Maybe types in haskell

maybe.hs

-- example of a dumb function which adds one to its input, and throws exception if given 0

addOne :: Int -> Int
addOne n = case n of
    0 -> error "dont add 0"
    _ -> 1 + n

-- λ> 2 + (addOne 1)
-- 4

-- λ> 2 + (addOne 0)
-- *** Exception: dont add 0
-- CallStack (from HasCallStack):
--   error, called at maybe.hs:3:10 in main:Main

-- above is not desirable, because it breaks
-- and it does not compose well, don't want to have exception handling blocks... 

addOneBetter :: Int -> Maybe Int
addOneBetter n = case n of
    0 -> Nothing
    _ -> Just (1 + n)

-- λ> addOneBetter 1
-- Just 2

-- λ> addOneBetter 0
-- Nothing

-- λ> map addOneBetter [1,2,3,0]
-- [Just 2,Just 3,Just 4,Nothing]

-- above does not break! however, Maybe types are a bit clunky:

-- λ> (addOneBetter 1) + (addOneBetter 1)
-- <interactive>:104:1: error:
--     • No instance for (Num (Maybe Int)) arising from a use of ‘+’
--     • In the expression: (addOneBetter 1) + (addOneBetter 1)
--       In an equation for ‘it’: it = (addOneBetter 1) + (addOneBetter 1)

-- '+' doesn't know how to handle Maybe types...

sumM :: Maybe Int -> Maybe Int -> Maybe Int
sumM (Just a) (Just b) = Just (a + b)
sumM Nothing (Just b) = Nothing
sumM (Just a) Nothing = Nothing

-- λ> sumM (addOneBetter 1) (addOneBetter 1)
-- Just 4

-- λ> sumM (addOneBetter 1) (addOneBetter 0)
-- Nothing

multM :: Maybe Int -> Maybe Int -> Maybe Int
multM (Just a) (Just b) = Just (a * b)
multM Nothing (Just b) = Nothing
multM (Just a) Nothing = Nothing

-- this works, but it's annoying to have to make functions work for Maybe

-- λ> (+) <$> (addOneBetter 1) <*> (addOneBetter 1)
-- Just 4

-- λ> (+) <$> (addOneBetter 1) <*> (addOneBetter 1)
-- Nothing

-- haskell has lots of tricks for abstracting more generally

main = print $ addOne 5