Return all permutations of given list

perms.hs

module Main where

-- inserts a to all possible places in list bs
-- insert 1 [2,3] -> [[1,2,3], [2,1,3], [2,3,1]]
insert :: t -> [t] -> [[t]]
insert a [] = [[a]]
insert a bs@(b:rest) = (a:bs):fmap (b:) (insert a rest)

-- Returns all permitations of given list
-- perms [1,2,3] -> [[1,2,3],[2,1,3],[2,3,1],[1,3,2],[3,1,2],[3,2,1]]
perms :: [t] -> [[t]]
perms [] = [[]]
perms (a:as) = [val |rest <- perms as, val <- insert a rest]

main :: IO ()
main = do
  putStrLn ("Response:" ++ (show $ perms [1..3]))
  -- Response:[[1,2,3],[2,1,3],[2,3,1],[1,3,2],[3,1,2],[3,2,1]]