[swift-evolution] [swift-evolution-announce] [Review] SE 0192 - Non-Exhaustive Enums

Nevin Brackett-Rozinsky nevin.brackettrozinsky at gmail.com
Tue Jan 2 22:45:07 CST 2018

On Tue, Jan 2, 2018 at 9:07 PM, Jordan Rose via swift-evolution <
swift-evolution at swift.org> wrote:

> [Proposal: https://github.com/apple/swift-evolution/blob/
> master/proposals/0192-non-exhaustive-enums.md]
> Whew! Thanks for your feedback, everyone. On the lighter side of
> feedback—naming things—it seems that most people seem to like '*@frozen*',
> and that does in fact have the connotations we want it to have. I like it
> too.
> More seriously, this discussion has convinced me that it's worth including
> what the proposal discusses as a *'future' case*. The key point that
> swayed me is that this can produce a *warning* when the switch is missing
> a case rather than an *error,* which both provides the necessary compiler
> feedback to update your code and allows your dependencies to continue
> compiling when you update to a newer SDK. I know people on both sides won't
> be 100% satisfied with this, but does it seem like a reasonable compromise?
> The next question is how to spell it. I'm leaning towards `unexpected
> case:`, which (a) is backwards-compatible, and (b) also handles "private
> cases", either the fake kind that you can do in C (as described in the
> proposal), or some real feature we might add to Swift some day. `unknown
> case:` isn't bad either.
> I too would like to just do `unknown:` or `unexpected:` but that's
> technically a source-breaking change:
> switch foo {
> case bar:
>   unknown:
>   while baz() {
>     while garply() {
>       if quux() {
>         break unknown
>       }
>     }
>   }
> }
> Another downside of the `unexpected case:` spelling is that it doesn't
> work as part of a larger pattern. I don't have a good answer for that one,
> but perhaps it's acceptable for now.
> I'll write up a revision of the proposal soon and make sure the core team
> gets my recommendation when they discuss the results of the review.
> ---
> I'll respond to a few of the more intricate discussions tomorrow,
> including the syntax of putting a new declaration inside the enum rather
> than outside. Thank you again, everyone, and happy new year!
> Jordan

+1 to warning instead of error
+1 to unknown/unexpected case
+1 to “@frozen” or any other reasonable spelling, they are all fine by me.

The one remaining problem to solve is making sure multi-module apps can
leave out the unknown/unexpected case on enums from modules which are part
of the app itself and thus cannot be updated independently of it. John
McCall’s version-locking plan sounds promising, though we should explore
the available options before finalizing a course.

Perhaps we need a concept of submodules, or supermodules, or some other way
to demarcate the boundaries of a resilience domain.

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