Path: utzoo!attcan!uunet!mcsun!ukc!edcastle!lfcs!jha
From: jha@lfcs.ed.ac.uk (Jamie Andrews)
Newsgroups: comp.lang.prolog
Subject: Re: Prolog's pitfalls
Keywords: cut, Prolog efficiency
Message-ID: <5135@castle.ed.ac.uk>
Date: 12 Jul 90 11:39:15 GMT
References: <1396@quintus.UUCP>
Reply-To: jha@lfcs.ed.ac.uk (Jamie Andrews)
Organization: Laboratory for the Foundations of Computer Science, Edinburgh U
Lines: 23

In article <1396@quintus.UUCP> dave@quintus.UUCP (David Bowen) writes:
>...  If the Prolog implementation isn't able to spot the determinacy
>it has to keep backtracking information around indefinitely, which can waste
>arbitrary amounts of space.  The simplest way to correct this is to add cuts at
>all appropriate places....

     This reminds me of something I've been wondering about for
a while.  Built-in "if-then-else" and "case" constructs are
often touted as a way of getting around using cuts.  What is the
current wisdom about this?  I know that not all uses of cut can
be replaced by if-then-else, but what proportion of them can?
(I seem to remember hearing some figure like 70% at one point --
don't know how this was measured, though.)

     On a related point, what proportion of instances of
negation can be eliminated by using if-then-elses?  (The
problems of negation wouldn't be eliminated, of course, but if
we could replace negation by if-then-else there would be more
uniformity with other programming languages.)

--Jamie.		Original material copyright (c) 1990 by Jamie Andrews;
  jha@lfcs.ed.ac.uk	for distribution only on unmoderated USENET newsgroups.
 "If you try to leave this room, I'll tackle you." -- Don Getty to Clyde Wells