> On 12 Mar 2017, at 06:51, Austin Zheng <[email protected]> wrote:
>
> I think you want higher-kinded types.
> https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#higher-kinded-types
>
> <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#higher-kinded-types>
>
> Best,
> Austin
>
>> On Mar 11, 2017, at 9:49 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
>
Not necessarily. Higher-kinded types (IIUC) are per-instance - e.g. every
instance of a Collection could have a unique type of Index.
I want to constrain a return value, whose type is an associated type, based on
a generic parameter to the function. SE-0142 only allows constraining entire
associated types, and allows no interaction with per-function generic
parameters.
- Karl
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