I’m not saying vital support is missing, just that it is more awkward. String 
doesn’t conform to collection, String.UTF8View does; so if you change some 
implementation detail (in StringCore, because that’s where they live for 
String), you get the error popping up somewhere other than the place you just 
changed. That’s what I mean when I say “language support”. If you do what 
StringCore does, and you’re changing stuff which is ultimately going to be used 
to conform to, say, Collection, you have to build distance between the 
implementation and the (only) conformance it is used for, and it’s less optimal.

Let’s say I have an object MyComplexDataType, it implements 
“InternalStructureView” and “Collection”:

```
protocol InternalStructureView {
    associatedtype Index
    var count : Int { get } // whatever, just some stuff that will cause a name 
conflict
    func doInternalMagic(at: Index)
}

struct MyComplexDataType {
    var __collection_count : Int {
        // This is quite a complex operation which we’d rather leave inside the 
type, otherwise we’d need to expose a bunch of implementation details internally
    }
   var __internal_count : Int {
       // Again, best left here
   }

   struct CollectionView  : Collection {
       init(parent: MyComplexDataType) { … }
        var count { return parent.__collection_count }
        // ...etc
   }

  struct InternalStructure : InternalStructureView {
      init(parent: MyComplexDataType) { … }
      var count { return parent.__internal_count }
      // ...etc
  }

  var collection : CollectionView { return CollectionView(self) }
  var internalStructure : InternalStructure { return InternalStructure(self) }
}
```

This is basically what String does (except that it wants to conform to 
Collection multiple times with different indexes and results). It’s a lot of 
work to maintain, especially if you have evolving protocols that are conformed 
to in several places.
We should have a better solution. We should be able to define:

“protocol UTF8Collection : Collection {}
 protocol UTF16Collection : Collection {}”

and have String conform to both of them at the same time. At the same time, 
since we’re now being explicit about which protocol requirement is satisfied 
where - we should also be able to delegate our conformance, telling the 
compiler to dispatch any unimplemented methods to another object. For example, 
lets say you want to wrap a Collection and observe mutations to it; you might 
override replaceSubrange(), but every other method (such as count, 
index(after:)… all the rest) is just a forwarding function.

Interestingly, we could do this safely (from a code legibility perspective) if 
we say that every scope which adds a conformance must completely satisfy its 
requirements, which we would more reasonably be able to require if we made this 
change (so your internal functions can still be wherever you like, but they 
won’t automatically satisfy a protocol requirement if they happen to have the 
same name). Then we could reasonably say that if you add an extension which 
adds a conformance to, say, Collection, you have to tell us where to find every 
one of its requirements. That’s where we could put the forwarding syntax for 
retroactive modelling. Stored properties can’t be defined in extensions, so if 
you want to back an implementation with one, you’ll need to make its 
conformance explicit in the main body (or we loosen that to at least extensions 
in the same file).

```
// generates thunks to members of this extension in the base type; 
// so MyComplexDataType.count —> MyComplexDataType.InternalStructure.count,
// to account for conformance being added in later version. Can also be used 
for renamed protocols, and be tagged on individual members.

@makeAvailable(as: _)
extension MyComplexDataType : InternalStructure {

   typealias Index = InternalIndexType
    var count : Int {
        // We have access to all of the private members because we’re inside 
MyComplexDataType
        // No need to pollute internal API with conformance implementation 
details.
        // Also, we get errors about non-conformance where we want them — where 
the implementation is.
    }
    func doInternalMagic(at: Index) {
       ...
    }
}
```

> On 20 Sep 2016, at 23:46, Xiaodi Wu <xiaodi...@gmail.com> wrote:
> 
> I'm not sure I understand. What compiler or language support is missing for 
> StringCore?
> On Tue, Sep 20, 2016 at 16:42 Karl via swift-evolution 
> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>> On 20 Sep 2016, at 23:28, Karl <raziel.im+swift-...@gmail.com 
>> <mailto:raziel.im+swift-...@gmail.com>> wrote:
>> 
>> 
>>> On 20 Sep 2016, at 18:43, Nevin Brackett-Rozinsky via swift-evolution 
>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>> 
>>> I have been following this discussion (as well as similar threads earlier 
>>> this year) and listening to the ideas put forth by all sides.
>>> 
>>> It seems to me that the fundamental difference between classes and 
>>> protocols is that classes inherit implementation whereas protocol 
>>> conformance is a promise about interface.
>>> 
>>> When a class or struct or enum declares itself as conforming to a protocol, 
>>> that means it has all the members specified in the protocol. The protocol 
>>> conformance simply codifies a fact about the type itself: namely that all 
>>> those members are present.
>>> 
>>> In this model, any keyword such as `implements` on each conforming member 
>>> would introduce substantial boilerplate for negligible gain. The purpose of 
>>> a protocol is to communicate that certain members are available, not to 
>>> make declaring those members more onerous.
>>> 
>>> However, default implementations for protocols blur the line. Now there is 
>>> actual implementation being inherited. A conforming type may choose to roll 
>>> its own version of a method, or to utilize the default provided by the 
>>> protocol. This is closer to the situation with subclassing.
>>> 
>>> Moreover, a protocol which conforms to another protocol may itself define 
>>> (or redefine!) default implementations for members of that other protocol. 
>>> This can create “inheritance chains” of protocol default implementations. I 
>>> think there is value in being able to refer to (and call) the inherited 
>>> default implementation through some sort of `super` functionality.
>>> 
>>> On the other hand, the existence of a default implementation in a protocol 
>>> is in large part merely a convenience: a courtesy so that each conforming 
>>> type need not rewrite the same boilerplate code.
>>> 
>>> A type which conforms to a protocol may accept the default or it may 
>>> provide its own implementation, but it is not “overriding” anything. The 
>>> default implementation was offered as a convenience, to be taken or left as 
>>> needed. Thus I do not think any keyword (neither `override` nor 
>>> `implements`) should be required in that case either.
>>> 
>>> The frequently-raised point regarding near-miss member names deserves some 
>>> attention. Several people have expressed a desire for the compiler to 
>>> assist them in determining whether a given member does or does not meet a 
>>> protocol requirement. Specifically, when a type conforms to a protocol with 
>>> a default implementation, and the type defines a member with a similar 
>>> signature, it is not obvious at glance if that member matches the protocol.
>>> 
>>> I think this is a job for linters and IDEs. For example, syntax 
>>> highlighting could distinguish members which satisfy a protocol 
>>> requirement, thereby providing immediate visual confirmation of success.
>>> 
>>> Having followed the lengthy discussion and weighed the numerous ideas put 
>>> forth, I come down firmly on the side of no keyword for protocol 
>>> conformance.
>>> 
>>> A protocol describes an interface and provides a set of customization 
>>> points. It may also, as a convenience, offer default implementations. The 
>>> protocol simply describes the capabilities of its conforming types, and any 
>>> default implementations are there to make things easier for them.
>>> 
>>> Conforming types should not be afflicted with extraneous keywords: that 
>>> would run contrary to the purpose of having protocols in the first place.
>>> 
>>> Nevin
>>> 
>>> 
>>> On Tue, Sep 20, 2016 at 11:16 AM, Xiaodi Wu via swift-evolution 
>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>> As I mentioned above, I agree that better diagnostics for near-misses are 
>>> necessary, but they are possible without new syntax. There is no win in 
>>> avoiding unintentional behavior because, without a default implementation, 
>>> these issues are caught at compile time already.
>>> 
>>> On Tue, Sep 20, 2016 at 10:14 Vladimir.S via swift-evolution 
>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>> 
>>>  > extension P {
>>>  > implement func foo() -> [String : String] { return [:] }
>>>  > }
>>> 
>>> Yes, it seems like we need `implement` (or `override` as another
>>> suggestion) in protocol extension also just for the same reasons - be clear
>>> about our intention regarding implementing the requirement, to show that
>>> this func *depends* on the previous definition of P protocol and to avoid
>>> possible mistakes related to protocol conformance.
>>> 
>>> On 20.09.2016 17:38, Charles Srstka wrote:
>>> >> On Sep 20, 2016, at 8:17 AM, Vladimir.S via swift-evolution
>>> >> <swift-evolution@swift.org <mailto:swift-evolution@swift.org> 
>>> >> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>> 
>>> >> wrote:
>>> >>
>>> >> On 20.09.2016 3:03, Xiaodi Wu via swift-evolution wrote:
>>> >>> 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
>>> >>
>>> >> Yes, *at the moment of writing* the type's code there could be no default
>>> >> implementation for protocol requirement. But, *at the moment of
>>> >> compilation* such default implementation could appear.
>>> >>
>>> >> Let's discuss such scenario in case we'll take your suggestion:
>>> >>
>>> >> You got SomeClass.swift file, 3rd party file you don't want to change or
>>> >> changes are not allowed. Content:
>>> >>
>>> >> public protocol SomeProtocol {
>>> >> func foo()
>>> >> }
>>> >>
>>> >> public class SomeClass : SomeProtocol {
>>> >> func foo() {...} // no default implementation *at the moment of writing*,
>>> >> no need in `overload`
>>> >> }
>>> >>
>>> >> Now, you adds SomeClass.swift file to your project and in some *other*
>>> >> file you write:
>>> >>
>>> >> extension SomeProtocol {
>>> >> func foo() {...}
>>> >> }
>>> >>
>>> >> As you see, you don't control the SomeClass.swift but you suggest in this
>>> >> case SomeClass.foo() should be defined with `override`.
>>> >>
>>> >> With 'implement' SomeClass.foo() will be marked initially and will save
>>> >> us if protocol's requirement PLUS default implementation changed.
>>> >
>>> > Requiring the ‘implement’ keyword can help us even if no default
>>> > implementation is involved. Consider:
>>> >
>>> > protocol P {
>>> > func foo() -> [String : Any]
>>> > }
>>> >
>>> > struct S : P {
>>> > func foo() -> [String : String] { return [:] }
>>> > }
>>> >
>>> > We will get an error here that S does not conform to P. However, this is
>>> > not the correct error, since S in fact *tries* to conform to P, but it has
>>> > a mistake in a method signature. This misleads us as to the true nature of
>>> > the problem, and if S has enough members in it that we fail to spot the
>>> > existing foo(), we might solve the problem by reimplementing foo(), and
>>> > leaving the original foo() as dangling dead code. Having an ‘implement’
>>> > keyword on the existing foo() function would change the compiler error to
>>> > let us know that we have an existing foo() that is incorrectly declared.
>>> >
>>> > In addition, ‘implement’ can help us when the declaration in question *is*
>>> > the default implementation:
>>> >
>>> > protocol P {
>>> > func foo() -> [String : Any]
>>> > }
>>> >
>>> > extension P {
>>> > implement func foo() -> [String : String] { return [:] }
>>> > }
>>> >
>>> > Here we will get an error with the proposed ‘implement’ keyword, because
>>> > foo() does not have a signature matching anything in the protocol, whereas
>>> > without ‘implement’ we would happily and silently generate a useless
>>> > dangling function that would never be used, and then pass the buck to the
>>> > concrete type that implements P:
>>> >
>>> > protocol P {
>>> > func foo() -> [String : Any]
>>> > }
>>> >
>>> > extension P {
>>> > func foo() -> [String : String] { return [:] } // The error is here:
>>> > }
>>> >
>>> > struct S : P {} // But it gets reported here.
>>> >
>>> > Charles
>>> >
>>> _______________________________________________
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>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution 
>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>> 
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>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>> 
>>> 
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>> 
>> 
>> I agree that a new keyword is unwanted. Conforming to protocols is quite a 
>> common thing, so you want it to be easy to remember.
>> 
>> I think the best way is to prefix the member name with the protocol, e.g:
>> 
>> protocol MyProto {
>>     var aVariable : Int
>>     func aFunction()
>> }
>> class MyClass : MyProto {
>>     var MyProto.aVariable : Int
>>     func MyProto.aFunction() { … }
>> }
>> 
>> This is consistent with how we refer to other members of types (e.g. 
>> “extension MyClass.MyInternalClass”). It will be easy for autocompletion to 
>> provide good suggestions, too.
>> As I see it, the only problem is what if `MyClass` wants its own function 
>> called `aFunction()`? What if the same name satisfies 2 protocols, which do 
>> you write?
>> 
>> The way to solve all of the problems in a consistent way is to make the 
>> function actually called “MyProto.aFunction”, and for it to be a separate 
>> function from plain “aFunction()” or from “SomeotherProto.aFunction”.
>> 
>> I believe it is crucial to protocols that we can do this. Maybe I have some 
>> complex data structure and it has its own API, but I want people to be able 
>> to view it as a Collection. By conforming to Collection, I reserve lots of 
>> keywords and indexing operations which I now can’t use in my own API. Maybe 
>> I’m just providing Collection as a convenience to work with generic 
>> algorithms, but my own API has more efficient semantics for some operations. 
>> We’re relegated to using less-obvious and legible names in order to avoid 
>> conflicts.
>> 
>> We have a way to work around this, which String uses - create a struct which 
>> references your object and calls internal methods such as 
>> “_collection_count” so you can have separate interfaces. This adds up to 
>> quite a lot of boilerplate and maintenance overhead.
> 
> Also to add here: you’re basically implementing what I’m proposing manually 
> if you do this; only you don’t get language/compiler support.
> String basically does this - it shares StringCore with UTF8View and defines 
> some internal functions to support it.
> 
> The String views could then be made in to protocols on String, turning 
> “UTF8View” in to “UTF8Representable”, and opening up algorithms which can 
> work on generic sequences of UTF8 bytes. I think that’s pretty cool, and 
> could open up better integration with other types which are (for example) 
> UTF8Representable — for example a stream of UTF8 bytes (depending on how 
> flexible implementation allows us to make the protocol).
> 
>> 
>> I don’t agree that Protocol conformances are kind-of incidental, as others 
>> here have written. This isn’t like Objective-C where anything that has the 
>> correctly-named methods conforms. Protocol conformances are completely 
>> explicit, and in fact we have empty protocols (“marker protocols”) for 
>> exactly that purpose. I think it is consistent that we make every member of 
>> a conformance specify which protocol it belongs to, and to have its name 
>> scoped to that protocol.
>> 
>> Karl
>> 
>> 
>> CC-ing Dave A, to understand better if this fits with the vision of protocols
>> 
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