foo.hs

data List a =
    Nil
  | Cons a (List a)
  deriving (Eq, Show)

instance Monoid (List a) where
  mempty :: List a
  mempty = Nil

  mappend :: List a -> List a -> List a
  mappend xs Nil = xs
  mappend Nil ys = ys
  mappend (Cons x xs) ys = Cons x (xs `mappend` ys)

instance Functor List where
  fmap :: (a -> b) -> List a -> List b
  fmap _ Nil = Nil
  fmap f (Cons x xs) = Cons (f x) (fmap f xs)