86 lines
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86 lines
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<TITLE>CLHS: Section 2.4.8.19.1</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="02_dhs.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="02_dhs.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="02_dhsb.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>
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2.4.8.19.1 Examples of Sharpsign Vertical-Bar</H2> <P>
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The following are some examples that exploit the <TT>#</TT>|<TT>...</TT>|<TT>#</TT> notation: <P>
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<PRE>
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;;; In this example, some debugging code is commented out with #|...|#
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;;; Note that this kind of comment can occur in the middle of a line
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;;; (because a delimiter marks where the end of the comment occurs)
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;;; where a semicolon comment can only occur at the end of a line
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;;; (because it comments out the rest of the line).
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(defun add3 (n) #|(format t "~&Adding 3 to ~D." n)|# (+ n 3))
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;;; The examples that follow show issues related to #| ... |# nesting.
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;;; In this first example, #| and |# always occur properly paired,
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;;; so nesting works naturally.
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(defun mention-fun-fact-1a ()
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(format t "CL uses ; and #|...|# in comments."))
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=> MENTION-FUN-FACT-1A
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(mention-fun-fact-1a)
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>> CL uses ; and #|...|# in comments.
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=> NIL
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#| (defun mention-fun-fact-1b ()
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(format t "CL uses ; and #|...|# in comments.")) |#
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(fboundp 'mention-fun-fact-1b) => NIL
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;;; In this example, vertical-bar followed by sharpsign needed to appear
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;;; in a string without any matching sharpsign followed by vertical-bar
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;;; having preceded this. To compensate, the programmer has included a
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;;; slash separating the two characters. In case 2a, the slash is
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;;; unnecessary but harmless, but in case 2b, the slash is critical to
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;;; allowing the outer #| ... |# pair match. If the slash were not present,
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;;; the outer comment would terminate prematurely.
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(defun mention-fun-fact-2a ()
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(format t "Don't use |\# unmatched or you'll get in trouble!"))
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=> MENTION-FUN-FACT-2A
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(mention-fun-fact-2a)
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>> Don't use |# unmatched or you'll get in trouble!
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=> NIL
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#| (defun mention-fun-fact-2b ()
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(format t "Don't use |\# unmatched or you'll get in trouble!") |#
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(fboundp 'mention-fun-fact-2b) => NIL
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;;; In this example, the programmer attacks the mismatch problem in a
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;;; different way. The sharpsign vertical bar in the comment is not needed
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;;; for the correct parsing of the program normally (as in case 3a), but
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;;; becomes important to avoid premature termination of a comment when such
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;;; a program is commented out (as in case 3b).
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(defun mention-fun-fact-3a () ; #|
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(format t "Don't use |# unmatched or you'll get in trouble!"))
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=> MENTION-FUN-FACT-3A
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(mention-fun-fact-3a)
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>> Don't use |# unmatched or you'll get in trouble!
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=> NIL
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#|
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(defun mention-fun-fact-3b () ; #|
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(format t "Don't use |# unmatched or you'll get in trouble!"))
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|#
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(fboundp 'mention-fun-fact-3b) => NIL
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</PRE>
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</TT> <P>
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