map2_Maybe :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c
map2_Maybe f Nothing _ = Nothing
map2_Maybe f (Just a) Nothing = Nothing
map2_Maybe f (Just a) (Just b) = Just ((f a) b)
-- Or: map2_Maybe f (Just a) mb = fmap (f a) mb
map2_Either :: (a -> b -> c) -> Either e a -> Either e b -> Either e c
map2_Either f (Left e) _ = Left e
map2_Either f (Right a) (Left e) = Left e
map2_Either f (Right a) (Right b) = Right (f a b)
-- Or: map2_Either f (Right a) eb = fmap (f a) eb
In these two examples, Is ((f a) b) the same as (f a b) since every function in Haskell can only take one argument?