perms.hs

import Data.List (inits, tails)

perms :: [a] -> [[a]]
perms [] = [[]]
perms (x : xs) = perms xs >>= spread x
  where
    spread :: a -> [a] -> [[a]]
    spread el lst = zipWith (\x y -> x ++ [el] ++ y) (inits lst) (tails lst)

main :: IO ()
main = print $ perms [1, 2, 3] 
-- evaluates to [[1,2,3],[2,1,3],[2,3,1],[1,3,2],[3,1,2],[3,2,1]]