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From: lee@munnari.oz.au (Lee Naish)
Newsgroups: comp.lang.prolog
Subject: Re: Delayed Inequality Predicate
Message-ID: <6565@munnari.oz.au>
Date: 30 Jan 91 06:30:15 GMT
References: <9101260626.AA06894@ucbvax.Berkeley.EDU>
Sender: news@cs.mu.oz.au
Reply-To: lee@munmurra.UUCP (Lee Naish)
Organization: Comp Sci, University of Melbourne
Lines: 45

In article <9101260626.AA06894@ucbvax.Berkeley.EDU> ludemann@mlpvm1.iinus1.ibm.com writes:
>takahash@rex.cs.tulane.edu (Silvia Takahashi) asks:
>
>> I am looking for a Prolog that implements the inequality
>> predicate using delayed evaluation.  ...
>
>As you mentioned, MU-Prolog and IC-Prolog have this.  Others
>include:  the NU-Prolog (MU-Prolog's successor), Prolog-II
>Prolog-III, Prolog/MALI, and IBM-Prolog

Also Sepia and Sicstus.

One difference between systems is whether quantifiers are supported.
Reasonably often I want to express things such as X is not a cons cell;
in other words, for all H and T, X is not equal to H.T.  This can be
expressed in NU-Prolog as follows:

	all [H,T] X ~= H.T

The implementation limits the scopes of H and T to this call and they
are allowed to be instantated by the unification with X.  The original
dif/2 prediate from Prolog-II (and adopted in several systems) cannot
express this (you can do it by using functor; other more complex cases
need disjunction also).

It is also often desirable to check for equality and return true or
false, rather than fail or succeed (dif and ~= are not flexible enough
for this).  It can be done in NU-Prolog by using termCompare/3 (like
compare/3, but it delays until the terms are sufficiently instantiated).
If-then-else is also treaded specially in NU-Prolog if the condition is
a call to =/2 (possibly with quantifiers).

One final thing which is sometimes nice (but non-logical) is to treat
variables as being quantified (so they are allowed to be instantated by
the unification), without limiting their scope.  This can be done in
NU-Prolog by using gAll instead of all.  One use is meta interpreters:

	solve((all Vars T1 ~= T2)) :- gAll Vars T1 ~= T2.
	...

Here all variables in Vars are treated as being quantified and the scope
of Vars is the whole clause.  There is a paper in the London ICLP, LNCS
225, (1986) discussing the negation facilities of NU-Prolog.

	lee