newtype Constant a b =
    Constant { getConstant :: a } deriving (Eq, Ord, Show)

instance Functor (Constant a) where
  fmap :: (c -> d) -> Constant a c -> Constant a d
  fmap _ c = Constant (getConstant c)

instance Monoid a => Applicative (Constant a) where
  pure :: a -> Constant a a
  pure = Constant