Path: utzoo!mnetor!uunet!husc6!mailrus!ames!ucsd!sdcsvax!ucsdhub!hp-sdd!hplabs!otter!sfk
From: sfk@otter.hple.hp.com (Stephen Knight)
Newsgroups: comp.lang.prolog
Subject: Re: Programming Quickie
Message-ID: <1600012@otter.hple.hp.com>
Date: 28 Mar 88 13:03:51 GMT
References: <1600011@otter.hple.hp.com>
Organization: Hewlett-Packard Laboratories, Bristol, UK.
Lines: 22

Dear Ian,

Here is my solution, which I submit for elegance points ...

exactly_one( X, Y ) :-
    X contains Y,            /* contains is infix in my version */
    p( Y ),
    forall(
        ( X contains Y1, Y /= Y1 ),
        q( Y1 )
    ).

forall( X, Y ) :- not( (X, not(Y)) ).

The inefficiency of this programs is the repeated calculation of q( Term ).
One approach would be to patch this by making "q" cache, but other solutions
on this problem are more efficient than that anyway.

Steve Knight

PS: I have tested this on some simple examples, although I could have introduced
errors in transcribing it to this message.