<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">+1 on @frozen and John’s reasoning.</div><div class=""><br class=""></div><br class=""><div><blockquote type="cite" class=""><div class="">On 21. Dec 2017, at 04:32, John McCall via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><blockquote type="cite" style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class=""><br class="Apple-interchange-newline">On Dec 20, 2017, at 10:16 PM, Brent Royal-Gordon via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""><blockquote type="cite" class="">On Dec 19, 2017, at 2:58 PM, Ted Kremenek via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>• What is your evaluation of the proposal?<br class=""></blockquote><br class="">I am pleased with the broad strokes of this design. I have quibbles with three areas:<br class=""><br class="">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.<br class=""></blockquote><br style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">I rather like @frozen. We could use that across language features, so that we don't end up with a keyword per kind of declaration.</span><br style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">John.</span><br style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><blockquote type="cite" style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class=""><br class="">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.<br class=""><br class="">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.<br class=""><br class="">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.<br class=""><br class="">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.<br class=""><br class="">None of these are showstoppers if left unaddressed, but I think the design would be better if we fixed them.<br class=""><br class=""><blockquote type="cite" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>• Is the problem being addressed significant enough to warrant a change to Swift?<br class=""></blockquote><br class="">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.<br class=""><br class=""><blockquote type="cite" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>• Does this proposal fit well with the feel and direction of Swift?<br class=""></blockquote><br class="">Yes, with the exception of conflating `default` and `future`, which removes useful correctness checks.<br class=""><br class=""><blockquote type="cite" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>• If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?<br class=""></blockquote><br class="">I've experienced bugs in Objective-C caused by the compiler not knowing an enum might have additional, unknown cases. Debugging them sucked.<br class=""><br class=""><blockquote type="cite" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>• How much effort did you put into your review? A glance, a quick reading, or an in-depth study?<br class=""></blockquote><br class="">I've participated in multiple rounds of discussion on this topic, and read the proposal top-to-bottom for this review.<br class=""><br class="">--<span class="Apple-converted-space"> </span><br class="">Brent Royal-Gordon<br class="">Architechies<br class=""><br class="">_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class="">https://lists.swift.org/mailman/listinfo/swift-evolution<br class=""></blockquote><br style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">_______________________________________________</span><br style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">swift-evolution mailing list</span><br style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><a href="mailto:swift-evolution@swift.org" style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">swift-evolution@swift.org</a><br style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" style="font-family: Helvetica; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a></div></blockquote></div><br class=""></body></html>