object test { trait A { type T; } trait B { type T; } /** def functor(x: A): B { type T = x.T } */ abstract class functor() { val arg: A; val res: B { type T = arg.T } = new B { type T = arg.T; }; } val a = new A { type T = String }; /** val b: B { type T = String } = functor(a) */ val b: B { type T = String } = { val tmp = new functor() { val arg = a }; tmp.res } }