another Haskell task

combinations.hs

-- Return a list of all combinations (i.e. order doesn't matter) of the given length.
-- Example: Given "abc" and 2 the answer is ["ab","ac","bc"] but order doesn't matter at either level.

combinations :: [a] -> Int -> [[a]]
combinations letters 0 = [[]]
combinations [] p = []
combinations (letter : letters) n = combinations letters n ++ map (letter :) (combinations letters (n -1))

main = print $ combinations "abc" 2