> On Sep 19, 2016, at 12:10 PM, Vladimir.S via swift-evolution 
> <swift-evolution@swift.org> wrote:
> 
> On 17.09.2016 6:32, Xiaodi Wu via swift-evolution wrote:
>> 
>> Let me give a concrete example of how retroactively modeling is used.
> 
> Karl is suggesting interesting but complex and IMO too much code-breaking 
> idea that I don't believe can be implemented at all in a reasonable amount of 
> time to be a part of Swift as soon as possible, to address the discussed 
> issue with protocols.
> 
> I wonder what objections could be made on the solution proposed below, which 
> should solve a major(IMO) number of issues with protocol conformance and 
> introduce only 1 keyword. Such solution will make Swift better as 
> Protocol-Oriented language and later we can even improve it, but it can 
> already solve a big number of issues:
> 
> 1. As soon as possible we add 'implement' keyword which is required to mark 
> method/property that was defined in type or extension exactly to conform to 
> some protocol.
> 
> 2. The 'implement' required only at a moment of 'direct' conformance, i.e. 
> when you declare methods/props of the type/extension that explicitly 
> conformed to protocol.
> 
> 3. Retrospective conformance will not require this keyword and will work for 
> now just like it is working today.
> 
> 4. Later, if this will be possible at all, we can extend this model to 
> support separate implementation of protocols with same requirements in the 
> same type, explicit protocol name in implemented methods/props and 
> improvements for retrospective conformance. For example some variants for 
> *future* improvements:
> 
> 4.1 Different implementation for different protocols
> class Foo : ProtocolA, ProtocolB {
>  implement(ProtocolA) func foo() {...}
>  implement(ProtocolB) func foo() {...}
> }
> class Foo : ProtocolA, ProtocolB {
>  implement ProtocolA {
>       func foo() {...}
>  }
>  implement ProtocolB {
>       func foo() {...}
>  }
> }
> etc
> 
> 4.2 Retrospective conformance: What is the main problem with retrospective 
> conformance? As I see it now(correct me, if I missing something), the problem 
> arises in such situation:
> * we *expect* that some method(s) in type will play the role of 
> implementation of protocol's requirements, so we retrospectively conform that 
> type to the protocol.
> * but protocol has default implementation for its requirements
> * and type's methods, that we *expect* to play roles for protocol 
> implementation, has different parameters or slightly different method name at 
> all.
> 
> I.e. when we have this set of code logic:
> 
> type T {
>  func foo()
> }
> 
> protocol P {
>  func foo(x: Int)
> }
> 
> extension P {
>  func foo(x: Int) {...}
> }
> 
> extension T : P { // expect foo in T will play role of P.foo
> }
> 
> I support the opinion that it is not an option to require to explicitly list 
> conformed methods/props in type extension for retrospective conformance.
> But I do believe we need a way to *express our intention* regarding the 
> retrospective conformance: do we expect that type already contains 
> implementation for some protocol's requirements OR we are aware that protocol 
> can have defaults for some methods and our type does not contains some 
> implementations.
> 
> So, the solution here IMO is some syntax to express that intention. Right now 
> I think that we can use current syntax "extension T : P" to keep it working 
> as it now works: "I'm aware of all the names, defaults etc. Treat this as 
> usually you did". But for example something like "extension T: implement P 
> {..}" or "extension T: P(implement *) {..}" will say that we *expect* that 
> all requirements of P protocol should be implemented inside T type. Or some 
> syntax inside extension to specify the list of methods/props we expect to be 
> implemented in T. Or "extension T : P(implement foo, bar(x:y:)) {..}".. 
> Should be discussed.
> 
> But again, IMO this could be discussed later, after we'll have 'implement' 
> for most important place - in type definition for method/prop that we created 
> exactly for the conformed protocol.

I would be completely +1 on this.

Charles

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