[swift-evolution] Enums and Source Compatibility

Zach Waldowski zach at waldowski.me
Wed Aug 9 10:18:57 CDT 2017

I disagree. Closed is indeed the stronger guarantee, but APIs are
designed differently in Swift; closed is a sensible default. We
shouldn’t need to define new keywords and increase the surface area of
the language for something that has verisimilitude with the existing
open syntax.
  Zachary Waldowski
  zach at waldowski.me

On Wed, Aug 9, 2017, at 06:23 AM, David Hart via swift-evolution wrote:> 
> On 9 Aug 2017, at 09:21, Adrian Zubarev via swift-evolution <swift-
> evolution at swift.org> wrote:>> Hi Jordan, is that only me or haven't you metioned the default should
>> be applied to all new enums? Personally I'd say that 'closed' should
>> be the default and the 'open' enum would require an extra keyword.> 
> I think it should definitely be the other way round for public enums
> because closed is the stronger guarantee. Final is the default for
> classes because open is the stronger guarantee. That’s probably why we
> should not use the same keywords.> 
>> Now about the keyword itself. Here are two keywords that IMHO nail
>> their behavior down to the point:>> 
>> finite enum A {} - so to say a closed enum (default)
>> infinite enum B {} - so to say an open enum (requires default case in
>> a switch statement)>> 
>> If you think the default should be the other way around, than feel
>> free to switch that. 'finite' also implies that the enum connot ever
>> be extended with more cases (to become infinite), which was also
>> mentioned in your email.>> 
>> -- 
>> Adrian Zubarev
>> Sent with Airmail
>> Am 9. August 2017 um 00:27:53, Jordan Rose via swift-evolution (swift-
>> evolution at swift.org) schrieb:>>> 
>>> 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[1]" 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
>>> _______________________________________________
>>> swift-evolution mailing list swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution>>> 
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  1. http://jrose-apple.github.io/swift-library-evolution/
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