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
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