permutations in haskell

permu.hs

import Data.List (nub)

disperse :: a -> [a] -> [[a]]
disperse value arr =
  map (\(start, ending) -> start ++ [value] ++ ending) .
  map (\char -> splitAt char arr) $ [0..length arr]

permu :: Ord a => [a] -> [[a]]
permu []     = [[]]
permu (x:xs) = concatMap (disperse x) $ permu xs

main = mapM_ putStrLn $ nub $ permu ['a'..'d']