+1 for @frozen

> On Dec 20, 2017, at 7:16 PM, Brent Royal-Gordon via swift-evolution 
> <swift-evolution@swift.org> wrote:
>> On Dec 19, 2017, at 2:58 PM, Ted Kremenek via swift-evolution 
>> <swift-evolution@swift.org> wrote:
>>      • What is your evaluation of the proposal?
> I am pleased with the broad strokes of this design. I have quibbles with 
> three areas:
> 1. The `@exhaustive` attribute may be confusing because the term doesn't 
> suggest versioning. My best alternative suggestion is `@frozen`, which 
> matches existing programming terminology: something that has been frozen will 
> not be changed in the future.
> 2. I think we need some kind of `future` keyword in `switch` statements. Even 
> though a nonexhaustive enum may gain additional cases in the future, it's 
> still useful for the compiler to diagnose that you forgot *known* cases.
> You say that "switches over non-exhaustive enums should be uncommon", and 
> this is true for many—perhaps most—non-exhaustive enums, but there is still a 
> large class of non-exhaustive enums which need to be switched over. These are 
> the ones I called "category 2" in my previous email in this thread. 
> `SKPaymentTransactionState` is the example I previously used; others might 
> include `Stream.Status` (if not exhaustive), `CLAuthorizationStatus`, 
> `EKParticipantType`, `PKPaymentMethodType`, and `MKMapType`. Each of these 
> could plausibly have more cases added; each has a good reason why you might 
> switch over cases (such as display in a user interface); and each ought to be 
> promptly updated when a new OS version introduces new cases. Without compiler 
> assistance, those updates won't happen.
> If we plan to add private cases in a future version of Swift, `future` may 
> not be the best keyword. `unknown`, `invalid` (or `case #invalid`), etc. may 
> be better.
> 3. I am very skeptical of treating all enums as exhaustive if imported by 
> `@testable import`. The only reason I can see not to do this is that forcing 
> you to provide `default` might hide tests that need to be updated for new 
> enum cases—but this is the exact problem that `future` is trying to solve. By 
> contrast, treating them as non-exhaustive forces you to actually notice when 
> an enum is published as nonexhaustive and consider whether that's the right 
> approach.
> None of these are showstoppers if left unaddressed, but I think the design 
> would be better if we fixed them.
>>      • Is the problem being addressed significant enough to warrant a change 
>> to Swift?
> Yes. I have no idea how Swift programs currently behave when a future 
> framework version adds a case, but I can't imagine they do anything good.
>>      • Does this proposal fit well with the feel and direction of Swift?
> Yes, with the exception of conflating `default` and `future`, which removes 
> useful correctness checks.
>>      • If you have used other languages or libraries with a similar feature, 
>> how do you feel that this proposal compares to those?
> I've experienced bugs in Objective-C caused by the compiler not knowing an 
> enum might have additional, unknown cases. Debugging them sucked.
>>      • How much effort did you put into your review? A glance, a quick 
>> reading, or an in-depth study?
> I've participated in multiple rounds of discussion on this topic, and read 
> the proposal top-to-bottom for this review.
> -- 
> Brent Royal-Gordon
> Architechies
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