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