[swift-users] Covariance in Generic Protocols & Proposal 0142

[Zhao Xin]

`protocol A: GenericProtocol where Instance == String { }` means, A is
something that `Instance` must be `String`.
However, it doesn't mean `Instance` has already been `String`.

So without assigning your `Instance`, the compiler doesn't know the type of
your `Instance`.

Zhao Xin

On Mon, Jun 26, 2017 at 11:46 AM, Justin Jia via swift-users <
swift-users at swift.org> wrote:

> Hi,
>
> I’m trying to implement something like this using Swift 4 after proposal
> 0142 is implemented:
>
> ```
> protocol GenericProtocol {
>     associatedtype Instance
>     func foo() -> Instance
>     func bar() -> Instance
>     // and more...
> }
>
> protocol A: GenericProtocol where Instance == String { }
> protocol B: GenericProtocol where Instance == Int { }
> protocol C: GenericProtocol where Instance == Double { }
> // and more…
>
> class Bar {
>     var a: A // Error: Protocol ‘A' can only be used as a generic
> constraint because it has Self or associated type requirements
>     var b: B // Error: Protocol ‘B' can only be used as a generic
> constraint because it has Self or associated type requirements
>     var c: C // Error: Protocol ‘C' can only be used as a generic
> constraint because it has Self or associated type requirements
>     // and more...
> }
> ```
>
> However, I’m still getting the `Protocol ‘A' can only be used as a generic
> constraint because it has Self or associated type requirements` error.
>
> Instead, the only thing I can do right now is to duplicate my code:
>
> (Just in case your are wondering, I need this syntax to simplify some code
> I’m working on https://github.com/TintPoint/Overlay/tree/swift-4.0)
>
> ```
> protocol A {
>     func foo() -> String
>     func bar() -> String
>     // and more...
> }
>
> protocol B {
>     func foo() -> Int
>     func bar() -> Int
>     // and more...
> }
>
> protocol B {
>     func foo() -> Double
>     func bar() -> Double
>     // and more...
> }
>
> // and more...
>
> class Bar {
>     var a: A // OK
>     var b: B // OK
>     var c: C // OK
>     // and more...
> }
> ```
>
> Am I doing something wrong here? Is it Swift’s current limitation that can
> be improved in a future version of Swift? Or it needs a special syntax and
> a separate proposal?
>
> Thank you in advance for your help!
>
> Sincerely,
> Justin
>
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>
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