2.4.8.19.1 Examples of Sharpsign Vertical-Bar

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