http://programmingpraxis.com/2011/07/19/sum-of-two-integers/

findSum.hs

import Data.List(find)

findSum :: Num a => a -> [a] -> Bool
findSum s ns = find (sumIs s) (pairs ns)
	where
		sumIs :: Num a => a -> (a, a) -> Bool
		sumIs s (x, y) = x+y == s
		
		pairs :: [a] -> [(a, a)]
		pairs [] = []
		pairs' (x:xs) = map (\y->(x,y)) xs ++ pairs xs