[swift-evolution] Enums and Source Compatibility

Thu Aug 10 09:25:46 CDT 2017

On 10.08.2017 16:46, Matthew Johnson via swift-evolution wrote:
> 
>> On Aug 10, 2017, at 7:46 AM, James Froggatt via swift-evolution
>> <swift-evolution at swift.org> wrote:
>> 
>> 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`.
> 
> I think Xiaodi’s syntax is very elegant if we want to avoid the access control
> style syntax.  However, it does one problem: the “error of omission” (not thinking
> about open vs closed) leaves a library author with a closed enum, preventing them
> from adding cases in the future without breaking compatibility.  I’m not sure this
> is acceptable.

Then, doesn't this mean that any 'usual' enum should be 'open' by default, and only 
enum declared with some marker (like 'final' or 'enum(sealed)') can be 'closed'?

Otherwise we need to require an explicit marker for *each* enum, and so break the 
source compatibility? (we'll have to append that marker to each enum in your current 
code)

Also I'd suggest this for closed enum:

enum MyClosedEnum {
   case a
   case b
   case c
   final
}

So, for public closed enum it will looks like:

public enum MyClosedEnum {
   case a
   case b
   case c
   final
}

Also, if we need to explicitly mark open enum, probably we can consider 'continue' 
keyword, as IMO is not clear what 'default' is saying on declaration site('you must 
insert `default` in switch'? 'there are other `default` cases'?) :

public enum MyOpenEnum {
   case a
   case b
   case c
   continue // to be continue...
}

> 
>> 
>> ------------ Begin Message ------------ Group: gmane.comp.lang.swift.evolution 
>> MsgID: <CAGY80u=kVQA1q=5TMxXxFgM4tLGFUQh61EN1daepEMAA_FoE9Q at mail.gmail.com>
>> 
>> On Tue, Aug 8, 2017 at 5:27 PM, Jordan Rose via swift-evolution < 
>> swift-evolution-m3FHrko0VLzYtjvyW6yDsg at public.gmane.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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