Kata

A combination is a way of selecting members from a group in which the order does not matter.

Write a function that returns all the combinations of a given size from a list.

combinations :: Int -> [a] -> [[a]]

examples:

combinations 1 [1,2,3] should be [[1],[2],[3]]

combinations 2 [1,2,3] should be [[1,2],[1,3],[2,3]]

http://en.wikipedia.org/wiki/Combination