On Wed, Feb 8, 2017 at 5:05 PM, Matthew Johnson via swift-evolution < [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/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/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. > 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'll leave aside closed protocols, which as per Jordan Rose's feedback may or may not have sufficient interestingness. - 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`. 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.
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