[Haskell-beginners] Combinations of choosing n items from a list

Hi all, This function took me a long time to write, getting my head around the double recursion. It returns a list of all possible sub-sets of a given length from a list. I have a couple of questions. 1. Can it be improved for efficiency or style? 2. Is there a library that already has this and other related functions? I assumed there would be but I couldn't find it on hoogle. combinations :: Int -> [a] -> [[a]] combinations _ [] = [] combinations 0 _ = [] combinations 1 x = map (:[]) x combinations n (x:xs) = (map (x:) $ combinations (n-1) xs) ++ combinations n xs e.g.

combinations 3 [1..5] [[1,2,3],[1,2,4],[1,2,5],[1,3,4],[1,3,5],[1,4,5],[2,3,4],[2,3,5],[2,4,5],[3,4,5]]

Thanks, Peter