Permutations.hs

perms []  = []                                                                                                  
perms [x] = [pure x]                                                                                            
perms k@(z:zs) = do                                                                                             
  (ls,rs) <-                                                                                                    
    splitAt                                                                                                     
      <$> [ 0 .. length zs ]                                                                                    
      <*> perms zs                                                                                              
  pure (ls ++ [z] ++ rs)