I don't think "override" is a good idea. It's not overriding.

Protocol is not Class.


Xiaodi Wu via swift-evolution <swift-evolution@swift.org>于2016年9月20日
周二08:04写道:

> I definitely think Vladimir's suggestion is a great starting point, IMO.
>
> However, I think it could be improved in one key respect where previous
> proposals using `override` are superior. Namely, the proposed `implement`
> keyword adds no additional safety when a type implements a protocol
> requirement that doesn't have a default implementation. This is because, if
> there's a "near-miss" typo in the function signature, an error is already
> generated at compile time because the type doesn't conform to the stated
> protocol. Currently, the error is very unhelpful, but IMO that's a
> straight-up bug; improving the diagnostics for that error can be done
> without evolution.
>
> On the other hand, if we require `implement`, the simplest use case of
> conforming a type to a protocol with no default implementations would take
> more effort but provide no benefit to justify that additional effort.
> Moreover (I think a core team member has expressed this more elegantly in
> the past), there's the philosophical point that POP represents the
> theoretical process by which we discover and express our discovery that
> certain types happen to share common semantic characteristics. In that
> conception of POP, it would be backwards to declare a certain member as
> fulfilling certain protocol requirements.
>
> So, if such a source breaking change were to be in scope for Swift, I
> would suggest modifying Vladimir's proposal to use `override` instead and
> requiring the keyword only when a default implementation is being
> overridden. To accommodate retroactive conformance, we could either propose
> that `extension Foo : Bar` is automatically understood to contain members
> that override default implementations (as Vladimir has essentially
> suggested), or stipulate that we must write `extension Foo : override Bar`.
> This has the advantage of not introducing an additional keyword and avoids
> the seemingly reduplicative spelling `extension Foo : implement Bar` (for
> what else would an `extension Foo : Bar` reasonably do but implement the
> requirements of Bar?).
>
>
> On Mon, Sep 19, 2016 at 3:10 PM, Goffredo Marocchi via swift-evolution <
> swift-evolution@swift.org> wrote:
>
>> If Swift 4 will make it impossible to tackle this again, I do not think
>> discussing this can be avoided for Swift 3.1... I am afraid we are rushing
>> into Swift 4 a bit too quickly, but perhaps it is just silly old me :).
>>
>> Sent from my iPhone
>>
>> On 19 Sep 2016, at 19:18, Charles Srstka via swift-evolution <
>> swift-evolution@swift.org> wrote:
>>
>> 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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