Hi Trevör,
What you’re asking for has been discussed extensively and a very thorough
proposal was drafted by Austin Zheng:
https://github.com/austinzheng/swift-evolution/blob/az-existentials/proposals/XXXX-enhanced-existentials.md
This is not something that is likely to make it into Swift 4 but is a feature
with a lot of community demand so it is likely to be added eventually.
Matthew
> On Feb 10, 2017, at 10:46 AM, Trevör ANNE DENISE via swift-users
> <swift-users@swift.org> wrote:
>
> Hello everyone !
>
> I am currently writing a project where I would like to do something similar
> to this:
>
> protocol ProtocolA {
> associatedtype U
> var someVariable : U { get set }
> func someFunction(_ x:U) -> U
> }
>
> class SomeClass<T> {
> var someProperty : ProtocolA where ProtocolA.U == T
> }
>
> But this isn't possible so I had to implement it as a generic class:
> class NewClass<U> { ... }
>
> And now I am using it like this:
> class SomeClass<T> {
> var someProperty: NewClass<T>
> }
>
> The issue here is that protocols aren't used any more with this approach, and
> in my case there is no "default" implementation of myFunction, so protocols
> where really useful!!
>
> Is there a better way to do that in the current Swift version?
>
> Today, I came across the proposal SE-0142 (Permit where clauses to constrain
> associated types), but this only seems to apply inside of protocols, what is
> your opinion about having similar mechanisms in classes and structs ?
>
> Thank you!
>
> Trevör
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