Ocaml object system can also achieve this in a quite lightweight way type foo = < foo:int > type bar = < bar:int > type k = < foo; bar > type u

Ocaml object system can also achieve this in a quite lightweight way type foo = < foo:int > type bar = < bar:int > type k = < foo; bar > type u = < k; baz:int > let f (x: <u; ..>) (\* the type annotation is not needed \*) = x#m