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From: andrews@ubc-cs.UUCP (Jamie Andrews)
Newsgroups: net.lang.prolog
Subject: Re: Standard behavior?
Message-ID: <240@ubc-cs.UUCP>
Date: Mon, 12-May-86 16:57:35 EDT
Article-I.D.: ubc-cs.240
Posted: Mon May 12 16:57:35 1986
Date-Received: Wed, 14-May-86 11:29:16 EDT
References: <6500005@uicsl>
Reply-To: andrews@ubc-cs.UUCP (Jamie Andrews)
Organization: UBC Department of Computer Science, Vancouver, B.C., Canada
Lines: 20

In article <6500005@uicsl> gooley@uicsl.UUCP writes:
>Consider the following trivial predicate:
>a([]).
>a(_).
>Given the query :-a([]). , C-Prolog finds one match and UNSW Prolog finds two.
>Which is standard behavior?  How do other implementations behave?

     It depends what you mean by "one match".  I should think all Prolog
systems would just tell you that the query succeeds, since there are no free
variables to get bindings for in the query.

     The standard reading of the predicate in FOL with identity would be
a(X) <- X = []  \/  X = X.
...which, given the standard computation algorithm, should probably give
two responses to the query 
:-a(X).
... namely, X=[] and X=X.

--Jamie.
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