T4093a.hs:8:8: error:
    • Could not deduce: e ~ ()
      from the context: Foo e ~ Maybe e
        bound by the type signature for:
                   hang :: forall e. (Foo e ~ Maybe e) => Foo e
        at T4093a.hs:7:1-34
      ‘e’ is a rigid type variable bound by
        the type signature for:
          hang :: forall e. (Foo e ~ Maybe e) => Foo e
        at T4093a.hs:7:1-34
      Expected type: Foo e
        Actual type: Maybe ()
    • In the expression: Just ()
      In an equation for ‘hang’: hang = Just ()
    • Relevant bindings include hang :: Foo e (bound at T4093a.hs:8:1)