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From: weyand@csli.Stanford.EDU (Chris Weyand)
Newsgroups: comp.lang.prolog
Subject: Re: Does It Unify?
Message-ID: <13473@csli.Stanford.EDU>
Date: 7 May 90 19:54:11 GMT
References: <1990May7.132305.15989@agate.berkeley.edu>
Sender: weyand@csli.Stanford.EDU (Chris Weyand)
Distribution: usa
Organization: Center for the Study of Language and Information, Stanford U.
Lines: 27

c60c-2ca@WEB.berkeley.edu (Andrew Choi) writes:

>Hi Gurus.  I have a simple question about unification...
>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?

This is like querying with:

?- Y = foo(Y).
In which case what will be returned is a very long predicate with the 
following form:
	Y = foo(foo(foo(foo(foo(foo(...

It's just a recursive definiition of Y that happens to be somewhat nasty.  
Sort of like setting the cdr of a list to the list itself.

Chris Weyand
weyand@csli.Stanford.edu