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From: HEP@smectos.gang.umass.edu (High Energy Physics Group)
Newsgroups: comp.lang.lisp
Subject: Re: Destructive Operations
Message-ID: <29982@dime.cs.umass.edu>
Date: 2 May 91 06:36:07 GMT
References: <1351@ai.cs.utexas.edu> <1991Apr25.052624.14541@Think.COM>
Sender: root@dime.cs.umass.edu
Reply-To: HEP@smectos.CS.UMASS.EDU (High Energy Physics Group)
Organization: University of Massachusetts, Amherst
Lines: 27

I have managed to get myself into trouble many times by using
destructive operators.  The following example is probably my favorite:

  <cl> (setf *ht* (make-hash-table :test #'equal))
  #<EQUAL hash-table with 0 entries @ #x-ebca20e> 

  <cl> (setf *x* '(3))
  (3) 

  <cl> (setf (gethash *x* *ht*) :hi-there)
  :hi-there 

  <cl> (gethash '(3) *ht*)
  :hi-there 
  t 

  <cl> (setf (first *x*) 92)
  92 

  <cl> (gethash '(3) *ht*)
  nil 
  nil 

Sure, it's obvious in retrospect, but not at the time when I did this!


	--kleanthes