[swift-evolution] [Idea] Generic associated types

[Douglas Gregor]

> On Mar 11, 2017, at 9:49 PM, Karl Wagner <razielim at gmail.com> wrote:
> 
> I have a model like this:
> 
> protocol Promise {
>     associatedtype Result
> }
> 
> protocol Scanner {
>     associatedtype ScanPromise: Promise
> 
>     func promiseScan<T>(from: Offset, until: (Offset, Item) -> T?) -> ScanPromise // where Result == T?
> }
> 
> The thing that I’m trying to express is: whichever type implements the associated type ‘ScanPromise’ must be generic, and that parameter must be its result (i.e. something it got as a result of calling the “until” closure).
> 
> Even with SE-0142, this kind of constraint would not be possible. What I would like to write is something like this:
> 
> protocol Promise {
>     associatedtype Result
> }
> 
> protocol Scanner {
>     associatedtype ScanPromise<T>: Promise // now generic. [SE-0142]: where Result == T
> 
>     func promiseScan<T>(from: Offset, until: (Offset, Item) -> T?) -> ScanPromise<T>
> }
> 
> Thoughts?

Some of us who work on the Swift compiler have talked about this extension to the model before, and we *think* it dodges some of the concerns about introducing more-general higher-rank types in Swift while enabling reasonable use cases like the one you provide. It seems like a reasonable direction.

Way out of scope for Swift 4 at this point, of course ;)

	- Doug


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