twoNumberSum.hs

module Main where

import qualified Data.Map as Map

twoNumberSum :: Map.Map Int Bool -> [Int] -> Int -> [Int]
twoNumberSum _ []     _ = []
twoNumberSum m (x:xs) n = case (Map.lookup y m) of
    Nothing  -> twoNumberSum (Map.insert x True m) xs n
    (Just _) -> [x,y]
    where y = n-x

main :: IO ()
main = print (twoNumberSum (Map.fromList []) [1, 2, 3, 4, 5, 6, 7, 8, 9, 15] 18)