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@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_fo...@mail.gmail.com>

On Tue, Aug 8, 2017 at 5:27 PM, Jordan Rose via swift-evolution < swift-evolution-m3fhrko0vlzytjvyw6y...@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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