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

[Jordan Rose]

> On Jan 2, 2018, at 18:07, Jordan Rose <jordan_rose at apple.com> wrote:
> 
> [Proposal: https://github.com/apple/swift-evolution/blob/master/proposals/0192-non-exhaustive-enums.md <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!

I ended up doing these in the opposite order, writing up the new proposal first and not yet responding to the discussion that's further out. You can read my revisions at https://github.com/apple/swift-evolution/pull/777.

In particular, I want to at least address:
- Dave D and Drew C's points about versioned libraries / linking semantics of modules.
- Jason M's point about migration
and I'll do one more pass over the thread to see if there's anything else I didn't address directly. (That doesn't mean everyone who disagrees, just messages where I think there's more I can do to explain why the proposal is the way it is.)

Jordan

P.S. Enjoying the Disney references. Thanks, Nevin and Dave. :-)
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