Permutations of a certain length of a list

ListPermutations.hs

module ListPermutations where

listPermutations :: Integer => [a] -> b -> [[a]]
listPermutations l n = snd $ permutate l n
  where permutate :: Integer => [a] -> b -> ([[a]], [[a]])
        permutate [] _ = ([[]], [])
        permutate (x:xs) n = let (pt, acc) = permutate xs n in permutateInner x pt pt acc n
          where permutateInner :: Integer => a -> [[a]] -> [[a]] -> [[a]] -> b -> ([[a]], [[a]])
                permutateInner _ [] pt acc _ = (pt, acc)
                permutateInner h (x:xs) pt acc n
                  | (fromIntegral $ length x) /=  n - 1 = permutateInner h xs ((h:x):pt) acc n
                  | otherwise = permutateInner h xs ((h:x):pt) ((h:x):acc) n

usage:

*ListPermutations> listPermutations [1,2,3] 2
[[1,3],[1,2],[2,3]]