How do you comment out all or part of a Lisp s-exp using Paredit?

Question

When editing Lisp code, occasionally it's useful to entirely comment out a top-level definition, like this:

;(defun some-fn-which-is-broken (x)
;  ...)

... or comment out only part of an s-expression, like this:

(foo x
;    y
     z)

... and then recompile the file and test something in the REPL, etc.

With paredit-mode enabled, this doesn't work. Here's what happens, if the point is right before the first paren below:

(defun some-fn (x)
  ...)

and you type a semicolon, what is entered is a semicolon and a newline:

;
(defun some-fn (x)
  ...)

Same with commenting out part of the s-expression:

(foo x
;    
     y
     z)

I think that if the definition is all on one line, this works:

;(defparameter *foo* 10)

... but otherwise I can't find out how to do this. Paredit is great, I would really like to keep using it. Are there any Lispers who know a way around this, or Emacs-wizards who can whip up a bit of Emacs Lisp to bind to something like paredit-comment-out-s-expr?

If there is a more Lispy or Emacsy way of accomplishing essentially the same thing, commenting out parts of source to recompile, please, don't hesitate to suggest them!

Answer 1

Position the point on the first character of the whole sexp, mark the whole sexp with C-M-space, and issue M-; to do the commenting. If it is necessary to do so, your source code will also be re-formatted so that only the sexp you marked, and nothing that was also on the same line, is in a comment.

You can very easily make a simple command or even a macro to do that:

(defun comment-sexp ()
  "Comment out the sexp at point."
  (interactive)
  (save-excursion
    (mark-sexp)
    (paredit-comment-dwim)))

Answer 2

Just a side note:

The #+ and #- reader macros are pretty nice for commenting out sexps. They allow ignoring the following sexp, if the given symbol isn't/is found in *FEATURES*. Just pick a symbol not in *FEATURES*, and use it with #+ like this:

#+nil
(defun foo ()
  ...)

Now, the function definition will be ignored (unless NIL is in *FEATURES*, which is not very likely).