I guess you mean (note the return type) fun with<F super E>(elt: F): FunctionalSet<F> That's an interesting suggestion, but I don't see any indication that

I guess you mean (note the return type) fun with<F super E>(elt: F): FunctionalSet<F> That's an interesting suggestion, but I don't see any indication that Kotlin provides for type lower bounds in generic function constraints. (It does have type upper bounds; see the bottom of this page: http://kotlinlang.org/docs/reference/generics.html http://kotlinlang.org/docs/reference/generics.html).