Hmm, this does seem like a pretty tasteful design in many ways. The declaration-site variance for generics is a pretty good idea, but is still inadequate to de

Hmm, this does seem like a pretty tasteful design in many ways. The declaration-site variance for generics is a pretty good idea, but is still inadequate to deal with functional collections. (I use the term "functional" as opposed to simply "immutable": an immutable collection has no update operations at all, while a functional collection has functional update operations: for example, given a set S and an element X, to compute a new set whose elements are those of S together with X (S ∪ {X}). This usage is certainly not universal, but the distinction needs to be drawn somehow.) Anyway, the point is this. Kotlin has in its collections API interface Set<out E> ... where the use of 'out' says that given types A and B where B is a subtype of A, 'Set<B>' is a subtype of 'Set<A>'. But then if you wanted to add a 'with' operation as described above: interface FunctionalSet<out E> : Set<E> { fun with(E elt) } you can't do it, because type parameters marked with 'out' can't be used for method parameters.