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From: bd@zyx.UUCP
Newsgroups: comp.lang.prolog
Subject: compare/3 problem
Message-ID: <373@zyx.UUCP>
Date: Thu, 26-Mar-87 21:10:21 EST
Article-I.D.: zyx.373
Posted: Thu Mar 26 21:10:21 1987
Date-Received: Mon, 30-Mar-87 03:24:05 EST
Reply-To: bd@zyx.UUCP (Bjorn Danielsson)
Organization: ZYX Sweden AB, Stockholm
Lines: 26

In the process of implementing Prolog, I and the people I am
working with have been considering the problem of comparing
variables with compare/3. The goal is that two variables should
always be sorted in the same order, as long as their bindings
don't change.

In an implementation that is based on a Warren Abstract Machine,
the natural way to compare variables is to compare their addresses.
This normally sorts older variables to be < newer ones.
However, it occurred to us that put_unsafe_value might cause
problems with permanent variables (in Warren's terminology) that
become globalized. Try the following program on your favourite
WAM-based Prolog:

	test :- foo(X), bar(X).

	foo(X) :- compare(Z,Y,X), compare(Z,Y,X).

	bar(_).

The query "?- test." should preferably succeed. (It didn't on one
of the commercially available, widely popular Prologs we tried :-)

The obvious fix is to let compare/3 globalize variables.
-- 
Bjorn Danielsson, ZYX, +46 8 653205, ...mcvax!enea!zyx!bd