python 示例分享---逻辑推理编程解决八皇后

可以和Haskell ， Prolog 一样做到模式匹配，

建立逻辑推到规则，描述问题，得出答案。

    from pyDatalog import pyDatalog

    pyDatalog.create_atoms( 'N, N1, X, Y, X0, X1, X2, X3, X4, X5, X6, X7' )
    pyDatalog.create_atoms( 'ok, queens, next_queen, pred, pred2' )

    size = 8
    ok( X1, N, X2 ) <= ( X1 != X2 ) & ( X1 != X2 + N ) & ( X1 != X2 - N )

    pred( N, N1 )  <= ( N > 1 ) & ( N1 == N - 1 )
    queens( 1, X ) <= ( X1._in( range( size ) ) ) & ( X1 == X[0] )
    queens( N, X ) <= pred( N, N1 ) & queens( N1, X[:-1] ) & next_queen( N, X )

    pred2( N, N1 )   <= ( N > 2 ) & ( N1 == N - 1 )
    next_queen( 2, X ) <= ( X1._in( range( 8 ) ) ) & ok( X[0], 1, X1 ) & ( X1 == X[1] )
    next_queen( N, X ) <= pred2( N, N1 ) & next_queen( N1, X[1:] ) & ok( X[0], N1, X[-1] ) 

    print( queens( size, ( X0, X1, X2, X3, X4, X5, X6, X7 ) ) )