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

[Vladimir.S]

On 12.01.2018 10:30, Chris Lattner via swift-evolution wrote:
> 
>> On Jan 11, 2018, at 11:15 PM, Jean-Daniel via swift-evolution <swift-evolution at swift.org> wrote:
>>
>> A question about the new #unknown behavior. Is it intended to be used for error handling too ?
>> Will it be possible to use in catch clause ?
> 
> If we go with the #unknown approach, then yes of course it will work in catch clauses.  They are patterns, so it naturally falls out.
> 
> If we go with the “unknown default:” / “unknown case:"  approach, then no, this has nothing to do with error handling.
> 
> IMO, this pivots on the desired semantics for “unknown cases in enums”: if you intentionally try to match on this, do we get a warning or error if you don’t handle all the cases?  If we can get to consensus on that point, then the design is pretty obvious IMO.

For me the other question is what "all the cases" means for enum with private cases(if we'll have them). I.e. if switch 
contains all the "public" cases of frozen enum - does this mean "all the cases" were processed? As I understand, the 
answer is no, because we *can* have 'private' case value here and so we need to react to this. How switch will look in 
this case?

switch frozenEnumWithPrivateCases {
   case .one: ..
   case .two: ..
   unknown default: ..  // or 'case #unknown:' depending on our decision, or 'unknown case:' etc
}
?
But then such switch looks exactly as switch for non-frozen enum value, no? It looks like we are reacting on future new 
cases, while enum is frozen.

Moreover. How the switch for non-frozed enum with private cases should looks like?

switch nonfrozenEnumWithPrivateCases {
   case .one: ..
   case .two: ..
   unknown default: ..  // or 'case #unknown:' depending on our decision, or 'unknown case:' etc
}
? But then, is that 'unknown default' for reacting on "future" cases we didn't know about during the compilation OR it 
is for reacting on private cases?

Or the main idea that we don't want to separate "future" cases and "private" cases? Then still the first 'switch' on 
frozen enum confuses the reader.

I understand that we are not discussing the 'private' cases yet, but IMO we should consider how new 'case 
#unknown/unknown default' will be used in case we'll add that feature.

Vladimir.

> 
> -Chris
> 
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