Code to generate list permutations

gistfile1.hs

import Data.Tree
import Data.List
import Control.Applicative

permutationForest :: [a] -> Forest a
permutationForest = zipWith3 build <*> inits <*> tail . tails
    where build a xs ys = Node a (permutationForest $ xs ++ ys)

traverse :: Forest a -> [[a]]
traverse [] = [[]]
traverse nodes = do Node x xs <- nodes
                    next <- traverse xs
                    return (x:next)

maxDepth :: Int -> Forest a -> Forest a
maxDepth n xs | n > 0     = map (\(Node y ys) -> Node y (maxDepth (n-1) ys)) xs
              | otherwise = []