> On Feb 12, 2017, at 12:50 AM, David Hart <da...@hartbit.com> wrote: > > Hi Matthew, > > I've read your proposal ideas and most of the discussions on the thread, and > I'd like to provide some personal feedback. > > Swift already has a complicated "access modifier" story so I think we really > want a good reason to introduce a new one. And the problem I see is that > `closed` has much less semantic weight than the other modifiers.
How so? I’m not sure if I catch your meaning here. It feels to me like it has the same semantic weight as `open`: prohibiting future versions of a module from adding cases / subclasses / conformances is roughly the inverse of lifting the restriction that clients cannot add those things. Therefore it has roughly the same degree of additional meaning over `public` as `open` does. > > First of all, the Library Evolution document you linked says toward at the > top that "this document is primarily concerned with binary compatibility, > i.e. what changes can safely be made to a library between releases that will > not break memory-safety or type-safety, or cause clients to fail to run at > all." It seems to me that the @closed introduced in that document is much > more about library resilience than about only closing down the addition of > new cases: that's why it also talks about reordering and all other changes > that can change the memory layout. > > Swift 3 having introduced both fileprivate and open has complexified the > access level story for developers and library authors. That complexity is the > cost that we have paid for more expressiveness. But if we continue adding new > access control modifiers to express new semantics, we may be going too far: > perfect is the enemy of good. > > Both of those arguments explain why I think closed should be introduced, but > only as a rarely-used attribute for library authors which need to express ABI > resilience, and not as an extra access modifier. `closed` is about much more than binary compatibility. Any time a library publishes an enum that clients can reasonably be expected to switch statements over the library should strive to make it `closed` wherever possible. Otherwise clients are expected to handle unknown future cases by design. That is a design smell if you ask me. This means that we can expect libraries to often carefully design such enums in a way that allows them to be `closed`. The use case for resilient enums is in things like mutually exclusive option sets received as input to the module and for which it would be unusual for clients of the library to write a switch statement over. With this in mind, `closed` should not be a rarely-used attribute at all. In fact it will often be the best choice. This is a big motivation behind my desire to see it on equal footing with `public` and `open`. In regards to the complexity of the access model - if you look closely, `public` has three subtly different meanings today. That kind of inconsistency is part of the complexity of it. And as noted, `closed` is a concept that *will* play a significant role in Swift, regardless of how we spell it. What my proposal aims to do is to incorporate it into a consistent system of outside-the-module access modifiers. One can make a very reasonable argument that access modifiers should *only* be in the business of talking about visibility and should stay out of the business of talking about “who can add to the set of cases / subclasses / conformances”. The time for that argument was when we had the `open` discussion last year. I happen to like the direction we went because it places `public` and `open` on equal footing. And now that we *have* decided to go in this direction, I think we should stick with it when we introduce `closed`. > > David > > On 9 Feb 2017, at 00:05, Matthew Johnson via swift-evolution > <swift-evolution@swift.org <mailto: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/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 >> swift-evolution@swift.org <mailto:swift-evolution@swift.org> >> https://lists.swift.org/mailman/listinfo/swift-evolution >> <https://lists.swift.org/mailman/listinfo/swift-evolution>
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