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From: finin@prc.unisys.com (Tim Finin)
Newsgroups: comp.lang.prolog
Subject: Re: Duplicate Solutions
Message-ID: <13167@burdvax.PRC.Unisys.COM>
Date: 15 Mar 90 14:35:17 GMT
References: <11500023@hpldola.HP.COM>
Sender: news@PRC.Unisys.COM
Reply-To: finin@prc.unisys.com (Tim Finin)
Organization: Unisys Center for Advanced Information Technology
Lines: 34

In article <11500023@hpldola.HP.COM>, patch@hpldola (Pat Chkoreff) writes:
>Occasionally I have written a predicate that yields duplicate solutions, and
>I've wondered how to make each solution occur uniquely.  For example, I have
>a multigraph: ...
>
>Usually, the existence of duplicate solutions makes me change either my data
>model or my algorithms to avoid them.  But in this example, my data model is
>straightforward, joined/2 seems like a natural thing to have, and duplicate
>solutions bother me (I'm not sure why, though).  What would you do to avoid
>duplicate solutions, or would you even bother?

You are correct, I think, in looking to change your data model and/or
algorithm to avoid generating duplicate solutions when they are not
appropriate.  Other than that, there is no good way to do this in a
pure logic programming language. In practice, this is usually done
with a cut or some related operator, such as if-then-else.  You might
use one of the following approaches, for example:

	joined(X,Y) :- edge(_,X,Y),!.

	joined(X,Y) :- edge(_,X,Y) -> true ; fail.

	joined(X,Y) :- once(edge(_,X,Y))
	once(P) :- P,!.

All of these have problems, however, if you call joined/2 with
uninstantiated arguments and expect it to generate a stream of
answers.  To support this use, you could use bagof/setof (or the
equivalent) or, as you suggest, revise your representation scheme.
The latter is preferable in my mind.  
-- 
 Tim Finin                                   finin@prc.unisys.com
 Center for Advanced Information Technology  215-648-2840, 215-648-2288 (fax)
 Unisys, PO Box 517, Paoli, PA 19301         215-386-1749 (home)