T7264.hs:13:19: error:
    • Couldn't match type ‘a’ with ‘forall r. r -> String’
      Expected: a -> Foo
        Actual: (forall r. r -> String) -> Foo
      ‘a’ is a rigid type variable bound by
        the inferred type of mkFoo2 :: a -> Maybe Foo
        at T7264.hs:13:1-32
    • 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)
    • Relevant bindings include
        val :: a (bound at T7264.hs:13:8)
        mkFoo2 :: a -> Maybe Foo (bound at T7264.hs:13:1)