> On Mar 11, 2017, at 9:49 PM, Karl Wagner <[email protected]> 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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