[swift-evolution] Handling unknown cases in enums [RE: SE-0192]

Chris Lattner clattner at nondot.org
Sat Jan 13 20:33:18 CST 2018

I don’t understand why #unknown wouldn’t work in catch clauses.  In the absence of typed throws you can’t match on an enums case without the enums base:  you can’t use .foo, you have to use MyEnum.foo.

Similarly, catch wouldn’t allow .#unknown, it would require MyEnum.#unknown.  This is perfectly well defined and just falls out of the model.

That said, I agree that the issue of source dependencies that might use this is a significant problem.  IMO, that argues strongly for “unknown default:” producing a warning.


> On Jan 12, 2018, at 10:49 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
>> On Jan 12, 2018, at 3:08 PM, Jordan Rose <jordan_rose at apple.com> wrote:
>> Okay, I went back to `unknown case` in the proposal, but mentioned Chris's point very specifically: if the compiler emits an error, we should go with `case #unknown` instead. (I'm very strongly in the "warning" camp, though.)
> Thanks!
> Out of curiosity, why not “unknown default:”?  The “warning” behavior is a customization of default, so this seems like a more logical model.  It also fits into the existing Swift grammar, unlike “unknown case:” which requires adding a new special case production.
> -Chris
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