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From: roland@sics.se (Roland Karlsson)
Newsgroups: comp.lang.prolog
Subject: Re: Arrays in Prolog
Message-ID: <1990Aug29.095308.18522@sics.se>
Date: 29 Aug 90 09:53:08 GMT
References: <90239.175243SCHMIED@DB0TUI11.BITNET>
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Organization: Swedish Institute of Computer Science, Kista
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In-Reply-To: vu0310@bingvaxu.cc.binghamton.edu's message of 28 Aug 90 19:24:52 GMT


OK Kym. I am lost. I do not understand what you mean by ! making the
destructive change made by setarg permanent.  This is how it works
in SICStus though.

When doing a setarg you will first save code on heap that will restore
the old value at backtracking.  Then you put a pointer on trail that
refers to this code.  Then you do the change.  Neither the heap or
the trail will be effected at !.  So at backtracking (untrailing) you
will trap at the pointer to the heap.  The code on the heap will be
executed and thereby the old value restored.  So a ! will NOT make
the change permanent.

--
Roland Karlsson
SICS, PO Box 1263, S-164 28 KISTA, SWEDEN	Internet: roland@sics.se
Tel: +46 8 752 15 40	Ttx: 812 61 54 SICS S	Fax: +46 8 751 72 30