[swift-evolution] [Discussion] Generic protocols

[Daniel Leping]

Ok. Let's say I have a generic protocol P<T> with two methods:
func foo()
func bar(o:T)

I have a default implementation of foo, which internally calls bar.

How would work foo once a class implements P<Int> and P<String>?

On Sat, 3 Dec 2016 at 15:26 Adrian Zubarev via swift-evolution <
swift-evolution at swift.org> wrote:

> I believe generic protocols could be used as a shortcut for protocols with
> associated types.
>
> // Protocol with associated type
>
> protocol Foo {
>
>     associatedtype F
>     func foo(_ f: F)
> }
>
> // Existential
> typealias IntFoo = Any<Foo> where F == Int
>
> struct Test : IntFoo {} // error
>
> struct Test : Foo { func foo(_: Int) {…} }
>
> let intFoo: IntFoo = Test() // fine
>
> // SE-0142
>
> protocol IntFooProtocol : Foo where F == Int {}
>
> // Generic protocols
> // Autogenerated with all associated types present in the parameter list
>
> protocol GenericFoo<F> : Foo { … }
>
> Instead of creating new protocol for a different subset of types
> constrained by the where clause, this approach could come really handy.
>
> Does it affect stdlib and/or ABI somehow? When SE–0142 is implemented to
> improve the stdlib, wouldn’t that mean that more types like IntFooProtocol
> from above will spawn?
>
>
>
> --
> Adrian Zubarev
> Sent with Airmail
>
> Am 3. Dezember 2016 um 13:47:13, Anders Ha (hello at andersio.co) schrieb:
>
> This is called generalized existentials. It is included in the Generic
> Manifesto, has been discussed quite a few times with long email chains
> before, and spawned the change to the `protocol<>` syntax as kinda a
> precursor. It would be surprised if Swift 4 Phase 2 doesn't have it given
> its popularity.
>
> Regards
> Anders
>
> On 2 Dec 2016, at 20:13, Charles Srstka via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> On Dec 2, 2016, at 12:34 PM, Adrian Zubarev via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> I just overlooked that the subsection about generic protocols was inside
> the *Unlikely* section.
>
> The problem is that I need a way to refer to a function with a specific
> name. Plus the connection type has to have a specific API, like having a
> DispatchQueue and know the router object if there is any (sounds like a
> protocol right?!). The function reference should also keep the connection
> object alive with a strong reference.
>
> associatedtype does not solve that problem for me.
>
> I clearly see that generic protocols overlap with associatedtype but
> couldn’t we find a compromise here? For instance like Chris Lattner
> introduced generic type aliases without the ability of constants.
>
> Why don’t we just use angle brackets to specify associated types?
> Protocols aren’t using them for anything anyway. Then you could:
>
> if let someSequence as? Sequence<Iterator.Element == Int> { // do
> something }
>
> which would form a nice parallel to the syntax for casting things to
> generic types.
>
> Charles
>
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