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From: eliot@phoenix.Princeton.EDU (Eliot Handelman)
Newsgroups: comp.lang.lisp
Subject: Re: Amusing Code
Message-ID: <3520@phoenix.Princeton.EDU>
Date: 24 Aug 88 17:12:03 GMT
References: <389@soi.UUCP> <450@oravax.UUCP>
Reply-To: eliot@phoenix.Princeton.EDU (Eliot Handelman)
Organization: Princeton University, NJ
Lines: 26

In article <389@soi.UUCP>, alex@soi.UUCP (Alex Zatsman) writes:
; It took me and few of my friends a while to figure out
; seemingly strange effect of TEST-STACK below. I thought
; other people will find it amusing too:
; 
; (defvar *Stack*)
; 
; (defun Push-Object (Object) (push Object (car *Stack*)))
; 
; (defun Init-Stack () (setf *Stack* '((:Bottom-Frame))))
; 
; (defun Test-Stack ()
;   (Init-Stack)     (print *Stack*)
;   (Push-Object 11) (print *Stack*)
;   (Init-Stack)     (print *Stack*)  (values))
; 

After having called test-stack, have a look at #'init-stack. Miracle of
miracles! Well, they all warned you about RPLACA, which is being called
by PUSH. Of course it's trivial to obviate this effect, but at least
it proves the existence of pointers. Thanks for an interesting example.

-Eliot

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