Thanks for the heads up. I wonder what’s the root cause of the difficulty to make "such an existential conform to Hashable in a general way”. Is it because objects of different types have local definitions for hash value, thus making it impossible to compare them directly?
Go is able to handle heterogeneous items defined by a protocol: https://play.golang.org/p/8xtrZyTW40 <https://play.golang.org/p/8xtrZyTW40> I’m no expert in language design, wonder how they are able to implement it. > On 19 Jul 2017, at 2:34 PM, Slava Pestov <spes...@apple.com> wrote: > > Hopefully we will one day have generalized existentials which would at least > allow a heterogeneous array of protocols with an associated type; however it > is not clear how such an existential could conform to Hashable in a general > way, given that Hashable implies Equatable which has a Self requirement. > > Slava > >> On Jul 18, 2017, at 10:36 PM, Glen Huang via swift-users >> <swift-users@swift.org <mailto:swift-users@swift.org>> wrote: >> >> Do you think if there will be any evolution proposal to address this >> limitation? Or it’s an inherent tradeoff that is unlikely to be changed? >> >>> On 19 Jul 2017, at 8:49 AM, Jordan Rose <jordan_r...@apple.com >>> <mailto:jordan_r...@apple.com>> wrote: >>> >>> >>> >>>> On Jul 18, 2017, at 10:33, Vladimir.S via swift-users >>>> <swift-users@swift.org <mailto:swift-users@swift.org>> wrote: >>>> >>>> On 17.07.2017 4:51, Glen Huang via swift-users wrote: >>>>> Thanks for the code sample and link, but if I’m not wrong, this pattern >>>>> doesn’t allow heterogeneous items. >>>> >>>> Support the question. Trying to understand if we can have something like >>>> [AnyHashable] for our custom protocol(with associated type) or AnyHashable >>>> has a very special support from compiler and we can use only [Any] or such >>>> kind of wrapper: >>>> >>>> struct AnyMyProtocol { >>>> let actualInstance: Any >>>> init<T: MyProtocol>(_ instance: T) { actualInstance = instance} >>>> } >>>> >>>> let instances: [AnyMyProtocol] = [AnyMyProtocol(...), AnyMyProtocol(...)] >>>> >>>> if let some = instances[0].actualInstance as? >>>> SpecificImplementationOfMyProtocol { >>>> // use 'some' as SpecificImplementationMyProtocol instance >>>> // seems like no way to refer to just MyProtocol >>>> } >>> >>> AnyHashable is special, sorry. You'll have to use this sort of indirect >>> unwrapping instead. You can write a little convenience method for it >>> though, if you want: >>> >>> extension AnyMyProtocol { >>> func get<T: MyProtocol>(as: T.Type) -> T? { >>> return self.actualInstance as? T >>> } >>> } >>> >>> if let some = instances[0].get(as: SpecificImplementationOfMyProtocol.self) >>> { >>> // use 'some' here >>> } >>> >>> Jordan >> >> _______________________________________________ >> swift-users mailing list >> swift-users@swift.org <mailto:swift-users@swift.org> >> https://lists.swift.org/mailman/listinfo/swift-users >
_______________________________________________ swift-users mailing list swift-users@swift.org https://lists.swift.org/mailman/listinfo/swift-users
