maybe.hs

{-# OPTIONS -fwarn-incomplete-patterns #-}

-- return a Maybe value either containing the first number
-- over 20, or Nothing
over20 :: [Int] -> Maybe Int
over20 (h:tl)
    | h > 20 = Just h
    | otherwise = over20 tl
over20 [] = Nothing

-- how do we add two Maybe Ints?
maybePlus :: Maybe Int -> Maybe Int -> Maybe Int
maybePlus (Just a) (Just b) = Just (a + b)
maybePlus _ _ = Nothing

-- writing a wrapper function for every function we
-- want to apply to a Maybe value is annoying...
--
-- Instead, we can use a higher order function to
-- generate these functions
-- use this like so: lift2 (+) (Just 1) (Just 2)
lift2 :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c
lift2 f (Just a) (Just b) = Just (f a b)
lift2 _ _ _ = Nothing