bifunctors

A.hs

data Gabuzomeu a b = Gabu a | Zomeu b
  deriving Show

shadok :: (a -> c) -> (b -> d) -> Gabuzomeu a b -> Gabuzomeu c d
shadok f _ (Gabu a) = Gabu (f a)
shadok _ g (Zomeu b) = Zomeu (g b)

newtype G b a = G {first :: Gabuzomeu a b}
  deriving Show

instance Functor (G b) where
  fmap f = G . shadok f id . first

  -- Written explicitly, the above looks like this:
  --
  -- fmap f (G (Gabu x)) = G (Gabu (f x))

newtype Z a b = Z {second :: Gabuzomeu a b}
  deriving Show

instance Functor (Z a) where
  fmap f = Z . shadok id f . second