[Pitch] Parameter Packs

[Slava_Pestov]

@ksluder

The fact that nominal types can only have one parameter pack but function types can have more than one is surprising to me. It makes it difficult to design a type like struct Functor<Args…, Result…>. Is this purely due to the syntax ambiguity when naming the concrete type?

Yeah. Perhaps this limitation can be removed if we introduced labeled generic arguments at the same time, but that feels like a separate feature.

Another trick is that you can use a conformance requirement to a protocol with associated types to turn one type parameter into several. Eg,

protocol P {
  associatedtype A
  associatedtype B
}

struct G<T...> where T: P {}

Now inside the body of G, T.A and T.B are two pack type parameters with the same length.