module Queens where

safe p n = and [ not (check (i,j) (m,n))
               | (i, j) <- zip [1..lp] p
               ]
    where
        m = lp + 1
        lp = length p

check (i, j) (m, n) = j == n
                      || i + j == m + n
                      || i - j == m - n

queens 0     = [[]]
queens (m+1) = [ p ++ [n]
               | p <- queens m,
                 n <- [1..8],
                 safe p n
               ]