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

Brent Royal-Gordon brent at architechies.com
Wed Dec 20 21:16:46 CST 2017

> On Dec 19, 2017, at 2:58 PM, Ted Kremenek via swift-evolution <swift-evolution at 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

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