> On 12 Mar 2017, at 14:32, Matthew Johnson <[email protected]> wrote:
>
> This is a really important feature IMO, but as others have pointed out it
> basically amounts to higher-kinded types. I would love to be wrong about
> this but I am reasonably sure this is out of scope for Swift 4 (otherwise I
> would be working on a proposal already).
>
> Sent from my iPad
>
I’m not an expert on this stuff, but are they still higher-kinded types if we
don’t express a relationship between Self and the associated type? I don’t
think it’s quite the same conceptual leap as HKT.
Consider that every associated type must be backed by a typealias (explicit or
inferred) in the conforming type. We can already have generic typealiases. This
would be a more targeted thing which required those
associatedtype-implementing-typealiases to contain generic parameters. It would
also extend the constraints from SE-0142 to allow constraints to refer to those
parameters and bind them to other associated types.
The workaround is basically to erase and dynamic-cast your way out:
//NOTE: dynamic type of ScanPromise.Result *must* be same as closure
result. No static enforcement though :(
extension Scanner where ScanPromise.Result == Any? {
func scan<T>(from f: Offset, until u: (Offset, Item) -> T?) -> T? {
return withoutActuallyEscaping(u) { _u -> T? in
return promiseScan(from: f, until: _u).await() as? T // downcast from
Any? to T?
}
}
}
class MyPromise<R>: Promise {
typealias Result = R?
let offset: Offset
let block: (Offset, Item) -> R?
}
class MyScanner: Scanner {
typealias ScanPromise = MyPromise<Any> // want this to be “typealias
ScanPromise<X> = MyPromise<X>"
func promiseScan<T>(from: Offset, until: @escaping (Offset, Item) -> T?) ->
ScanPromise {
return MyPromise(offset: from, block: until) // upcast from T? to Any?
}
}
- Karl
> On Mar 11, 2017, at 11:49 PM, Karl Wagner via swift-evolution
> <[email protected] <mailto:[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?
>>
>> - Karl
>> _______________________________________________
>> swift-evolution mailing list
>> [email protected] <mailto:[email protected]>
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
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