[swift-evolution] [Pitch] consistent public access modifiers

Thu Feb 9 09:57:13 CST 2017

> On Feb 8, 2017, at 5:48 PM, Xiaodi Wu <xiaodi.wu at gmail.com> 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 <swift-evolution at swift.org <mailto:swift-evolution at 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.
> 
> 
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