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From: patch@hpldoda.UUCP (Pat Chkoreff)
Newsgroups: comp.lang.prolog
Subject: A Challenge
Message-ID: <11500022@hpldoda.UUCP>
Date: 10 Dec 89 06:53:58 GMT
Organization: Hunga Dunga U.
Lines: 40

Here's a challenge for you.  It's a stripped-down version of a problem I've
been working on, and I haven't solved it yet.

Write a predicate good/1 which succeeds if and only if its argument is a
list of lists, where all of the lists have the same length.

All of these queries should work:

    ?- good([]).
    Yes
    ?- good([[a,b],[c,d]]).
    Yes
    ?- good([[a],[b,c]]).
    No
    ?- good([_,_,_,[a],_,_,_,[b,c],_,_,_|_]).       % this one's tough.
    No

The predicate should be written using pure Horn clauses -- no calls to !/0
or var/1.

'Same length' is defined by:

    same_length([], []).
    same_length([_|Xs], [_|Ys]) :-
        same_length(Xs, Ys).


Maybe I have no business even trying to solve this problem, but it bothers
me that I can't do it.

Please e-mail candidate solutions (or proofs of intractability) to me:  I'll
post the winner.  Thanks.


Patrick Chkoreff   719-590-5983
{hpfcla,hplabs}!hpldola!patch


P.S.  I'm not really at "Hunga Dunga U.", I'm really at Hewlett-Packard,
Electronic Design Division.