How do I implement minimum with foldr (or foldMap)?

Question

I want to implement minimum with foldr or foldMap. According to the exercise, it should have this definition:

mini :: (Foldable t, Ord a) => t a -> Maybe a -- named "mini" to avoid name clash

It sounded pretty straightforward, but I do not know what I can substiture for X below to make it work. Help please?

mini xs = Just (foldr min X xs)

And you get bonus points for showing me how to do it with foldMap too, but that seems harder.

Answer 1

You could do:

mini :: (Foldable f, Ord a) => f a -> Maybe a
mini = foldr maybeMin Nothing
  where
    maybeMin x Nothing = Just x
    maybeMin x (Just y) = Just (min x y)