返回对应的字段。 See below for how to set the fields of a structure. 以后讨论如何设置一个结构的字段。 Setf Certain forms in LISP naturally define a memory location. For example, if the value of x is a structure of type foo, then (foo-bar x) defines the bar field of the value of x. Or, if the value of y is a one- dimensional array, (aref y 2) defines the third element of y. LISP 确认语句的时间,自然就会定义一个内存区域。例如,如果 x 的值是 foo 类型 的一个结构,(foo-bar x)定义 x 中 bar 字段的值。或者,如果 y 是一个一维数组, (aref y 2)定义 y 的第三个元素。 The setf special form uses its first argument to define a place in memory, evaluates its second argument, and stores the resulting value in the resulting memory location. For example, setf 语句用于在内存中将它的第一个参数定位到第二个参数上,并且返回内存 区域的结果值。例如: > (setq a (make-array 3)) #(NIL NIL NIL) > (aref a 1) NIL > (setf (aref a 1) 3) 3 > a #(NIL 3 NIL) > (aref a 1) 3 > (defstruct foo bar) FOO > (setq a (make-foo)) #s(FOO :BAR NIL) > (foo-bar a) NIL > (setf (foo-bar a) 3) 3 > a #s(FOO :BAR 3) > (foo-bar a) 3 Setf is the only way to set the fields of a structure or the elements of an array. setf 是给结构的字段或者数组的元素赋值的唯一方法。 Here are some more examples of setf and related functions. 这里还有一些关于 setf 和相关函数的示例。 > (setf a (make-array 1)) ;setf on a variable is equivalent to setq #(NIL) > (push 5 (aref a 1)) ;push can act like setf (5) > (pop (aref a 1)) ;so can pop 5 > (setf (aref a 1) 5) 5 > (incf (aref a 1)) ;incf reads from a place, increments, 6 ;and writes back > (aref a 1) 6 Booleans and Conditionals LISP uses the self-evaluating symbol nil to mean false. Anything other than nil means true. Unless we have a reason not to, we usually use the self-evaluating symbol t to stand for true. LISP 用自解析符号 nil 来代表 false。任何 nil 之外的其它东西都代表 true。 除非有什么特别的原因,我们总是使用 true 的标准自解析符号 t。 LISP provides a standard set of logical functions, for example and, or, and not. The and and or connectives are short-circuiting: and will not evaluate any arguments to the right of the first one which evaluates to nil, while or will not evaluate any arguments to the right of the first one which evaluates to t. LISP 提供了一个逻辑运算的标准函数集,例如,与、或、非。与和或支持短路 算法: 在遇到第一个 nil 后不再解析右面的参数,而与在遇到第一个t 后也不再解析右 面的 参数。 LISP also provides several special forms for conditional execution. The simplest of these is if. The first argument of if determines whether the second or third argument will be executed: LISP 还为条件控制提供了几个特殊语句。最简单的是 if。if 语句的第一个参数 决定了 执行第二或第三个参数中的哪一个: > (if t 5 6) 5 > (if nil 5 6) 6 > (if 4 5 6) 5 If you need to put more than one statement in the then or else clause of an if statement, you can use the progn special form. Progn executes each statement in its body, then returns the value of the final one. 如果你需要在 then 或 else 块中放置超过一行的语句,可以使用特殊语句 progn。 Progn 在它的程序体内依次执行每一条语句,返回最后一个结果。 > (setq a 7) 7 > (setq b 0) 0 > (setq c 5) 5 > (if (> a 5) (progn (setq a (+ b 7)) (setq b (+ c 8))) (setq b 4)) 13 An if statement which lacks either a then or an else clause can be written using the when or unless special form: 一个没有 else 或 when 的语句可以写为 when 或 unless 语句。 > (when t 3) 3 > (when nil 3) NIL > (unless t 3) NIL > (unless nil 3) 3 When and unless, unlike if, allow any number of statements in their bodies. (Eg, (when x a b c) is equivalent to (if x (progn a b c)).) 与 if 不同,when 和 unless 允许在程序体内放置任意多的语句。(例如, (when x a b c) 等价于 (if x (progn a b c))) > (when t (setq a 5) (+ a 6)) 11 More complicated conditionals can be defined using the cond special form, which is equivalent to an if ... else if ... fi construction. 更复杂的条件控制可以用 cond 语句定义实现,它定价于一个 if ... else if ... fi 条件。 A cond consists of the symbol cond followed by a number of cond clauses, each of which is a list. The first element of a cond clause is the condition; the remaining elements (if any) are the action. The cond form finds the first clause whose condition evaluates to true (ie, doesn't evaluate to nil); it then executes the corresponding action and returns the resulting value. None of the remaining conditions are evaluated; nor are any actions except the one corresponding to the selected condition. For example: 一个 cond 由 symbol 符号和其后的若干条件分支组成,每一个分支是一个 list。 cond 分支的第一个元素是条件;其它的元素(如果有的话)是动作。cond 语句查 找第一个条件解析为 true 的分支,执行其响应动作并返回结果值。其它条件不会 被解析,除了这个响应之外的分支也不会执行。例如: > (setq a 3) 3 > (cond ((evenp a) a) ;if a is even return a ((> a 7) (/ a 2)) ;else if a is bigger than 7 return a/2 ((< a 5) (- a 1)) ;else if a is smaller than 5 return a-1 (t 17)) ;else return 17 2 If the action in the selected cond clause is missing, cond returns what the condition evaluated to: 如果选定的分支没有响应动作,cond 返回条件的解析结果。 > (cond ((+ 3 4))) 7 Here's a clever little recursive function which uses cond. You might be interested in trying to prove that it terminates for all integers x at least 1. (If you succeed, please publish the result.) 这里用 cond 巧妙的实现了一个递归函数。你可能会有兴趣证明它对于所有的整数 x 都少有个终结。(如果你成功了,请发表出来。) > (defun hotpo (x steps) ;hotpo stands for Half Or Triple Plus One (cond ((= x 1) steps) ((oddp x) (hotpo (+ 1 (* x 3)) (+ 1 steps))) (t (hotpo (/ x 2) (+ 1 steps))))) A > (hotpo 7 0) 16 The LISP case statement is like a C switch statement: LISP 的 case 语句类似于 C 的 switch 语句: > (setq x 'b) B > (case x (a 5) ((d e) 7) ((b f) 3) (otherwise 9)) 3 The otherwise clause at the end means that if x is not a, b, d, e, or f, the case statement will return 9. 末尾的 otherwise 语句意味着 x 不是 a、b、c、d、e、f,case 语句将会返回 9。 Iteration The simplest iteration construct in LISP is loop: a loop construct repeatedly executes its body until it hits a return special form. For example, LISP 中最简单的迭代结构是 loop:loop 结构反复执行它的程序体直到到达 一个返回语句,例如: > (setq a 4) 4 > (loop (setq a (+ a 1)) (when (> a 7) (return a))) 8 > (loop (setq a (- a 1)) (when (< a 3) (return))) NIL The next simplest is dolist: dolist binds a variable to the elements of a list in order and stops when it hits the end of the list. dolist 是下一个最简单的:dolist 把一个变量依次绑定到一个列表的各个元素 上, 在到达列表结尾时结束。 > (dolist (x '(a b c)) (print x)) A B C NIL Dolist always returns nil. Note that the value of x in the above example was never nil: the NIL below the C was the value that dolist returned, printed by the read-eval-print loop. Dolist 总是返回 nil。注意上例中的 x 永远不会为 nil:C 后面的 NIL 是 dolist 返回的,它被读取-解析-打印循环所打印。 The most complicated iteration primitive is called do. A do statement looks like this: 更复杂的迭代称为 do。do 语句的示例如下: > (do ((x 1 (+ x 1)) (y 1 (* y 2))) ((> x 5) y) (print y) (print 'working)) 1 WORKING 2 WORKING 4 WORKING 8 WORKING 16 WORKING 32 The first part of a do specifies what variables to bind, what their initial values are, and how to update them. The second part specifies a termination condition and a return value. The last part is the body. A do form binds its variables to their initial values like a let, then checks the termination condition. As long as the condition is false, it executes the body repeatedly; when the condition becomes true, it returns the value of the return-value form. do 的第一步是指定绑定的变量,它们的初始值,以及如何更新。第二步是指定 一 个终止条件和返回值。最后是程序体。do 语句像 let 一样把它的变量绑定到初 始值, 然后校验终止条件。条件为 false 时,它重复执行程序体;当条件为 true,它 返回 return-value 语句的值。 The do* form is to do as let* is to let. do* 语句之于 do 如同 let* 之于 let。 Non-local Exits The return special form mentioned in the section on iteration is an example of a nonlocal return. Another example is the return-from form, which returns a value from the surrounding function: 前一节中迭代示例里的 renturn 语句是一个无定位返回的示例,另一个是 return-from,它从包围它的函数中返回指定值。 > (defun foo (x) (return-from foo 3) x) FOO > (foo 17) 3 Actually, the return-from form can return from any named block -- it's just that functions are the only blocks which are named by default. You can create a named block with the block special form: 实际上,return-from 语句可以从任何已命名的语句块中退出--只是默认情况下 函数是唯一的命名语句块而已。我们可以用 block 语句自己定义一个命名语句块。 > (block foo (return-from foo 7) 3) 7 The return special form can return from any block named nil. Loops are by default labelled nil, but you can make your own nil-labelled blocks: return 语句可以从任何 nil 命名的语句块中返回。默认情况下循环是 nil 命名,而 我们可以创建自己的 nil 标记语句块。 > (block nil (return 7) 3) 7 Another form which causes a nonlocal exit is the error form: 另外一个无定位退出语句是 error 语句: > (error "This is an error") Error: This is an error The error form applies format to its arguments, then places you in the debugger. error 语句格式化它的参数,然后进入调试器。 Funcall, Apply, and Mapcar Earlier I promised to give some functions which take functions as arguments. Here they are: 早先我承诺介绍一些可以将函数作为参数调用函数,它们在这里: > (funcall #'+ 3 4) 7 > (apply #'+ 3 4 '(3 4)) 14 > (mapcar #'not '(t nil t nil t nil)) (NIL T NIL T NIL T) Funcall calls its first argument on its remaining arguments. funcall 用它的其它参数调用它的第一个参数。 Apply is just like funcall, except that its final argument should be a list; the elements of that list are treated as if they were additional arguments to a funcall. Apply 和 funcall 很相像,不过它的最后一个参数可以是一个列表;这个列表被 看 作是 funcall 的附加参数。 The first argument to mapcar must be a function of one argument; mapcar applies this function to each element of a list and collects the results in another list. mapcar 的第一个参数必须是一个单参数的函数;mapcar 在列表上逐个元素应用 该 函数,并将返回值包含如另一个链表。 Funcall and apply are chiefly useful when their first argument is a variable. For instance, a search engine could take a heuristic function as a parameter and use funcall or apply to call that function on a state description. The sorting functions described later use funcall to call their comparison functions. Funcall 和 apply 主要用于第一个参数是变量的场合。例如,搜索引擎把一个启 发式 函数作为参数,在一个状态描述上应用 funcall 或者 apply。 Mapcar, along with nameless functions (see below), can replace many loops. Mapcar 配合匿名函数(后面介绍),可以取代很多循环。 Lambda If you just want to create a temporary function and don't want to bother giving it a name, lambda is what you need. 如果你只是想创建一个临时函数,并不想给它一个命名,lambda 如你所愿。 > #'(lambda (x) (+ x 3)) (LAMBDA (X) (+ X 3)) > (funcall * 5) 8 The combination of lambda and mapcar can replace many loops. For example, the following two forms are equivalent: lambda 和 mapcar 的组合可以取代很多循环,例如,如下的两个语句是等价的: > (do ((x '(1 2 3 4 5) (cdr x)) (y nil)) ((null x) (reverse y)) (push (+ (car x) 2) y)) (3 4 5 6 7) > (mapcar #'(lambda (x) (+ x 2)) '(1 2 3 4 5)) (3 4 5 6 7) Sorting LISP provides two primitives for sorting: sort and stable-sort. LISP 提供两种主要的排序:排序和稳定排序。 > (sort '(2 1 5 4 6) #'<) (1 2 4 5 6) > (sort '(2 1 5 4 6) #'>) (6 5 4 2 1) The first argument to sort is a list; the second is a comparison function. The sort function does not guarantee stability: if there are two elements a and b such that (and (not (< a b)) (not (< b a))), sort may arrange them in either order. The stable-sort function is exactly like sort, except that it guarantees that two equivalent elements appear in the sorted list in the same order that they appeared in the original list. sort 的第一个参数是一个列表,第二个是一个比较函数。sort 函数不保证稳定 性: 如果这里有a和b两个元素,(and (not (< a b)) (not (< b a))),sort 可能会改 变它们的位置。stable-sort(稳定排序)与 sort 非常像,不过它确保两个相等 的元素在排序后的列表中的顺序与排序前列表中的顺序相同。 Be careful: sort is allowed to destroy its argument, so if the original sequence is important to you, make a copy with the copy-list or copy-seq function. 注意:sort 可能会破坏它的参数,如果原始序列对我们很重要,最好使用 copy-list 或 copy-seq 函数创建一个副本。 Equality LISP has many different ideas of equality. Numerical equality is denoted by =. Two symbols are eq if and only if they are identical. Two copies of the same list are not eq, but they are equal. 关于相等,LISP 有很多不同的观点。数值相等意味着=。两个符号当且仅当他们 完全 一样时相同。同一个列表的两个副本不相同,但是它们相等。 > (eq 'a 'a) T > (eq 'a 'b) NIL > (= 3 4) NIL > (eq '(a b c) '(a b c)) NIL > (equal '(a b c) '(a b c)) T > (eql 'a 'a) T > (eql 3 3) T The eql predicate is equivalent to eq for symbols and to = for numbers or the same type: 谓词 eql 代表相等,它对于符号表示相同,对于数值表示=。 > (eql 2.0 2) NIL > (= 2.0 2) T > (eq 12345678901234567890 12345678901234567890) NIL > (= 12345678901234567890 12345678901234567890) T > (eql 12345678901234567890 12345678901234567890) T The equal predicate is equivalent to eql for symbols and numbers. It is true for two conses if and only if their cars are equal and their cdrs are equal. It is true for two structures if and only if the structures are the same type and their corresponding fields are equal. equal 谓词对于符号和数值是相等。当且仅当两个 conse 的 car 和 cdr 都相等时它们 才是相等的。当且仅当两个结构是同类型而且各字段都相等时它们相等。 Some Useful List Functions These functions all manipulate lists. 这些函数都用来操作列表。 > (append '(1 2 3) '(4 5 6)) ;concatenate lists (1 2 3 4 5 6) > (reverse '(1 2 3)) ;reverse the elements of a list (3 2 1) > (member 'a '(b d a c)) ;set membership -- returns the first tail (A C) ;whose car is the desired element > (find 'a '(b d a c)) ;another way to do set membership A > (find '(a b) '((a d) (a d e) (a b d e) ()) :test #'subsetp) (A B D E) ;find is more flexible though > (subsetp '(a b) '(a d e)) ;set containment NIL > (intersection '(a b c) '(b)) ;set intersection (B) > (union '(a) '(b)) ;set union (A B) > (set-difference '(a b) '(a)) ;set difference (B) Subsetp, intersection, union, and set-difference all assume that each argument contains no duplicate elements -- (subsetp '(a a) '(a b b)) is allowed to fail, for example. Subsetp,intersection,union 和 set-difference 都允许各个参数包含不匹配 的元素 --例如,(subsetp '(a a) '(a b b))可以为 fail。 Find, subsetp, intersection, union, and set-difference can all take a :test keyword argument; by default, they all use eql. Find,subsetp,intersection,union和 set-difference 都可以接受一个:test 关键 字参数;默认情况下,它们是等价的。 Getting Started with Emacs You can use Emacs to edit LISP code: most Emacses are set up to enter LISP mode automatically when they find a file which ends in .lisp, but if yours isn't, you can type M-x lisp-mode. 你可以使用 Emacs 编辑 LISP 代码:Emaces 在打开.lisp 文件时总会自动进入 LISP 模式, 不过如果我们的 Emacs 没有成功进入这个状态,可以通过输入 M-x lisp-mode 做到。 You can run LISP under Emacs, too: make sure that there is a command in your path called "lisp" which runs your favorite LISP. For example, you could type 我们也可以在 Emacs 下运行 LISP:先确保在我们的私人路径下可以运行一个叫 "LISP"的命令。例如,我们可以输入: ln -s /usr/local/bin/clisp ~/bin/lisp Then in Emacs type M-x run-lisp. You can send LISP code to the LISP you just started, and do all sorts of other cool things; for more information, type C-h m from any buffer which is in LISP mode. 然后在 Emacs 中输入 M-x run-lip。我们可以向 LISP 发送先前的 LISP 代码,做其它 很酷的事情;在 LISP 模式下的任何缓冲输入 C-h m 可以得到进一步的信息。 Actually, you don't even need to make a link. Emacs has a variable called inferior-lisp-program; so if you add the line 实际上,我们甚至不需要建立链接。Emacs 有一个变量叫 inferior-lisp-program; 所以我们可以把下面这行 (setq inferior-lisp-program "/usr/local/bin/clisp") to your .emacs file, Emacs will know where to find CLISP when you type M-x run-lisp. 输入到自己的 .emacs 文件中,Emacs 就会知道在你输入 M-x run-lisp 时去哪里寻找 CLISP。
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