Generic parameter pack expansion only for return value

[Dmitriy_Ignatyev]

This thread is created on request of @scanon (Add splitMap function to Swift standard library - #10 by scanon)

The original intent is to address cases where single input Type is needed to be transformed into multiple Types, like this:

extension Sequence {
// curent imp:
  func splitMap<T1, T2, E: Error>(_ transform: (Element) throws(E) -> Either<T1, T2>) rethrows -> ([T1], [T2]) // we are currently limited by 2 generic Types

// what we need is something like:
  func splitMap<each T, E: Error>(_ transform: (Element) throws(E) -> OneOf<each T>) rethrows -> (repeat each Array<T>)
}

For now it is possible to implement a function with multiple input Types and single output Type by pack iteration which is already pitched and implemented.

func == <each Element: Equatable>(lhs: (repeat each Element), rhs: (repeat each Element)) -> Bool {

So some solution is also needed for the specified case.

[ensan-hcl (ensan)]

I don't have any good idea on this kind of things (maybe 'variadic sum type'?), but this post seemed to be facing somehow related issue.

How to update this code to use parameter packs?

I have this piece of code that used different arity for each number of generic type count, but I can't figure out a way to make it use variadic generics. protocol P { init(source: A) } class A { ... } @dynamicMemberLookup class C1<L1: P>: A { subscript(dynamicMember keyPath: KeyPath<L1, A>) -> A { return L1(source: self)[keyPath: keyPath] } } @dynamicMemberLookup class C2<L1: P, L2: P>: A { subscript(dynamicMember keyPath: KeyPath<L1, A>) -> A { return L1(source: self)[keyP…

[Dmitriy_Ignatyev]

Yes, thanks for handling this. I suppose the are needed to me merged in one thread.

[jrose (Jordan Rose)]

You can poorly implement a variadic sum type with repeat Optional<each T>, which might be sufficient for something like this where you hope it all gets inlined away. I don’t think we’d put that in the stdlib though.