[swift-evolution] Enums and Source Compatibility

[Jordan Rose]

Hi, Vladimir. I think framing this as a consumer decision is the wrong place to start. There are some enums that definitely make sense to be "closed", all the time, with no additional annotations, including Foundation.NSComparisonResult and, well, Swift.Optional. (Yes, Optional is special, and we could always handle it specially if we needed to, but it would be nice if it used the same logic as everything else.)

Beyond that, I think something like what you and Charlie describe (your '@exhaustive', Charlie's 'switch!') could make sense, as a way to provide a deterministic behavior while still getting compiler warnings for not being exhaustive. (Probably the only supported behavior here would be trapping, since there is no way to test such a scenario with the libraries you currently have.) But I'd like to see a real-world example of exhaustively switching over an enum in the SDK before designing a feature around this; I strongly suspect it's not something we need…

…in the binary framework case. It may still be interesting for source frameworks, particularly C enums in those source frameworks.

Jordan


> On Aug 9, 2017, at 09:57, Vladimir.S <svabox at gmail.com> wrote:
> 
> Sorry if I misunderstood the subject, but shouldn't this also be a *consumer* decision, when one wants to keep own switch exhaustive or it is OK to process all possible future cases in 'default'?
> 
> If enum is 'closed' - nothing is changed, as I understand, we have the same rules for switch as currently.
> 
> If 'external' enum, from other framework, was marked as 'closed' but was actually changed in new version of framework/module - our source code will fail to compile (because we need fix our switch cases) and our binary module(built with previous version of framework) will crash.
> 
> With 'open' enums, depending on situations, I as a consumer of external framework, can decide that it is important to me, in my code, check *each* value of external enum in switch. If new enum case added/changed in external framework - my code must fail to compile and notify me that new case should be processed.
> Once we added 'default' case in 'switch' in our code for 'open' enum - we lost compiler's support to keep our 'switch' exhaustive.
> 
> But from other side, in other situation, I want to process all new cases for 'open' enum in 'default' branch, and so allow my source/compiled code to work with new version of framework(with added cases).
> 
> So, it seems like in both situations we need to explicitly tell what is our decided behavior for new cases for 'open' enum in exhaustive switch : crash/fail to compile or process in 'default' block.
> 
> What if in case of exhaustive switch on 'open' enum, compiler warns us saying "this is an exhaustive switch on 'open' enum, but cases can be added later. clarify what is your decided behaviour. by default your code will crash if new cases are added"
> , then we can add 'default' block to process new future cases or somehow mark that switch as explicitly exhaustive... something like this:
> 
> @exhaustive
> switch openEnum {
>  case .yes : ...
>  case .no : ...
> }
> 
> What I'm missing?
> 
> On 09.08.2017 1:27, Jordan Rose via swift-evolution 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/ <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
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