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?
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