When you say permuations, I think of reorderings of a list, for example: permutations [1,2,3] = [ [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1] ] Here's an implementation: -- split [1,2,3] => [ -- ( 1, [2,3] ), -- ( 2, [1,3] ), -- ( 3, [1,2] ) ] split :: [a] -> [(a, [a])] split [] = error "split: empty list" split [a] = [(a, [])] split (a:as) = (a, as) : map prefix (split as) where prefix (x, xs) = (x, a : xs) permutations :: [a] -> [[a]] permutations [] = return [] permutations xs = do (first, rest) <- split xs rest' <- permutations rest return (first : rest') The problem you solved can be solved much more elegantly: pms : [a] -> Int -> [[a]] pms xs n = foldM combine [] (replicate n xs) where combine rest as = liftM (:rest) as or, for the unreadable version: pms xs n = foldM (map . flip (:)) [] $ replicate n xs (note that, in the list monad, liftM = map). -- ryan