T7264.hs:13:19:
    Couldn't match type ‘a’ with ‘forall r. r -> String’
      ‘a’ is a rigid type variable bound by
          the inferred type of mkFoo2 :: a -> Maybe Foo at T7264.hs:13:1
    Expected type: a -> Foo
      Actual type: (forall r. r -> String) -> Foo
    In the first argument of ‘mmap’, namely ‘Foo’
    In the expression: mmap Foo (Just val)
    Relevant bindings include
      val :: a (bound at T7264.hs:13:8)
      mkFoo2 :: a -> Maybe Foo (bound at T7264.hs:13:1)