[swift-users] ⁨Is it possible to store a set of heterogeneous items with protocol?

[David Hart]

> On 19 Jul 2017, at 09:22, Glen Huang via swift-users <swift-users at swift.org> wrote:
> 
> 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?

No. It’s because Hashable conforms to Equatable, and Equatable has a Self requirement. In English, it means that Equatable requires both of its arguments to be of the same type, which can’t be guaranteed in the context of an array of heterogenous items.

> 
> 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 <spestov at apple.com <mailto:spestov at 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 at swift.org <mailto:swift-users at 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_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Jul 18, 2017, at 10:33, Vladimir.S via swift-users <swift-users at swift.org <mailto:swift-users at 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 at swift.org <mailto:swift-users at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-users <https://lists.swift.org/mailman/listinfo/swift-users>
>> 
> 
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-users/attachments/20170719/f0e2e2c9/attachment.html>