[swift-evolution] [Idea] Generic associated types

[Karl Wagner]

> On 12 Mar 2017, at 06:51, Austin Zheng <austinzheng at gmail.com> 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 <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> 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
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> 


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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