The following are some examples that exploit the #|...|# notation:
;;; In this example, some debugging code is commented out with #|...|#
;;; Note that this kind of comment can occur in the middle of a line
;;; (because a delimiter marks where the end of the comment occurs)
;;; where a semicolon comment can only occur at the end of a line
;;; (because it comments out the rest of the line).
(defun add3 (n) #|(format t "~&Adding 3 to ~D." n)|# (+ n 3))
;;; The examples that follow show issues related to #| ... |# nesting.
;;; In this first example, #| and |# always occur properly paired,
;;; so nesting works naturally.
(defun mention-fun-fact-1a ()
(format t "CL uses ; and #|...|# in comments."))
=> MENTION-FUN-FACT-1A
(mention-fun-fact-1a)
|> CL uses ; and #|...|# in comments.
=> NIL
#| (defun mention-fun-fact-1b ()
(format t "CL uses ; and #|...|# in comments.")) |#
(fboundp 'mention-fun-fact-1b) => NIL
;;; In this example, vertical-bar followed by sharpsign needed to appear
;;; in a string without any matching sharpsign followed by vertical-bar
;;; having preceded this. To compensate, the programmer has included a
;;; slash separating the two characters. In case 2a, the slash is
;;; unnecessary but harmless, but in case 2b, the slash is critical to
;;; allowing the outer #| ... |# pair match. If the slash were not present,
;;; the outer comment would terminate prematurely.
(defun mention-fun-fact-2a ()
(format t "Don't use |\# unmatched or you'll get in trouble!"))
=> MENTION-FUN-FACT-2A
(mention-fun-fact-2a)
|> Don't use |# unmatched or you'll get in trouble!
=> NIL
#| (defun mention-fun-fact-2b ()
(format t "Don't use |\# unmatched or you'll get in trouble!") |#
(fboundp 'mention-fun-fact-2b) => NIL
;;; In this example, the programmer attacks the mismatch problem in a
;;; different way. The sharpsign vertical bar in the comment is not needed
;;; for the correct parsing of the program normally (as in case 3a), but
;;; becomes important to avoid premature termination of a comment when such
;;; a program is commented out (as in case 3b).
(defun mention-fun-fact-3a () ; #|
(format t "Don't use |# unmatched or you'll get in trouble!"))
=> MENTION-FUN-FACT-3A
(mention-fun-fact-3a)
|> Don't use |# unmatched or you'll get in trouble!
=> NIL
#|
(defun mention-fun-fact-3b () ; #|
(format t "Don't use |# unmatched or you'll get in trouble!"))
|#
(fboundp 'mention-fun-fact-3b) => NIL
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