ParEdit

ParEdit (paredit.el) is a minor mode for performing structured editing of S-expression data, e.g. Lisp or Scheme source code.

ParEdit helps keep parentheses balanced and adds many keys for moving S-expressions and moving around in S-expressions. Its behavior can be jarring for those who may want transient periods of unbalanced parentheses, such as when typing parentheses directly or commenting out code line by line.

Skeleton like matching paren insertion is a small part of ParEdit’s functionality; its true genius won’t be obvious until you code in some list oriented language.

Installing

You can install it through MELPA with M-x package-install RET paredit.

The stable release and beta release are available at paredit.org.

A spinoff of ParEdit (by the same author) is also now available in the current CVS version of EdWin, MIT Scheme’s Emacs clone. Type M-x paredit-mode RET to enable it, or add the following code to your .edwin file to enable it automatically in the Scheme mode:

    (add-event-receiver! (ref-variable scheme-mode-hook)
      (lambda (buffer)
        (enable-buffer-minor-mode! buffer (ref-mode-object paredit))))

(You can substitute other modes for scheme-mode also.)

Activate it automatically

To use ParEdit with Emacs’ Lisp modes, add the following to .emacs:

    (autoload 'enable-paredit-mode "paredit" "Turn on pseudo-structural editing of Lisp code." t)
    (add-hook 'emacs-lisp-mode-hook       #'enable-paredit-mode)
    (add-hook 'eval-expression-minibuffer-setup-hook #'enable-paredit-mode)
    (add-hook 'ielm-mode-hook             #'enable-paredit-mode)
    (add-hook 'lisp-mode-hook             #'enable-paredit-mode)
    (add-hook 'lisp-interaction-mode-hook #'enable-paredit-mode)
    (add-hook 'scheme-mode-hook           #'enable-paredit-mode)

ParEdit and ElDoc

To use ParEdit with ElDoc, you should make ElDoc aware of ParEdit’s most used commands:

    (require 'eldoc) ; if not already loaded
    (eldoc-add-command
     'paredit-backward-delete
     'paredit-close-round)

Whenever the listed commands are used, ElDoc will automatically refresh the minibuffer.

ParEdit and SLIME REPL

   (add-hook 'slime-repl-mode-hook (lambda () (paredit-mode +1)))

SLIME’s REPL has the very annoying habit of grabbing DEL which interferes with paredit’s normal operation. To alleviate this problem use the following code:

          ;; Stop SLIME's REPL from grabbing DEL,
          ;; which is annoying when backspacing over a '('
          (defun override-slime-repl-bindings-with-paredit ()
            (define-key slime-repl-mode-map
                (read-kbd-macro paredit-backward-delete-key) nil))
          (add-hook 'slime-repl-mode-hook 'override-slime-repl-bindings-with-paredit)

ParEdit and "Electric RETURN"

ParEdit works particularly well with an “electric” RETURN function.

If RETURN is pressed when the cursor is before a closing paren, the following code will add an extra newline. The extra newlines are re-gathered by paredit-close-round, which ParEdit binds to “)” by default.

The code is loosely based on code on the AutoPairs page:

  (defvar electrify-return-match
    "[\]}\)\"]"
    "If this regexp matches the text after the cursor, do an \"electric\"
  return.")
  (defun electrify-return-if-match (arg)
    "If the text after the cursor matches `electrify-return-match' then
  open and indent an empty line between the cursor and the text.  Move the
  cursor to the new line."
    (interactive "P")
    (let ((case-fold-search nil))
      (if (looking-at electrify-return-match)
      (save-excursion (newline-and-indent)))
      (newline arg)
      (indent-according-to-mode)))
  ;; Using local-set-key in a mode-hook is a better idea.
  (global-set-key (kbd "RET") 'electrify-return-if-match)

ParEdit, ElDoc, Show-Paren and "Electric RETURN"

Here’s a short video demonstration of ParEdit working with ElDoc and Show-Paren minor modes along with the electric RETURN function defined above.

Adding this code to your .emacs will set the modes as in the video:

  (add-hook 'emacs-lisp-mode-hook
        (lambda ()
          (paredit-mode t)
          (turn-on-eldoc-mode)
          (eldoc-add-command
           'paredit-backward-delete
           'paredit-close-round)
          (local-set-key (kbd "RET") 'electrify-return-if-match)
          (eldoc-add-command 'electrify-return-if-match)
          (show-paren-mode t)))

ParEdit, and extreme barfarge and slurpage

There are times when I’m in a list like (a b| c d e f g), and I know that want to barf out everything following point from the current form. Since C-} doesn’t take a prefix argument, there’s really no quick way to do this with the standard paredit.el mode.

To solve this problem I’ve written some extreme barfage and slurpage commands, so that C-M-} will do what I described and yield (a b) c d e f g. Evaluate the following after paredit is loaded:

  (defun paredit-barf-all-the-way-backward ()
    (interactive)
    (paredit-split-sexp)
    (paredit-backward-down)
    (paredit-splice-sexp))
  (defun paredit-barf-all-the-way-forward ()
    (interactive)
    (paredit-split-sexp)
    (paredit-forward-down)
    (paredit-splice-sexp)
    (if (eolp) (delete-horizontal-space)))
  (defun paredit-slurp-all-the-way-backward ()
    (interactive)
    (catch 'done
      (while (not (bobp))
        (save-excursion
          (paredit-backward-up)
          (if (eq (char-before) ?\()
              (throw 'done t)))
        (paredit-backward-slurp-sexp))))
  (defun paredit-slurp-all-the-way-forward ()
    (interactive)
    (catch 'done
      (while (not (eobp))
        (save-excursion
          (paredit-forward-up)
          (if (eq (char-after) ?\))
              (throw 'done t)))
        (paredit-forward-slurp-sexp))))
  (nconc paredit-commands
         '("Extreme Barfage & Slurpage"
           (("C-M-)")
                        paredit-slurp-all-the-way-forward
                        ("(foo (bar |baz) quux zot)"
                         "(foo (bar |baz quux zot))")
                        ("(a b ((c| d)) e f)"
                         "(a b ((c| d)) e f)"))
           (("C-M-}" "M-F")
                        paredit-barf-all-the-way-forward
                        ("(foo (bar |baz quux) zot)"
                         "(foo (bar|) baz quux zot)"))
           (("C-M-(")
                        paredit-slurp-all-the-way-backward
                        ("(foo bar (baz| quux) zot)"
                         "((foo bar baz| quux) zot)")
                        ("(a b ((c| d)) e f)"
                         "(a b ((c| d)) e f)"))
           (("C-M-{" "M-B")
                        paredit-barf-all-the-way-backward
                        ("(foo (bar baz |quux) zot)"
                         "(foo bar baz (|quux) zot)"))))
  (paredit-define-keys)
  (paredit-annotate-mode-with-examples)
  (paredit-annotate-functions-with-examples)

– JohnWiegley

ParEdit and delete-indentation

It’s currently possible to use delete-indentation (aka join-lines) to pull code up from one line and stick it into the end-of-line comment of another line, invalidating the code. The following replacement for delete-indentation ought to prevent that from happening by always making sure the comment is replaced at the end of the line.

  ;; FIXME Mishandles adjoining whole-line comments (reinserts preceding at end)
  (defun paredit-delete-indentation (&optional arg)
    "Handle joining lines that end in a comment."
    (interactive "*P")
    (let (comt)
      (save-excursion
        (move-beginning-of-line (if arg 1 0))
        ;; FIXME This misidentifies a semicolon inside a string as a comment
        ;; in lisp
        (when (skip-syntax-forward "^<" (point-at-eol))
      (setq comt (delete-and-extract-region (point) (point-at-eol)))))
      (delete-indentation arg)
      (when comt
        (save-excursion
          (move-end-of-line 1)
      (insert " ")
      (insert comt)))))
  (define-key paredit-mode-map (kbd "M-^") 'paredit-delete-indentation)

Discussion

I downloaded ParEdit a long time ago, but I did not succeed in using it daily. I have two questions here:

1. What is really useful with ParEdit that we can’t get with standard skeletons?

On the surface it looks like both ParEdit and skeleton are inserting matching parenthesis. However, a skeleton insertion is the end of the story. Think of a ParEdit insertion is just an indicator to show you the context. You can move past “)” to use it or just completely pretend it’s not there. The insertion is just a side-effect of ParEdit’s true purpose – clarify context while coding. – LeWang

Check out the ParEdit reference table mentioned above, especially the parts “Depth-Changing Commands” and “Barfage & Slurpage”. Skeletons are like templates, they’re static. ParEdit can modify the structure of Lisp code. So basically, they have different purposes. – PeterBarabas

2. How do you delete a parenthesis? I often end up with unbalanced parentheses when coding something (without paredit), and activating ParEdit prevents me from deleting the leading parenthesis.

It takes a bit of getting used to, but after that you’ll never end up with unbalanced parentheses. Just use ParEdit’s commands, e.g., paredit-open-parenthesis, paredit-wrap-sexp instead of manually typing/editing parentheses. ParEdit really makes transforming Lisp code easy. – PeterBarabas
Expression validity isn’t enforced with C-w; I find that’s the easiest way to make a quick fix that ParEdit won’t like.
C-u DEL falls back to ‘backward-delete-char’. Similarly, you can insert single parentheses with C-q ( and C-q ). – YannHodique
Works like a charm! – XavierMaillard

3. I’m working on a mode for a syntax where m#foo is a legal symbol but otherwise, # comments out the line (like in shell scripts). For some reason, putting (paredit-mode 1) in a run-hook for this mode, or in sh-mode-hook, makes it complain on a line like (foo m#bar) with condition-case: Unmatched bracket or quote. But, if I turn on paredit-mode after doing M-x sh-mode, it works fine, and I can edit around parens just as if they said (foo bar). Anyone know why this happens?

4. I’ll bite. Can anyone tell me what’s going on in the “Short video demonstration” video above? I turn on ParEdit and I don’t get newlines after my parens and I don’t get it showing me what comes next after typing “defun”. If these are other major/minor modes then the caption should be replaced with “Here’s a demonstration of ParEdit playing nice with secret-mode.el and fun-minor-mode-that-were-keeping-secret-from-dale.el.

– DaleWiles

It is implicit by the context, the two features you are seeing are “eldoc” and “electric RET” which has a snippet of code just under the video link for you to try it 😊 – GlauberPrado

5. Older version of Paredit managed newlines differently to the current beta version. I would like to suggest newline insertion context-based. It should add a newline after, for example, of close the right parenthesis of a last of arguments in defun. Many examples can be thought. I am aware it could be instrusive and some paredit users reject this feature. Although it could be an optional feature. What do you think?

I really see no need for this kind of complexity – I imagine it would be hard to implement and impossible to satisfy the varying users’ needs. Currently you have both ‘paredit-close-round’ and ‘paredit-close-round-and-newline’. You can bind these to any keys you like () and M-) by default). I find manually choosing the right behaviour much more useful than some kind of “AI” in this regard. – stepnem

CategoryCode CategoryEditing CategoryParentheses.