Since it seems to have been lost in the noise, I want to second with support for Xiaodi's syntax of having `default` appearing in the enum declaration itself.
It's much clearer in its intention, feels very ‘Swifty’, and more importantly it doesn't prompt whole threads debating the semantics of `open` vs `public`. ------------ Begin Message ------------ Group: gmane.comp.lang.swift.evolution MsgID: <CAGY80u=kVQA1q=5tmxxxfgm4tlgfuqh61en1daepemaa_fo...@mail.gmail.com> On Tue, Aug 8, 2017 at 5:27 PM, Jordan Rose via swift-evolution < [email protected]> wrote: >Hi, everyone. Now that Swift 5 is starting up, I'd like to circle back to >an issue that's been around for a while: the source compatibility of enums. >Today, it's an error to switch over an enum without handling all the cases, >but this breaks down in a number of ways: > >- A C enum may have "private cases" that aren't defined inside the >original enum declaration, and there's no way to detect these in a switch >without dropping down to the rawValue. >- For the same reason, the compiler-synthesized 'init(rawValue:)' on an >imported enum never produces 'nil', because who knows how anyone's using C >enums anyway? >- Adding a new case to a *Swift* enum in a library breaks any client code >that was trying to switch over it. > >(This list might sound familiar, and that's because it's from a message of >mine on a thread started by Matthew Johnson back in February called >"[Pitch] consistent public access modifiers". Most of the rest of this >email is going to go the same way, because we still need to make progress >here.) > >At the same time, we really like our exhaustive switches, especially over >enums we define ourselves. And there's a performance side to this whole >thing too; if all cases of an enum are known, it can be passed around much >more efficiently than if it might suddenly grow a new case containing a >struct with 5000 Strings in it. > > >*Behavior* > >I think there's certain behavior that is probably not *terribly* >controversial: > >- When enums are imported from Apple frameworks, they should always >require a default case, except for a few exceptions like NSRectEdge. (It's >Apple's job to handle this and get it right, but if we get it wrong with an >imported enum there's still the workaround of dropping down to the raw >value.) >- When I define Swift enums in the current framework, there's obviously no >compatibility issues; we should allow exhaustive switches. > >Everything else falls somewhere in the middle, both for enums defined in >Objective-C: > >- If I define an Objective-C enum in the current framework, should it >allow exhaustive switching, because there are no compatibility issues, or >not, because there could still be private cases defined in a .m file? >- If there's an Objective-C enum in *another* framework (that I built >locally with Xcode, Carthage, CocoaPods, SwiftPM, etc.), should it allow >exhaustive switching, because there are no *binary* compatibility issues, >or not, because there may be *source* compatibility issues? We'd really >like adding a new enum case to *not* be a breaking change even at the >source level. >- If there's an Objective-C enum coming in through a bridging header, >should it allow exhaustive switching, because I might have defined it >myself, or not, because it might be non-modular content I've used the >bridging header to import? > >And in Swift: > >- If there's a Swift enum in another framework I built locally, should it >allow exhaustive switching, because there are no binary compatibility >issues, or not, because there may be source compatibility issues? Again, >we'd really like adding a new enum case to *not* be a breaking change >even at the source level. > >Let's now flip this to the other side of the equation. I've been talking >about us disallowing exhaustive switching, i.e. "if the enum might grow new >cases you must have a 'default' in a switch". In previous (in-person) >discussions about this feature, it's been pointed out that the code in an >otherwise-fully-covered switch is, by definition, unreachable, and >therefore untestable. This also isn't a desirable situation to be in, but >it's mitigated somewhat by the fact that there probably aren't many >framework enums you should exhaustively switch over anyway. (Think about >Apple's frameworks again.) I don't have a great answer, though. > >For people who like exhaustive switches, we thought about adding a new >kind of 'default'—let's call it 'unknownCase' just to be able to talk about >it. This lets you get warnings when you update to a new SDK, but is even >more likely to be untested code. We didn't think this was worth the >complexity. > > >*Terminology* > >The "Library Evolution ><http://jrose-apple.github.io/swift-library-evolution/>" doc (mostly >written by me) originally called these "open" and "closed" enums ("requires >a default" and "allows exhaustive switching", respectively), but this >predated the use of 'open' to describe classes and class members. Matthew's >original thread did suggest using 'open' for enums as well, but I argued >against that, for a few reasons: > >- For classes, "open" and "non-open" restrict what the *client* can do. >For enums, it's more about providing the client with additional >guarantees—and "non-open" is the one with more guarantees. >- The "safe" default is backwards: a merely-public class can be made >'open', while an 'open' class cannot be made non-open. Conversely, an >"open" enum can be made "closed" (making default cases unnecessary), but a >"closed" enum cannot be made "open". > >That said, Clang now has an 'enum_extensibility' attribute that does take >'open' or 'closed' as an argument. > >On Matthew's thread, a few other possible names came up, though mostly >only for the "closed" case: > >- 'final': has the right meaning abstractly, but again it behaves >differently than 'final' on a class, which is a restriction on code >elsewhere in the same module. >- 'locked': reasonable, but not a standard term, and could get confused >with the concurrency concept >- 'exhaustive': matches how we've been explaining it (with an "exhaustive >switch"), but it's not exactly the *enum* that's exhaustive, and it's a >long keyword to actually write in source. > >- 'extensible': matches the Clang attribute, but also long > > >I don't have better names than "open" and "closed", so I'll continue using >them below even though I avoided them above. But I would *really like to >find some*. > > >*Proposal* > >Just to have something to work off of, I propose the following: > >1. All enums (NS_ENUMs) imported from Objective-C are "open" unless they >are declared "non-open" in some way (likely using the enum_extensibility >attribute mentioned above). >2. All public Swift enums in modules compiled "with resilience" (still to >be designed) have the option to be either "open" or "closed". This only >applies to libraries not distributed with an app, where binary >compatibility is a concern. >3. All public Swift enums in modules compiled from source have the option >to be either "open" or "closed". >4. In Swift 5 mode, a public enum should be *required* to declare if it >is "open" or "closed", so that it's a conscious decision on the part of the >library author. (I'm assuming we'll have a "Swift 4 compatibility mode" >next year that would leave unannotated enums as "closed".) >5. None of this affects non-public enums. > >(4) is the controversial one, I expect. "Open" enums are by far the common >case in Apple's frameworks, but that may be less true in Swift. > > >*Why now?* > >Source compatibility was a big issue in Swift 4, and will continue to be >an important requirement going into Swift 5. But this also has an impact on >the ABI: if an enum is "closed", it can be accessed more efficiently by a >client. We don't *have* to do this before ABI stability—we could access >all enums the slow way if the library cares about binary compatibility, and >add another attribute for this distinction later—but it would be nice™ (an >easy model for developers to understand) if "open" vs. "closed" was also >the primary distinction between "indirect access" vs. "direct access". > >I've written quite enough at this point. Looking forward to feedback! >Jordan > Jordan, I'm glad you're bringing this back up. I think it's clear that there's appetite for some forward movement in this area. With respect to syntax--which the conversation in this thread has tackled first--I agree with the discussion that "open" and "closed" are attractive but also potentially confusing. As discussed in earlier threads, both "open" and "closed" will constrain the enum author and/or user in ways above and beyond "public" currently does, but the terminology does not necessarily reflect that (as open is the antonym of closed); moreover, the implications of using these keywords with enums don't necessarily parallel the implications of using them with classes (for example, an open class can be subclassed; an open enum that gains additional cases is, if anything, something of a supertype of the original). I'd like to suggest a different direction for syntax; I'm putting it forward because I think the spelling itself naturally suggests a design as to which enums are (as you call it) "open" or "closed," and how to migrate existing enums: ``` enum MyClosedEnum { case a case b case c } enum MyOpenEnum { case a case b case c default } ``` In words, an enum that may have future cases will "leave room" for them by using the keyword `default`, sort of paralleling its use in a switch statement. All existing Swift enums can therefore continue to be switched over exhaustively; that is, this would be an additive, source-compatible change. For simplicity, we can leave the rules consistent for non-public and public enums; or, we could prohibit non-public enums from using the keyword `default` in the manner shown above. Obj-C enums would be imported as though they declare `default` unless some attribute like `enum_extensibility` is used to annotate them. Thoughts? ------------- End Message ------------- _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
