1 Feb 2012 danstowell   » (Journeyer)

Learning prolog, eight queens

I'm following the "7 languages in 7 weeks" book. This week, PROLOG! However, I'm failing on this task: solve the eight queens puzzle in prolog. Why does this fail:

      queens(List) :-
            List = [Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q8],

    valid([Head|Tail]) :-

    validone(One,[Head|[]]) :-
            pairok(One, Head).
    validone(One,[Head|Tail]) :-
            pairok(One, Head),
            validone(One, Tail).

    pairok((X1, Y1), (X2, Y2)) :-
            Range = [1,2,3,4,5,6,7,8],
            member(X1, Range),
            member(Y1, Range),
            member(X2, Range),
            member(Y2, Range),
            (X1 =\= X2),
            (Y1 =\= Y2),
            (X1+Y1 =\= X2+Y2),
            (X1-Y1 =\= X2-Y2).

I load it in gprolog using


then I ask it to find me the eight unknowns (A through to H) by executing this:


What it should do (I think) is suggest a set of values that the unknowns can take. What it does instead is say:


(which means it thinks there are no possible solutions.) Anyone spot my error?

Syndicated 2012-01-19 17:57:30 from Dan Stowell

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