First of all, MigMit has probably suggested the parameterization of Like by the constraint, something like the following: data Like ctx = forall a. (ctx a, Typeable a) => Like a instance ALike (Like ALike) where toA (Like x) = toA x instance CLike (Like CLike) where toC (Like x) = toC x get_mono :: Typeable b => [Like ALike] -> [b] get_mono = catMaybes . map ((\(Like x) -> cast x)) lst_a :: [Like ALike] lst_a = [Like a1, Like b1, Like c1, Like d1] lst_c :: [Like CLike] lst_c = [Like c1, Like d1] t1 = map print_a lst_a t2 = map print_a lst_c (The rest of the code is the same as in your first message). You need the flag ConstraintKinds. Second, all your examples so far used structural subtyping (objects with the same fields have the same type) rather than nominal subtyping of C++ (distinct classes have distinct types even if they have the same fields; the subtyping must be declared in the class declaration). For the structural subtyping, upcasts and downcasts can be done mostly automatically. See the OOHaskell paper or the code http://code.haskell.org/OOHaskell (see the files in the samples directory).