[swift-evolution] [Proposal] Associated Type and Generic One-to-One Mapping

[David Sweeris]

> On Jun 23, 2017, at 5:28 PM, David Moore via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I do indeed have quite a few real examples of this, such prompted me to bring this up. I think this could be done without any impact to existing code, but it would require some type of keyword. Take the following as a possible prototype.
> 
> protocol Foo { 
>     associatedtype T
> }
> 
> struct Bar<T> : Foo {
>     keyword typealias T // Or really any other syntactical implementation.
> }
> 
> With an opt-in method we could implement this without affecting existing code, thereby making this more viable. I will send some examples later.

At one point there was talk of just having generic parameters automatically becoming typealiases:
struct Bar<T> : Foo {
    // `T` is automatically an implicit typealias for, well, `T`
}

Dunno if that’s still the plan (or to what degree it ever was), but it’d work for me. I don’t even think it’d break source compatibility (though it may make a lot of typealiases unneeded.

- Dave Sweeris