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> On 19 Sep 2016, at 18:10, 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.
> Opinions?
>> Currently, there is a JIRA bug that Set does not conform to SetAlgebra. To
>> fix this issue, someone simply needs to write `extension Set : SetAlgebra {
>> }` and some tests. That's literally what the bug (filed by a core team
>> member) tells you to do. It's a starter bug, and if someone hasn't taken it
>> yet, you the reader could have a go at it. What's neat about Swift is that
>> it's super easy to provide the same functionality in your own project
>> without waiting on that bug to be fixed in Swift itself. You can simply
>> write a single line of code. By contrast, if your proposal were to be
>> implemented, this would become much more difficult.
>> This is actively used in Swift today. For example, in the Swift
>> implementation of NSScanner, you'll find the following lines:
>> ```
>> internal protocol _BitShiftable {
>>    static func >>(lhs: Self, rhs: Self) -> Self
>>    static func <<(lhs: Self, rhs: Self) -> Self
>> }
>> internal protocol _IntegerLike : Integer, _BitShiftable {
>>    init(_ value: Int)
>>    static var max: Self { get }
>>    static var min: Self { get }
>> }
>> extension Int : _IntegerLike { }
>> extension Int32 : _IntegerLike { }
>> extension Int64 : _IntegerLike { }
>> extension UInt32 : _IntegerLike { }
>> extension UInt64 : _IntegerLike { }
>> ```
>> If we adopted your proposed syntax below, it would take considerably more
>> lines of boilerplate code to express the same thing. The burden increases
>> significantly with the complexity of the retroactive modeling. For
>> instance, if the retroactively modeled protocol had 20 requirements and you
>> were retroactively conforming 20 types, that'd be at least 400 lines of
>> boilerplate.
>>    Basically, the way I see it, if my class MyClass implements MyProtocol,
>>    providing someRequiredFunc(), there’s an “ownership” chain there
>>    (reading it backwards).
>>    Now what happens if MyClass implements MyOtherProtocol, which also has
>>    someRequiredFunc()? In that case, I want to MyClass as a
>>    MyOtherProtocol and get another function pointer, which just happens to
>>    have the same human-readable name as some other property. Just because
>>    they have the same function signature, absolutely doesn’t mean they’re
>>    the same thing.
>>    Now, if we strongly bind all protocol conformances to the protocol they
>>    implement, what happens to instance methods? They don’t belong to any
>>    protocol, their parent is the class itself. If you have an instance
>>    method called someRequiredFunc(), and you later add a conformance to
>>    MyProtocol, you would need to declare that it belongs to MyProtocol. If
>>    you don’t want it to be an API-breaking change, you have to provide a
>>    thunk (or we could provide a shorthand syntax which emits thunks for
>>    you) to let us know that MyClass::someRequiredFunc() is the same thing
>>    as MyClass::MyProtocol::someRequiredFunc().
>> Your argument is that two methods with the same name should not in any way
>> conflict with each other. This is a fundamental change from the status quo.
>> If we were to take your argument to its logical conclusion, any member A of
>> a type T should be capable of being designated as the implementation of a
>> requirement B of protocol P. In the most general case, two functions A and
>> B shouldn't even need to take the same number of arguments, or arguments of
>> the same type; you should be able to supply default arguments, or even
>> write custom code that takes arguments for A and computes suitable
>> arguments for B in order to forward A to B, and the language should allow
>> you to designate A as an implementation of B. But that is simply not how
>> Swift protocols are designed.
>>    Let’s take an example where retroactive modelling could go wrong.
>>    You’ve got different teams working on different parts of an App, and
>>    they’ve all got their own convention for “copy()”. Sometimes it’s a
>>    deep-copy, sometimes a shallow-copy, sometimes it’s used in a fragile
>>    way for a specific case, whatever. Now you want to go and clean that up
>>    by creating a “Copyable” protocol with codified guarantees. Some
>>    objects may already conform, others may need tweaks, and some may want
>>    both behaviours simultaneously (preserving the old,
>>    non-Copytable-compliant behaviour until the next API break), depending
>>    on how you look at the object. A system like this allows all of those
>>    different ways of looking at the object live together. You could have
>>    the old, non-comforming API as an extension with a FIXME to delete it
>>    for version 2.
>> Even if you design a protocol called Copyable, you still need to explicitly
>> extend concrete types in order to conform to Copyable. Swift does not
>> automagically make anything conform to your protocol. If you choose
>> *explicitly* to conform different types that don't guarantee the same
>> semantics, and then you erroneously assume that they all have the same
>> semantics even though you *explicitly* chose types that don't have the same
>> semantics, you're the one who shot yourself in the foot, so to speak. It's
>> not the fault of Swift at all.
>>    I think it’s pretty arcane that members of a type are resolved only by
>>    their names. If you want to provide types which allow flexible views of
>>    data, each view of that data needs to be completely free in its
>>    expressivity.
>>    I would actually like to see a syntax like:
>>    ```
>>    let testObject = MyClass()
>>    let testMyProto = testObject.MyProtocol // the protocol-witness table
>>    for testObject as a MyProtocol.
>>    testObject.MyProtocol.someRequiredFunc() // that’s one function
>>    testObject.someRequiredFunc() // is a different function. May happen to
>>    have the same implementation as above if MyProtocol was retroactively
>>    modelled.
>>    ```
>>    I think it would fit well with the dispatch system for protocol
>>    extensions, too. I sometimes have code like the following:
>>    ```
>>    protocol Base {}
>>    protocol Derived : Base {}
>>    extension Base {
>>      func doSomething() { … }
>>    }
>>    extension Derived {
>>      func doSomething() {
>>       …
>>       (self as Base).doSomething() // Would be better if we could say
>>    “self.Base.doSomething()” to disambiguate instead of casting.
>>      }
>>    }
>>    ```
>> This is a complete redesign of protocols in Swift. With the emphasis on
>> minimizing source-breaking changes, I doubt such a change would be in scope
>> for any phase of Swift unless you could show an overwhelming benefit.
>>    So yeah, a big +1 to marking protocol methods with their protocol
>>    (whatever the syntax ends up looking like), and actually I’d take it
>>    further and bake them in to the ABI. That also makes it relevant for
>>    Swift 4 phase 1.
>>    Karl
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