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From: rggoebel@watdragon.UUCP (Randy Goebel LPAIG)
Newsgroups: net.lang.prolog
Subject: Re: Standard behavior?
Message-ID: <980@watdragon.UUCP>
Date: Tue, 13-May-86 12:39:01 EDT
Article-I.D.: watdrago.980
Posted: Tue May 13 12:39:01 1986
Date-Received: Wed, 14-May-86 11:20:24 EDT
Organization: U of Waterloo, Ontario
Lines: 22

> >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?
_____________
> I do not have the authority to say which is "standard" behavior, if by that
> you mean which is found more commonly in implementations.  However, any book
> on "vanilla" Prolog will tell you that "_" matches to anything, including a
> null list, so I would indeed be annoyed by C-Prolog if it did not find two
> matches.  Anybody disagree?
-------------
Hmmm.  The reason for logic programming's existence is to dispense with
guesses about what behaviour should be.  The formulae assert that the
individual constant named `[]' and everthing else (i.e., `_') is in the 
class named by the predicate `a'.  If you believe that the anonymous variable
is a universially quantified variable, then there are two resolution proofs
of the query a([]).

Implementors' treatment of `_' can produce non-standard behaviour; non-standard
means not consistent with the logical interpretation.