Blogger - HTIF - subtraction

-- |Subtracts second list from the first. O(2nlogn + 2n)
subtraction :: Ord a => [a] -> [a] -> [a]
subtraction = iter subtractF

subtractF :: Eq a => (Maybe a, Maybe a) -> [a] -> [a]
subtractF (Just x, Just y) acc = if x /= y then x:acc else acc
subtractF (Just x, Nothing) acc = x:acc
subtractF _ acc = acc