On Sat, Feb 11, 2017 at 6:41 AM, Matthew Johnson <matt...@anandabits.com> wrote:
> > > Sent from my iPad > > On Feb 10, 2017, at 9:48 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: > > On Wed, Feb 8, 2017 at 5:05 PM, Matthew Johnson via swift-evolution < > swift-evolution@swift.org> 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/piper >> mail/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/LibraryEvol >> ution.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. >> > > A different line of feedback here: > > As per previous reply, I now think if we clarify the mental model of the > access modifier hierarchy you're proposing and adopt or reject with that > clarity, we'll be fine whether we go with `closed` or with `@closed`. But I > don't think the source compatibility strategy you list is the most simple > or the most easy to understand for end users. > > > I'm pretty neutral on what kind of source compatibility strategy we would > adopt. I am happy to defer to the community and core team. > > > - I'll leave aside closed protocols, which as per Jordan Rose's feedback > may or may not have sufficient interestingness. > > > Jordan supported allowing protocols to have the same choice of contract > that classes do today. `public protocol` has the same meaning as `open > class` today so if we want consistency we need a breaking change. > Sure; I was specifically considering the phased introduction of `closed`. It's been a while since I've thought about how to phase in a change regarding public protocols and open protocols. That said, others make good points about _conforming to_ protocols by a type vs. _refining_ protocols by another protocol, and whether either or both of these is more akin to subclassing a class. - With respect to enums, I don't think we need such a drastic whiplash in > terms of what will compile in future versions. Instead, we could take a > more pragmatic approach: > > 1. In Swift 4, remove the warning (or is it error?) about `default` cases > in switch statements over public enums. Simultaneously, add `closed` or > `@closed` (whatever is the approved spelling) and start annotating standard > library APIs. The annotation will be purely future-proofing and have no > functional effect (i.e. the compiler will do nothing differently for a > `closed enum` or `@closed public enum` (as the case may be) versus a plain > `public enum`). > 2. In Swift 4.1, _warn_ if switch statements over public enums don't have > a `default` statement: offer a fix-it to insert `default: fatalError()` > and, if the enum is in the same project, offer a fix-it to insert `closed` > or `@closed`. > > > Why do you say "if the enum is in the same project, offer a fix-it to > insert `closed`? If the enum is in the same project we can perform an > exhaustive switch regardless of its public API contract (except for `open` > enums if we decide to add those). > Hmm, well now I'm not in favor of my own suggestion. A public enum, though it may gain or lose cases in future versions, can be exhaustively switched over in the present whether it's same-module or third-party. No warning or error should issue on attempting to switch over a public enum without a default case. > 3. In Swift 5, upgrade the warning to an error for non-exhaustiveness if a > switch statement over a public enum doesn't have a `default` statement. > Now, new syntax to extend an `open enum` can be introduced and the compiler > can treat closed and public enums differently. > > > If the community and core team support this strategy I will also. It > seems reasonable and speeds up the transition by using the point release. > That's a great idea! >
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