class Functor f where
    map :: (a -> b) -> f a -> f b

data Either a b = Left a | Right b

instance Functor (Either a) where
    map _ (Left x)  = Left x
    map f (Right y) = Right (f y)

id :: Integer -> Integer
id x = x

main = map id (Left (12 :: Integer))
--
(Left 12)