> On Feb 9, 2017, at 10:33 AM, Adrian Zubarev <[email protected]> > wrote: > > The last explanation is great, now I could follow the idea behind the > proposed closed keyword/access modifier. I do now understand the contract on > enums, but I’m struggling to understand how closed would work in my own > codebase. Assume I had a closed (not the public meaning of your proposal) > protocol. > > Am I allowed to conform to that protocol inside my library? If it’s a version > controlled feature, how to prevent further conformance to that protocol in a > newer version? > Yes, you are allowed to have as many conforming types inside the library as you wish, as long as the conformances are introduced alongside the protocol. New conformances may not be added in future versions.
I don’t know exactly what the enforcement mechanism would look like, but I imagine it would be similar or identical to the mechanism used for `closed` enums. Maybe someone from the core team can elaborate on what this will look like. > What would an access modifier like closed mean on type members (final > public)? If it’s only a type scope access modifier, it leans towards being an > attribute instead. > I hadn’t considered this yet but it’s a good question. I don’t have time to consider it deeply, but I think the implication for members is that all overrides are public and no new overrides will be added in the future. It isn’t likely to be useful in this case, but I think we could provide a clear definition. > If it’s not an access modifier, would closed have some versioning paramerters? > Bikeshedding: @closed(1.0.0) (The version, the type was closed) > On current enums: @closed(*) (From the start) Good question. This is what the Library Evolution document has to say: @closed is a versioned attribute. This is so that clients can deploy against older versions of the library, which may have non-public cases in the enum. (In this case the client must manipulate the enum as if the @closed attribute were absent.) All cases that are not versioned become implicitly versioned with this number. I would prefer to see `closed` be an access modifier so it isn’t penalized with verbosity, particularly for enums. I think the best approach is that `closed` with no version specifier is allowed if it is included in the initial release of a module. `public` enums may be changed to `closed(n.m.x)` in future versions of a module. There is already precedent for parameterized access modifiers: `private(set)`. > The latter implies in my head that each type needs an explicit version > annotation like @available(1.0.0, *)!? > No, I don’t think so. > How do we handle bug fixes on closed types? Assume all current enums would > get an annotation like @closed(*) public. After normalize enum case > representation proposal > <https://github.com/dduan/swift-evolution/blob/185b12711bfe1e3b7982d255b538f8dad9ae242c/proposals/NNNN-Normalize-Enum-Case-Representation.md> > you might also want to clean up the names of your cases. That means you have > to perform a breaking API change. Both proposals would be part of Swift 4. No module would be able to publish a production-ready version until Swift 4 is released. Also, `closed` wouldn’t have ABI implications until ABI is locked down, which won’t happen until Swift 4 is released (or later). So you would have the chance to clean up case names at the same time as you introduce `closed`. It’s also worth pointing out that changing a case name is going to be a breaking change regardless of whether an enum is `closed` or not. It might be possible to minimize the impact of these changes if there was a way to specify the prior, deprecated name for the case, but this issue is orthogonal to `closed`. > For example in one of my ‘just for fun’ projects I have two enum cases that > look as follows: > > case javaScript(String) > case scopedJavaScript(String, scope: Document) > If the mentioned proposal will be accepted I’d like to make these cases shiny. > > // v1: > case javaScript(String) > case javaScript(String, scope: Document) > > // or v2: > case javaScript(String, scope: Document? = nil) > > > > -- > Adrian Zubarev > Sent with Airmail > > Am 9. Februar 2017 um 16:57:40, Matthew Johnson via swift-evolution > ([email protected] <mailto:[email protected]>) schrieb: > >> >>> On Feb 8, 2017, at 5:48 PM, Xiaodi Wu <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>> I agree very much with rationalizing access levels, but I'm not sure I like >>> this proposal for public vs. closed. How would the compiler stop me from >>> editing my own code if something is closed? The answer must be that it >>> can't, so I can't see it as a co-equal to open but rather simply a >>> statement of intention. Therefore I think use cases for the proposed >>> behavior of closed would be better served by annotations and proper >>> semantic versioning. >> >> The most important point IMO is that they *are* co-equal in the sense that >> they define a contract between library authors, library users and the >> compiler. As you note, there are some differences in how the `closed` >> contract is supported. But that is far less important than the meaning of >> the contract itself. >> >> Dave's comment about tools to assist with contract-compatible API evolution >> is the right way to think about this. Of course you *can* make breaking >> changes, but we want to make it clear when you *are* making a breaking >> change, both for source and for ABI compatibility. This will help library >> authors, but it also helps users as well as the compiler reason about code >> when we are able to offer stronger guarantees. >> >> Most notably, the behavior of public enums *already* has the API contract of >> `closed` and we do not want to remove that capability. This proposal only >> formalizes how that contract is specified and makes it consistent across all >> kinds of types. It *does not* introduce the idea of a closed semantic >> contract for a type. >> >>> As this change didn't seem in scope for Swift 4 phase 1, I've held off on >>> discussing my own thoughts on access levels. The idea I was going to >>> propose in phase 2 was to have simply open and public enums (and >>> protocols). I really think that completes access levels in a rational way >>> without introducing another keyword. >> >> The reason I posted now is because formalizing this API contract for enums >> must happen before ABI is locked down, and also because there is at least >> one protocol in the standard library (`MirrorPath`) which is documented with >> the intent that it be `closed`. >> >> I understand the reluctance to introduce another keyword. It isn’t clear to >> me what semantics you assign to `open` and `public` enums. >> >> Are you suggesting that they match the semantics defined in my proposal and >> suggesting closed enums (i.e. matching the current behavior of `public` >> enums) would require an `@closed` annotation as suggested in the Library >> Evolution document? I am opposed to this approach because it penalizes the >> API contract that I think is often the most appropriate for enums. I >> strongly prefer that we adopt the same neutral stance that we when we >> introduced `open`. >> >> On the other hand, you might be suggesting that `public` enums maintain >> their current behavior and we simply introduce `open` as a modifier that >> reserves the right for the *library* to introduce new cases while continuing >> to prohibit *users* from introducing new cases. This approach has >> inconsistent semantics for both `public` and `open`. These keywords would >> indicate a different API contract for enums than they do for classes and >> protocols. In fact, `open` for enums would have a contract analagous with >> `public` for classes and protocols. This feels like a recipe for confusion. >> IMO, having consistent semantics for each keyword is pretty important. We >> already have, and desire to continue to have, three distinct semantic >> contracts. If we want keywords with consistent semantics we are going to >> have to introduce a new keyword for the third meaning. >> >> >>> On Wed, Feb 8, 2017 at 17:05 Matthew Johnson via swift-evolution >>> <[email protected] <mailto:[email protected]>> wrote: >>> I’ve been thinking a lot about our public access modifier story lately in >>> the context of both protocols and enums. I believe we should move further >>> in the direction we took when introducing the `open` keyword. I have >>> identified what I think is a promising direction and am interested in >>> feedback from the community. If community feedback is positive I will >>> flesh this out into a more complete proposal draft. >>> >>> >>> Background and Motivation: >>> >>> In Swift 3 we had an extended debate regarding whether or not to allow >>> inheritance of public classes by default or to require an annotation for >>> classes that could be subclassed outside the module. The decision we >>> reached was to avoid having a default at all, and instead make `open` an >>> access modifier. The result is library authors are required to consider >>> the behavior they wish for each class. Both behaviors are equally >>> convenient (neither is penalized by requiring an additional boilerplate-y >>> annotation). >>> >>> A recent thread >>> (https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170206/031566.html >>> >>> <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170206/031566.html>) >>> discussed a similar tradeoff regarding whether public enums should commit >>> to a fixed set of cases by default or not. The current behavior is that >>> they *do* commit to a fixed set of cases and there is no option (afaik) to >>> modify that behavior. The Library Evolution document >>> (https://github.com/apple/swift/blob/master/docs/LibraryEvolution.rst#enums >>> <https://github.com/apple/swift/blob/master/docs/LibraryEvolution.rst#enums>) >>> suggests a desire to change this before locking down ABI such that public >>> enums *do not* make this commitment by default, and are required to opt-in >>> to this behavior using an `@closed` annotation. >>> >>> In the previous discussion I stated a strong preference that closed enums >>> *not* be penalized with an additional annotation. This is because I feel >>> pretty strongly that it is a design smell to: 1) expose cases publicly if >>> consumers of the API are not expected to switch on them and 2) require >>> users to handle unknown future cases if they are likely to switch over the >>> cases in correct use of the API. >>> >>> The conclusion I came to in that thread is that we should adopt the same >>> strategy as we did with classes: there should not be a default. >>> >>> There have also been several discussions both on the list and via Twitter >>> regarding whether or not we should allow closed protocols. In a recent >>> Twitter discussion Joe Groff suggested that we don’t need them because we >>> should use an enum when there is a fixed set of conforming types. There >>> are at least two reasons why I still think we *should* add support for >>> closed protocols. >>> >>> As noted above (and in the previous thread in more detail), if the set of >>> types (cases) isn’t intended to be fixed (i.e. the library may add new >>> types in the future) an enum is likely not a good choice. Using a closed >>> protocol discourages the user from switching and prevents the user from >>> adding conformances that are not desired. >>> >>> Another use case supported by closed protocols is a design where users are >>> not allowed to conform directly to a protocol, but instead are required to >>> conform to one of several protocols which refine the closed protocol. >>> Enums are not a substitute for this use case. The only option is to resort >>> to documentation and runtime checks. >>> >>> >>> Proposal: >>> >>> This proposal introduces the new access modifier `closed` as well as >>> clarifying the meaning of `public` and expanding the use of `open`. This >>> provides consistent capabilities and semantics across enums, classes and >>> protocols. >>> >>> `open` is the most permissive modifier. The symbol is visible outside the >>> module and both users and future versions of the library are allowed to add >>> new cases, subclasses or conformances. (Note: this proposal does not >>> introduce user-extensible `open` enums, but provides the syntax that would >>> be used if they are added to the language) >>> >>> `public` makes the symbol visible without allowing the user to add new >>> cases, subclasses or conformances. The library reserves the right to add >>> new cases, subclasses or conformances in a future version. >>> >>> `closed` is the most restrictive modifier. The symbol is visible publicly >>> with the commitment that future versions of the library are *also* >>> prohibited from adding new cases, subclasses or conformances. >>> Additionally, all cases, subclasses or conformances must be visible outside >>> the module. >>> >>> Note: the `closed` modifier only applies to *direct* subclasses or >>> conformances. A subclass of a `closed` class need not be `closed`, in fact >>> it may be `open` if the design of the library requires that. A class that >>> conforms to a `closed` protocol also need not be `closed`. It may also be >>> `open`. Finally, a protocol that refines a `closed` protocol need not be >>> `closed`. It may also be `open`. >>> >>> This proposal is consistent with the principle that libraries should opt-in >>> to all public API contracts without taking a position on what that contract >>> should be. It does this in a way that offers semantically consistent >>> choices for API contract across classes, enums and protocols. The result >>> is that the language allows us to choose the best tool for the job without >>> restricting the designs we might consider because some kinds of types are >>> limited with respect to the `open`, `public` and `closed` semantics a >>> design might require. >>> >>> >>> Source compatibility: >>> >>> This proposal affects both public enums and public protocols. The current >>> behavior of enums is equivalent to a `closed` enum under this proposal and >>> the current behavior of protocols is equivalent to an `open` protocol under >>> this proposal. Both changes allow for a simple mechanical migration, but >>> that may not be sufficient given the source compatibility promise made for >>> Swift 4. We may need to identify a multi-release strategy for adopting >>> this proposal. >>> >>> Brent Royal-Gordon suggested such a strategy in a discussion regarding >>> closed protocols on Twitter: >>> >>> * In Swift 4: all unannotated public protocols receive a warning, possibly >>> with a fix-it to change the annotation to `open`. >>> * Also in Swift 4: an annotation is introduced to opt-in to the new >>> `public` behavior. Brent suggested `@closed`, but as this proposal >>> distinguishes `public` and `closed` we would need to identify something >>> else. I will use `@annotation` as a placeholder. >>> * Also In Swift 4: the `closed` modifier is introduced. >>> >>> * In Swift 5 the warning becomes a compiler error. `public protocol` is >>> not allowed. Users must use `@annotation public protocol`. >>> * In Swift 6 `public protocol` is allowed again, now with the new >>> semantics. `@annotation public protocol` is also allowed, now with a >>> warning and a fix-it to remove the warning. >>> * In Swift 7 `@annotation public protocol` is no longer allowed. >>> >>> A similar mult-release strategy would work for migrating public enums. >>> >>> >>> _______________________________________________ >>> swift-evolution mailing list >>> [email protected] <mailto:[email protected]> >>> https://lists.swift.org/mailman/listinfo/swift-evolution >>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] >> https://lists.swift.org/mailman/listinfo/swift-evolution >
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