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From: ludemann@mlpvm1.iinus1.ibm.com ("Peter Ludemann")
Newsgroups: comp.lang.prolog
Subject: Delayed Inequality Predicate
Message-ID: <9101260626.AA06894@ucbvax.Berkeley.EDU>
Date: 26 Jan 91 05:50:53 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: ludemann@mlpvm1.iinus1.ibm.com
Lines: 37

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 (apologies for those
I have missed).

IBM-Prolog provides delayed inequality and "not"; it also provides
a general delay mechanism.  For example, when writing
coroutining predicates, it is often handy to merge two streams
(that is, lists which may not yet be fully instantiated),
delaying until at least one of the streams has its head
instantiated.  For example, we want to handle:

   <- merge(A, B, C) & A = [a|X] & B = [c,d] & X = [b] .

The notation "P when (^var(X) | ^var(Y))" means "wait until
either X or Y is instantiated before calling goal P" (other
Prologs call this "freeze" or "geler"); the notation
"p(notvar$X)" in the head of a clause means "succeed only if X
is not a variable" (yes, Richard, I could have used var/1, etc.).

  merge(In1, In2, Out) <- merge2(In1, In2, Out) when (^var(In1) | ^var(In2)) .

  merge2(notvar$ In1,      In2, Out) <- merge3(In1, In2, Out) .
  merge2(var$ In1, notvar$ In2, Out) <- merge3(In2, In1, Out) .

  merge3([],      In2, In2) .
  merge3([I|In1], In2, [I|Out]) <- merge(In1, In2, Out) .

(This program does not run backwards; to handle that, I would need to
change the first "when" to include ^var(In3), and modify merge2/3.)

- Peter Ludemann    ludemann@mlpvm1.iinus1.ibm.com