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

[Jordan Rose]

> On Jan 9, 2018, at 21:39, Brent Royal-Gordon <brent at architechies.com> wrote:
> 
> 
> 
>> On Jan 9, 2018, at 4:46 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> - FWIW I can't actually think of a use case for using this with `if case` or anything else. I'm not against it, but I don't know why you would ever do it, just like I don't know why you would mix `case #unknown` with `default` when matching against a single value.
> 
> 	if case #unknown = someEnum {
> 		throw MyError.unknownValue
> 	}
> 	…

Yes, you can write that, but why would you do it? What are you going to do below now that you know 'someEnum' is a known value but the compiler doesn't?

(Also, it doesn't play well with recompiling…)


> 
>> That said, it sounds like people are happier with `case #unknown` than `unknown case`, and that leaves things a little more consistent if we ever do expand it to other pattern positions, so I'll change the proposal revision to use that spelling. (And if anyone comes up with a nicer spelling, great!)
> 
> I don't love the spelling of `#unknown`—particularly since some enums in the Apple frameworks actually *have* an ordinary case called "unknown"—but I think it's a nice, pragmatic solution which slots into pattern matching very nicely.
> 
> (And if we ever *do* decide to support @testable enum parameters, we'll already have the syntax to specify unknown values.)
> 
> -- 
> Brent Royal-Gordon
> Architechies
> 

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