Eight queens puzzle

gistfile1.hs

import Data.List

isUnique :: (Ord a) => [a] -> Bool
isUnique = all (null . drop 1) . group . sort

cols = [0..7]

solutions = [vec | vec <- permutations cols
                 , isUnique [vec!!i + i | i <- cols]
                 , isUnique [vec!!i - i | i <- cols]]

board :: [Int] -> String
board xs = unlines [replicate x '+' ++ "Q" ++ replicate (last cols - x) '+'
                   | x <- xs]

printSolution = putStrLn . board
printNthSolution n = printSolution (solutions !! n)