[swift-evolution] [Draft] Introducing StaticSelf, an Invariant Self

[Austin Zheng]

I'm not quite sure I understand the 'Additional Utility' section. A given class hierarchy can only have one implementation of a given static method; static  methods are effectively final. Therefore, the 'Self' introduced in the previous proposal should be completely sufficient for invoking a static method on a class. (Likewise, for moving around code.) In fact, the previous proposal for 'Self' used a very similar example.

I think the example would make more sense if it involved class methods. However, *without considering the protocol applications* I wouldn't consider that a compelling enough use case, by itself, to add StaticSelf.

As for StaticSelf's impact on protocols, I defer to the opinions of the other folks who've commented on the thread.

Austin


> On May 12, 2016, at 5:49 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Erica Sadun and I have written a proposal are following up the recent discussion thread "[RFC] #Self” with a proposal to introduce StaticSelf, an invariant Self.
> 
> The recent discussion can be found here: http://thread.gmane.org/gmane.comp.lang.swift.evolution/16565 <http://thread.gmane.org/gmane.comp.lang.swift.evolution/16565>
> 
> The proposal can be found here: https://github.com/anandabits/swift-evolution/blob/static-self/proposals/NNNN-static-self.md <https://github.com/anandabits/swift-evolution/blob/static-self/proposals/NNNN-static-self.md>
> 
> We look forward to continuing the discussion.  We plan to submit a PR in the near future after incorporating your final feedback.
> 
> Thanks,
> Matthew
> Introducing StaticSelf, an Invariant Self
> 
> Proposal: TBD
> Authors: Matthew Johnson <https://github.com/anandabits>, Erica Sadun <https://github.com/erica>
> Status: TBD
> Review manager: TBD
> Introduction
> 
> This proposal introduces a new keyword that provides consistent invariant type semantics in all contexts.
> 
> The Swift-evolution thread about this topic can be found here: [RFC] #Self <http://thread.gmane.org/gmane.comp.lang.swift.evolution/16565>
> Motivation
> 
> The distinction between covariant and non-covariant type references come into play when
> conforming non-final classes to protocols. Fixing a protocol requirement to a covarying type
> means that a method returning Self must be overriden by all subclasses in order to return
> the correct, matching type.
> 
> This proposal builds on the covariant construct Self accepted in SE–0068 <https://github.com/apple/swift-evolution/blob/master/proposals/0068-universal-self.md>
> to introduce an invariant type identifier. It enables protocol declarations to consistently
> refer to a type that is fixed at compile time. This ensures that subclasses can inherit
> protocol implementations without having to re-implement that code at each level of
> inheritance.
> 
> Under this proposal, a new identifier keyword is fixed in use at the point of protocol conformance
> to the static type of that construct. 
> 
> class A: MyProtocol
> The invariant StaticSelf identifier will always refer to A, unlike Self, which is covarying and refers to
> the type of the actual instance. Since multiple inheritance for non-protocol types is disallowed,
> this establishes this invariant type identifier with no possibility for conflict.
> 
> Consider the following example, under the current system:
> 
> protocol StringCreatable {
>     static func createWithString(s: String) -> Self
> }
> 
> extension NSURL: StringCreatable {
>  // cannot conform because NSURL is non-final
>  // error: method 'createWithString' in non-final class 'NSURL' must return `Self` to conform to protocol 'A'
> }
> Introducing a static, invariant version of Self permits the desired conformance:
> 
> protocol StringCreatable {
>     static func createWithString(s: String) -> StaticSelf
> }
> 
> extension NSURL: StringCreatable {
>  // can now conform conform because NSURL is fixed and matches the static
>  // type of the conforming construct. Subclasses need not re-implement
>  // NOTE: the return type can be declared as StaticSelf *or* as NSURL
>  //       they are interchangeable
>  static func createWithString(s: String) -> StaticSelf { 
>      // ...
>  }
> }
> Additional Utility
> 
> The utility of StaticSelf is not limited to protocols. A secondary use enables code to refer to the lexical context’s current type without explicitly mentioning its name. This provides a useful shortcut when referencing static type members with especially long names and when re-purposing code between types.
> 
> class StructWithAVeryLongName {
>     static func foo() -> String {
>       // ...
>     }
>     func bar() {
>       // ...
>       let s = StaticSelf.foo()
>       //
>     }
> }
> Detailed Design
> 
> This proposal introduces StaticSelf, a new keyword that may be used in protocols to refer to the invariant static type of a conforming construct. StaticSelf may also be used in the lexical context of any type declaration. In such use, the keyword is identical to spelling out the full name of that type.
> 
> Impact on existing code
> 
> Being additive, there should be no impact on existing code.
> 
> Alternatives considered
> 
> The keyword is not fixed at this time. Alternatives that have been discussed include StaticType, InvariantSelf, SelfType, or Type. The community is welcome to bikeshed on the most clear and concise name for this keyword.
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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