<div dir="ltr">On Sat, Feb 11, 2017 at 1:50 PM, Matthew Johnson <span dir="ltr"><<a href="mailto:matthew@anandabits.com" target="_blank">matthew@anandabits.com</a>></span> wrote:<br><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><br><div><div><div class="h5"><blockquote type="cite"><div>On Feb 11, 2017, at 12:40 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank">xiaodi.wu@gmail.com</a>> wrote:</div><br class="m_7332034061545598712Apple-interchange-newline"><div><div dir="ltr">On Sat, Feb 11, 2017 at 6:41 AM, Matthew Johnson <span dir="ltr"><<a href="mailto:matthew@anandabits.com" target="_blank">matthew@anandabits.com</a>></span> wrote:<br><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><div><br><br>Sent from my iPad</div><div><div class="m_7332034061545598712h5"><div><br>On Feb 10, 2017, at 9:48 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank">xiaodi.wu@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr">On Wed, Feb 8, 2017 at 5:05 PM, Matthew Johnson via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div>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.</div><div><br></div><div><br></div><div>Background and Motivation:</div><div><br></div>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).<div><br></div><div>A recent thread (<a href="https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170206/031566.html" target="_blank">https://lists.swift.org/piper<wbr>mail/swift-evolution/Week-of-M<wbr>on-20170206/031566.html</a>) 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 (<a href="https://github.com/apple/swift/blob/master/docs/LibraryEvolution.rst#enums" target="_blank">https://github.com/apple/swif<wbr>t/blob/master/docs/LibraryEvol<wbr>ution.rst#enums</a>) 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.</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div><br></div><div>Proposal:</div><div><br></div><div>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.</div><div><br></div><div>`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)</div><div><br></div><div>`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.</div><div><br></div><div>`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.</div><div><br></div><div>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`.</div><div><br></div><div>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.</div><div><br></div><div><br></div><div>Source compatibility:</div><div><br></div><div>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.</div><div><br></div><div>Brent Royal-Gordon suggested such a strategy in a discussion regarding closed protocols on Twitter:</div><div><br></div><div>* In Swift 4: all unannotated public protocols receive a warning, possibly with a fix-it to change the annotation to `open`.</div><div>* 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.</div><div>* Also In Swift 4: the `closed` modifier is introduced.</div><div><br></div><div>* In Swift 5 the warning becomes a compiler error. `public protocol` is not allowed. Users must use `@annotation public protocol`.</div><div>* 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.</div><div>* In Swift 7 `@annotation public protocol` is no longer allowed.</div><div><br></div><div>A similar mult-release strategy would work for migrating public enums.</div></div></blockquote><div><br></div><div>A different line of feedback here:</div><div><br></div><div>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.</div></div></div></div></div></blockquote><div><br></div></div></div>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.<div><span><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><br></div><div>- I'll leave aside closed protocols, which as per Jordan Rose's feedback may or may not have sufficient interestingness.</div></div></div></div></div></blockquote><div><br></div></span><div>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.</div></div></div></blockquote><div><br></div><div>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.</div><div><br></div><div>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.</div></div></div></div></div></blockquote><div><br></div></div></div><div>This is something that was in the back of my mind for months (I’ve thought about this off and on since last summer). My conclusion is that *conforming* is the important relationship, at least in terms of the `open`, and `closed` discussion. </div><div><br></div><div>As I mentioned in my reply to Karl, I can’t think of any benefit that would be afforded to either a library or its clients by restricting refinement. Obviously clients get more flexibility if they *can* refine protocols defined by a library. From the perspective of a library author nothing changes if a client refines a protocol it defines. All of the semantics of the code in the library is identical either way, as is it’s options for future evolution.</div></div></div></blockquote><div><br></div><div>Perfect; I'm on board with that rationale.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><span class=""><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><div><span><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>- 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:</div><div><br></div><div>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`).<br></div><div>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`.</div></div></div></div></div></blockquote><div><br></div></span><div>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).</div></div></div></blockquote><div><br></div><div>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.</div></div></div></div></div></blockquote><div><br></div></span><div>This is true for the current semantics of `public enum`. But what I am suggesting is that this semantic be called `closed enum`. `public enum` would allow libraries to add new cases resiliently. This is the same semantic for `public enum` that is mentioned in the Library Evolution document (which spells my `closed enum` as `@closed public enum`). </div><div><br></div><div>We have to require a default case for resilient enums because the client may run against a future version of the library with a new case. I think a couple people have mentioned either allowing an implicit default case with `break` or `fatalError` to be synthesized but I am strongly opposed to this. The only other option is a compiler error for a switch over a resilient enum that does not have a default clause.</div></div></div></blockquote><div><br></div><div>You're quite right. I was clearly thinking more clearly yesterday than today.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><span class=""><blockquote type="cite"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><div><span><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>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.</div></div></div></div>
</div></blockquote><br></span></div><div>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!</div></div></blockquote></div><br></div></div>
</blockquote></span></div><br></div></blockquote></div><br></div></div>