Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!pasteur!agate!labrea!sri-unix!quintus!ok
From: ok@quintus.uucp (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: The double-cut.
Message-ID: <211@quintus.UUCP>
Date: 30 Jul 88 04:52:43 GMT
References: <1162@ttds.UUCP>
Sender: news@quintus.UUCP
Reply-To: ok@quintus.UUCP (Richard A. O'Keefe)
Organization: Quintus Computer Systems, Inc.
Lines: 15

In article <1162@ttds.UUCP> jonasn@ttds.UUCP (Jonas Nygren) writes:
>When trying to write procedural code for a simple curvplotting routine
>I often encountered problems with backtracking over system-calls. The
>remedy for this was code like:
>	a :- !, b, !, c, !, d, !, e, !.
>to avoid backtracking of goals a,b,c,d,e but this looks rather awkward.

Forgive me, but I cannot imagine this cropping up.  Why do you WANT to
prevent backtracking over system-calls?  You can backtrack over read/1,
write/1, var/1, assert/1 &c all you like, it doesn't hurt.  As a general
rule, code with side-effects should be determinate, so backtracking over
it will do nothing at all.  (retract/1 is an exception, but I doubt that
anyone would commend as an example to follow.)

Please let's see a real example of where you think you need to do this.