FLOLAC '18 Week of Code #3: Goldbach's conjecture

goldbach.hs

sieve :: [Int] -> [Int]
sieve (p:xs) = p : sieve [x | x <- xs, x `mod` p /= 0]

primes :: [Int]
primes = sieve [2..]

goldbachs :: Int -> [(Int, Int)]
goldbachs n = [(x, y) | x <- primesSmallerThenHalfOfN, y <- primesSmallerThenN, x < y, x + y == n]
  where primesSmallerThenHalfOfN = takeWhile (< n `div` 2) primes
        primesSmallerThenN = takeWhile (< n) primes

goldbach :: Int -> Maybe (Int, Int)
goldbach n = case take 1 $ goldbachs n of pair:_ -> Just(pair)
                                          otherwise -> Nothing