[Pitch] Parameter Packs

[hborla (Holly Borla)]

@filip-sakel

@Slava_Pestov

protocol P {
  associatedtype A
  associatedtype B
}

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

How would this work? Wouldn’t T need to be named either A or B to fulfill the respective requirement?

In the above code T: P says that "every type in the pack T has to conform to P". In the implementation of G, T.A is a pack that contains the associated A type of every type in the pack T. Similarly, T.B is a pack that contains the associated type B of every type in the pack T.

For example:

struct S: P {
  typealias A = Int
  typealias B = String
}

struct R<A, B>: P {}

let g = G<S, R>()
// T := {S, R}
// T.A := {Int, R.A}
// T.B := {String, R.B}