[swift-evolution] Mark protocol methods with their protocol
Thorsten Seitz
tseitz42 at icloud.com
Tue Sep 27 09:22:38 CDT 2016
> Am 21.09.2016 um 01:58 schrieb Karl via swift-evolution <swift-evolution at swift.org>:
>
> I’ve explained it so many times, do you really not understand it?
>
> Basically, protocols are explicit things. They are **not** things that you just “discover” about types at all. We have protocols with no members (“marker protocols”) which are basically used to tag certain types with no other guarantees. That is the important difference between protocols in Swift and in Objective-C. Even if you have implementations for every requirement, if you never explicitly declared conformance you don’t conform. Period.
>
> However, the requirements themselves don’t need to be explicitly marked-up. They just get automatically assigned by the compiler; maybe my property named “count” doesn’t mean the same thing as Collection’s “count”?
>
> I’ve written a lot trying to explain this; please tell me what you don’t understand. To summarise (again):
>
> - Members which satisfy a protocol requirement will have to be explicit about which protocol they belong to
> ——> They will no longer be automatically assigned as the witness for a protocol requirement
>
> - Members which satisfy a protocol requirement will have the protocol name in their symbolic names (e.g. MyType.someFunction() —> MyType.AProtocol.someFunction())
> ——> this will allow the same type to conform to protocols with overlapping names, free protocol designers from naming constraints, and reduce the need for “helper structs” to represent alternate views of the same data.
While this solves the problem of overlapping names with different semantics, how would you express the case where the names actually have the same semantics, so that you only want to have one implementor?
protocol A {
func specficSemantics()
func sameSemantics()
}
protocol B {
func specficSemantics()
func sameSemantics()
}
struct C : A, B {
// these implementations have to be specific for A and B so Karl’s approach allows to differentiate them
func A.specficSemantics() { … }
func B.specficSemantics() { … }
// this implementation should be shared by A and B because both share the same semantic here
func sameSemantics() { … } // ??
// would I have to write it like that (manual delegation to the shared implementation)?
func sameSemantics() { … }
func A.sameSemantics() { sameSemantics() }
func B.sameSemantics() { sameSemantics() }
}
-Thorsten
> ——> Extensions which declare conformance have to satisfy all of their requirements explicitly - by implementing, forwarding to other functions, stored properties or other objects. We can have a shorthand to make this easy.
>
> IMO, we definitely need to solve the overlapping naming problem. If we explicitly assign protocol methods to protocols (as the thread was originally about), this falls out of it.
>
>
>> On 21 Sep 2016, at 01:25, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>
>> This was the conversation we were having in the other thread. Perhaps I'm still not understanding something, but I'm not convinced that this feature is an improvement.
>>
>> Currently, protocols represent a declaration that you have discovered that your type has certain semantics and guarantees a certain API, and you declare that fulfillment "by name only," just as you invoke members "by name only"; unintentional failure to fulfill the contract is a compile-time error. The status quo is precisely my idea of what protocols should be. What are the two ideas you have about them?
>> On Tue, Sep 20, 2016 at 18:02 Karl <razielim at gmail.com <mailto:razielim at gmail.com>> wrote:
>> I’m using String as an example of where this issue of conformance conflicts crops up in the standard library. Ideally, String (or any data type) should be able to conform to protocols whose requirements have conflicting names.
>> Currently, in order to work around this limitation, you have to delegate the conformance to a supporting type. This is more complicated to write and maintain, and pollutes your internal API. String gives us an example of this, but it’s not the worst example.
>> It basically implements what I’m talking about anyway, but manually via a supporting type instead of directly inside the data-type (String). As an ABI consideration, we should scope all protocol members to their protocols. This would resolve all naming conflicts.
>>
>> I can’t understand how anybody would argue for the status quo - we’re currently in-between two ideas of what protocols should be - they are explicit but their conformances are resolved by name only and can overlap without warning.
>>
>>> On 21 Sep 2016, at 00:48, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>>
>>> Sorry, I'm still not sure I understand what you're getting at about this. How would String conforming to Collection multiple times simplify or improve the implementation of String? Or are you arguing it would be better for users of String? If so, how?
>>>
>>> On Tue, Sep 20, 2016 at 17:26 Karl <razielim at gmail.com <mailto:razielim at gmail.com>> wrote:
>>> 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.wu at gmail.com <mailto:xiaodi.wu at 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 at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> On 20 Sep 2016, at 23:28, Karl <raziel.im+swift-evo at gmail.com <mailto:raziel.im+swift-evo at gmail.com>> wrote:
>>>>>
>>>>>
>>>>>> On 20 Sep 2016, at 18:43, Nevin Brackett-Rozinsky via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at 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 at swift.org <mailto:swift-evolution at 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 at swift.org <mailto:swift-evolution at 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 at swift.org <mailto:swift-evolution at swift.org> <mailto:swift-evolution at swift.org <mailto:swift-evolution at 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
>>>>>> >
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>>
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>>>>>>
>>>>>>
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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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>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>
>>
>
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