FLOLAC 2018 0-3: Goldbach's conjecture

goldbach.hs

goldbach :: Int -> Maybe (Int, Int)
goldbach = listToMaybe . goldbachFull
    where listToMaybe = foldr (const . Just) Nothing

goldbachFull :: Int -> [(Int, Int)]
goldbachFull n = mkPairs . filter isNegPrime . takeHalfN $ primes
    where mkPairs = map (\p -> (p, n-p))
          isNegPrime p = isPrime (n - p)
          takeHalfN = takeWhile (\p -> p*2 <= n)

primes :: [Int]
primes = 2 : filter isPrime [3, 5 ..]

isPrime :: Int -> Bool
isPrime n | n <= 2    = n == 2
          | otherwise = all notDivisor . takeSqrtN $ primes
    where notDivisor p = mod n p /= 0
          takeSqrtN = takeWhile (\p -> p^2 <= n)