def inc (a: Int): Int { a + 1 } class Eq a { equals : a -> a -> Bool } instance Eq Int { \a -> \b -> a == b } class Functor f { fmap : (a -> b) -> f a -> f b } /* Maybe */ type Maybe a = Just a | Nothing instance (Eq a) => Eq (Maybe a) { \maybeA -> \maybeB -> case (maybeA, maybeB) { (Just a, Just b) -> equals a b, (Nothing, Nothing) -> True, _ -> False } } instance Functor Maybe { \f -> \maybe -> case maybe { Just a -> Just (f a), Nothing -> Nothing } } test "fmap works with Just" { let result = fmap inc (Just 1); let expected = Just (2 : Int); equals result expected }