<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On 20. Dec 2017, at 19:54, Jordan Rose <<a href="mailto:jordan_rose@apple.com" class="">jordan_rose@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html; charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On Dec 20, 2017, at 05:36, Karl Wagner 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=""><meta http-equiv="Content-Type" content="text/html; charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On 19. Dec 2017, at 23:58, Ted Kremenek 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=""><meta http-equiv="Content-Type" content="text/html; charset=us-ascii" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><p style="-webkit-print-color-adjust: exact; margin-right: 0px; margin-bottom: 15px; margin-left: 0px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255); margin-top: 0px !important;" class="">The review of "SE 0192 - Non-Exhaustive Enums" begins now and runs through <strong style="-webkit-print-color-adjust: exact;" class="">January 3, 2018</strong>.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">The proposal is available here:</p><blockquote style="-webkit-print-color-adjust: exact; margin: 15px 0px; border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221); padding: 0px 15px; color: rgb(119, 119, 119); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class=""><div style="-webkit-print-color-adjust: exact; margin: 0px;" class=""><a href="https://github.com/apple/swift-evolution/blob/master/proposals/0192-non-exhaustive-enums.md" class="">https://github.com/apple/swift-evolution/blob/master/proposals/0192-non-exhaustive-enums.md</a></div></blockquote></div></div></blockquote><div class="">+1, it needs to happen (and ASAP, since it _will_ introduce source-breaking changes one way or the other).</div><div class=""><br class=""></div><div class="">I think non-exhaustive is the correct default. However, does this not mean that, by default, enums will be boxed because the receiver doesn’t know their potential size?</div></div></div></div></blockquote><div class=""><br class=""></div><div class="">It's not always boxing, but yes, there will be more indirection if the <i class="">compiler</i> can't see the contents of the enum. (More on that below.)</div><div class=""><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class=""><div class="">That would mean that the best transition path for multi-module Apps would be to make your enums @exhaustive, rather than adding “default” statements (which is unfortunate, because I imagine when this change hits, the way you’ll notice will be complaints about missing “default” statements).</div></div></div></div></blockquote><div class=""><br class=""></div><div class="">Yep, that's going to be the recommendation. The current minimal-for-review implementation does not do this but I'd like to figure out how to improve that; at the very least it might be a sensible thing to do in the migrator.</div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class=""><div class=""><br class=""></div><div class="">I do have some thoughts about how we could ease the transition (for this and other resilience-related changes), but it’s best to leave that to a separate discussion.</div><div class=""><br class=""></div><div class="">The one thing I’m still not overly fond of is the name - I would like us to keep the set of resilience/optimisation related keywords to a minimum. “exhaustive” for enums feels an awful lot like “fixed_contents” for structs - couldn’t we come up with a single name which could be used for both? I don’t think anybody’s going to want to use “exhaustive” for structs.</div></div></div></div></blockquote><br class=""></div><div class="">The core team was very focused on this too, but I contend that "exhaustive" is not about optimization and really isn't even about "resilience" (i.e. the ability to evolve a library's API while preserving binary compatibility). It's a semantic feature of an enum, much like 'open' or 'final' is for classes, and it affects what a client can or can't do with an enum. For libaries compiled from source, it won't affect performance at all—the <i class="">compiler</i> still knows the full set of cases in the <i class="">current</i> version of the library even if the programmer is forced to consider future versions.</div><div class=""><br class=""></div><div class="">I'm working on the fixed-contents proposal now, though it won't be ready for a while, and the same thing applies there: for structs compiled from source, the compiler can still do all the same optimizations. It's only when the library has binary compatibility concerns that we need to use extra indirection, and then "fixed-contents" becomes important. (As currently designed, it doesn't affect what clients can do with the struct at all.) This means that I don't expect a "normal" package author to write "fixed-contents" at all (however it ends up being spelled), whereas "exhaustive" is a fairly normal thing to consider whenever you make an enum public.</div><div class=""><br class=""></div><div class="">I hope that convinces you that "fixed-contents" and "exhaustive" don't need to have the same name. I don't think anyone loves the <i class="">particular</i> name "exhaustive", but as you see in the "Alternatives considered" we didn't manage to come up with anything significantly better. If reviewers all prefer something else we'd consider changing it.</div><div class=""><br class=""></div><div class="">Thanks for responding!</div><div class="">Jordan</div><br class=""></div></div></blockquote><br class=""></div><div>When you say “libraries compiled from source”, what do you mean?</div><div><br class=""></div><div>As for whether its a resilience feature: actually it is completely a resilience feature. The effects on switching are only side-effects; really what “exhaustive” or “nonexhaustive” are saying is literally that cases may be added later. Even if we added private cases, you wouldn’t need to mark those enums as specially exhaustive or not; that would be implied. It’s an accommodation for things which don’t exist yet, so really, it is all about resilience IMO.</div><div><br class=""></div><div>Anyway, as I see it, library authors in general ought to be happy about this:</div><div>+ Their libraries become safer by default, so they can make changes in the future without having to worry about breakage</div><div>+ It doesn’t affect your code inside of a module, so it only affects types they already explicitly marked “public”</div><div><br class=""></div><div>The only people who lose are multi-module App developers, because they are “library authors” who don’t need to care about evolution, and now need to add attributes to things they wouldn’t have to before, or suffer language and performance penalties. Their libraries become less reusable and not resilient-by-default.</div><div><br class=""></div><div>For example, I have an App for which I wrote a cross-platform model framework in Swift. When I compile it as a framework inside my App, it is bundled there forever. However, I use the same code to build libraries for Linux, which I would like to ship in binary form to 3rd-parties. Am I supposed to litter my code with annotations to mark those types as final, just to make the App fast and convenient to code? What happens when I need to fix a bug and distribute an updated copy, this means the 3rd-parties need to recompile (which they won’t do…).</div><div><br class=""></div><div>Typically, for such a problem, I would recommend using a static library instead. But we don’t have those, and anyway they’re not always the best thing these days. So that’s why I started a new thread about creating a “@static” import, so App developers can go back to all the conveniences they had before.</div><div><br class=""></div><div>- Karl</div><div><br class=""></div><div><br class=""></div><div><br class=""></div><br class=""></body></html>