Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!bradley
From: bradley@cs.utexas.edu (Bradley L. Richards)
Newsgroups: comp.lang.prolog
Subject: Re: Does It Unify?
Message-ID: <189@qt.cs.utexas.edu>
Date: 8 May 90 19:32:03 GMT
References: <1990May7.132305.15989@agate.berkeley.edu>
Distribution: usa
Organization: U. Texas CS Dept., Austin, Texas
Lines: 17

In article <1990May7.132305.15989@agate.berkeley.edu> c60c-2ca@WEB.berkeley.edu (Andrew Choi) writes:
>Assume I have the following predicate defined:
>equal(X, X).
>Does the following unify?
>?-  equal(foo(Y), Y).
>
>On my Prolog interpreter, it unifies but gives very strange
>instantiation for Y.  What should the "correct" answer be?

According to the proper definition for unification, these terms should
not unify.  However, for "efficiency" reasons, most Prolog implementations
omit the "occurs" check (i.e. here Y appears in foo(Y)), and blindly
unify these terms.  This is, in fact, an error--and one worth fussing
at the manufacturer of your Prolog about.  Implementing the occurs
check isn't really that costly....

Bradley