[swift-evolution] Enums and Source Compatibility

[Xiaodi Wu]

On Tue, Aug 8, 2017 at 5:27 PM, Jordan Rose via swift-evolution <
swift-evolution at swift.org> 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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