Node: Examples of Sharpsign Vertical-Bar, Next: , Previous: Sharpsign Vertical-Bar, Up: Sharpsign



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