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
    Relevant bindings include
      mkFoo2 :: a -> Maybe Foo (bound at T7264.hs:13:1)
      val :: a (bound at T7264.hs:13:8)
    In the first argument of `mmap', namely `Foo'
    In the expression: mmap Foo (Just val)
    In an equation for `mkFoo2': mkFoo2 val = mmap Foo (Just val)