T7748a.hs:16:24: error:
    • Couldn't match expected type ‘a’
                  with actual type ‘Maybe (Maybe (r -> ()))’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          test :: forall a r. a -> r -> ()
        at T7748a.hs:11:1-20
    • In the pattern: Just (Just p)
      In a case alternative: Just (Just p) -> p
      In the expression:
        case zd of
          Nothing -> const ()
          Just Nothing -> const ()
          Just (Just p) -> p
    • Relevant bindings include
        g :: r -> () (bound at T7748a.hs:13:16)
        f :: r -> () (bound at T7748a.hs:13:8)
        zd :: a (bound at T7748a.hs:12:6)
        test :: a -> r -> () (bound at T7748a.hs:12:1)