> On Nov 25, 2016, at 12:06 PM, Nevin Brackett-Rozinsky via swift-evolution
> <[email protected]> wrote:
>
> Protocol forwarding was discussed on-list near the end of 2015 in the thread
> “[swift-evolution] [Proposal Draft] automatic protocol forwarding”. Feedback
> was generally positive, though there were disagreements about whether the
> forwarder and/or forwardee should have to conform to the protocol. Also,
> complications regarding Self requirements were brought up.
I was the author of the draft proposal discussed in that thread. I’m sitting
on a second draft which is about 2/3 complete. I will finish the draft and
bring it back up when the core team is ready to consider proposals along these
lines again.
The ideas I have run in a different and more general direction than what you
suggest below. But let’s wait until the time is right to dive into the details.
>
> For simplicity, if we are going to do this, I propose the following:
>
> Direct forwarding
> • Both the forwarder and the forwardee must conform to the protocol being
> forwarded.
> • The forwarder’s associated types are inferred to (and must) match the
> forwardee’s.
> • Requirements the forwarder implements take precedence and are not forwarded.
>
> Self requirements
> • A return type of Self cannot be forwarded.
> • A return type of, eg. Self.Element, *can* be forwarded, since the
> associated types match.
> • A parameter type of Self is forwarded in the natural way, by calling the
> forwardee’s implementation with the argument’s forwardee.
>
> Example of that last point:
>
> protocol P { func f(_: Self) }
> struct Q: P { func f(_: Q){ } }
> class C: P {
> var q: Q implements P
>
> // synthesized:
> func f(_ a: C) {
> q.f(a.q)
> }
> }
>
> Some of these restrictions could potentially be lifted in the future, however
> for the time being I think they serve an important purpose.
>
> All that said, I am not yet entirely convinced that protocol forwarding
> carries sufficient benefits to justify its addition to the language.
>
> Nevin
>
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