ToInt.hs

data ToInt a = ToInt (a -> Int)

instance Functor ToInt where
  -- note, f :: a -> b; g :: a -> Int, and the todo :: b -> Int
  fmap f (ToInt g) = ToInt (error "todo")

class Contravariant f where
  contramap :: (b -> a) -> f a -> f b

instance Contravariant ToInt where
  -- note, f :: b -> a, g :: a -> Int, and the todo :: b -> Int
  contramap f (ToInt g) = ToInt (error "todo")