40.hs

goldbach a = head $
                    filter (\(x,y) -> isPrime x && isPrime y) $
                    map (\e -> (e, a - e)) [1,3..a `div` 2]
  where
  factors a = filter (isFactor a) [2..a-1]
  isFactor a b = a `mod` b == 0
  isPrime 1 = False
  isPrime a = null $ factors a

*Main> goldbach 28
(5,23)
*Main> goldbach 14
(1,13)