Hmm, with these complications, I regret suggesting that this could be extended to classes so easily. Definitely some interesting solutions, but my inclination as well would be to leave classes out entirely for the moment and let it percolate for Swift 4.1+. After all, it's an additive convenience.<br><br>Any thought as to including tuples though?<br><div class="gmail_quote"><div dir="ltr">On Fri, May 5, 2017 at 13:51 Matthew Johnson via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><blockquote type="cite"><div>On May 5, 2017, at 1:33 PM, Tony Allevato <<a href="mailto:tony.allevato@gmail.com" target="_blank">tony.allevato@gmail.com</a>> wrote:</div><br class="m_7959734635982325664Apple-interchange-newline"><div><div dir="ltr"><br><br><div class="gmail_quote"><div dir="ltr">On Fri, May 5, 2017 at 11:07 AM Matthew Johnson <<a href="mailto:matthew@anandabits.com" target="_blank">matthew@anandabits.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><blockquote type="cite"><div>On May 5, 2017, at 10:45 AM, Tony Allevato via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br class="m_7959734635982325664m_4844033761213030745Apple-interchange-newline"><div><div dir="ltr">Thanks for your feedback, everybody!</div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div>Thanks for continuing to drive this forward!</div><div></div></div><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div dir="ltr"><div><br></div><div>I've updated the <a href="https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad" target="_blank">gist</a> to reflect what seems to be a consensus here:</div><div><br></div><div>* Derived conformances are now opt-in (this makes the recursive case *much* cleaner, and the complexity involved in that section has been completely removed)</div></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div><div>Can the opt-in conformance be declared in an extension? If so, can the extension be in a different module than the original declaration? If so, do you intend any restrictions, such as requiring all members of the type declared in a different module to be public? My initial thought is that this should be possible as long as all members are visible.</div></div></div></blockquote><div><br></div><div>Declaring the conformance in an extension in the same module should definitely be allowed; I believe this would currently be the only way to support conditional conformances (such as the `Optional: Hashable where Wrapped: Hashable` example in the updated draft), without requiring deeper syntactic changes.</div><div><br></div><div>I'm less sure about conformances being added in other modules, but I'm inclined to agree with your assessment. I could see two ways of interpreting it:</div><div><br></div><div>* E/H can only be derived in an extension in an external module if all the members are accessible (and the other conditions are met).</div><div>* E/H can be derived in an extension in an external module using only the subset of accessible members (if the other conditions are met).</div><div><br></div><div>These are subtly different. The argument for the first would be "if you want to add E/H to a type in a different module, you must *consciously* decide which members you want to use in those computations". The argument for the second would be "you can already make a type in a different module conform to E/H and you'd be restricted to the accessible members there, so let's make that path easier for users too."</div><div><br></div><div>The first case is probably the safer choice. I'm not sure about the implementation difficulty of each.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div dir="ltr"><div>* Classes are supported now as well</div><div><br></div><div>Please take a look at the updated version and let me know if there are any concerns! If folks like it, I'll prepare a pull request.</div></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div><div>Will the synthesis for classes dispatch through a non-final method which is expected to be overridden by subclasses? You don’t explicitly state this but it seems implied. If so, what if the subclass requires a custom implementation? This would require the signature of the non-final method to be part of the synthesis contract.</div></div></div></blockquote><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><div><br></div><div>Supporting non-final classes introduces enough complexity (especially when multiple modules are involved). I would hate to see it get sidetracked in discussions regarding non-final classes and miss the Swift 4 window because of that. Given the limited time left for Swift 4 it might be better to keep the initial proposal simpler and consider a followup in the Swift 5 timeframe to build on the initial proposal.</div></div></div></blockquote><div><br></div><div>For ==, the operator must already be "class final" or "static" regardless of this proposal, and it can't be "overridden" as such in subclasses because the arguments would be different (lhs and rhs would be the subclass, not the superclass). So the compiler should be able to generate the correct implementation for subclasses in all cases, right?</div></div></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div>This won’t work because Equatable has a `Self` requirement so the `==` defined by the initial conforming class would be called. In order to support non-final classes you would need to have that dispatch through something like an `isEqual` method which *can* be overridden.</div><div></div></div><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_quote"><div><br></div><div>For hashValue, I think the possibilities are:</div><div><br></div><div>* Sub is a subclass of Super. Super conforms to Hashable and implements non-final hashValue. The compiler can derive it for Sub and call super.hashValue in its implementation.</div></div></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div><div>Yes, this makes sense. The primary difficulty with Hashable is that it refines Equatable. Refining a non-final implementation of `hashValue` is relatively straightforward.</div></div></div><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_quote"><div>* Sub is a subclass of Super. Super conforms to Hashable and implements a final hashValue. The compiler cannot derive one for Super and would silently not do so.</div></div></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div><div>Do you mean “the compiler cannot derive one for Sub”?</div></div></div><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_quote"><div>* Sub is a subclass of Super. Super does not conform to Hashable, but Sub asks to derive it. This can either (1) not be allowed, telling the user that they need to write it manually in this case, or (2) be allowed and use all accessible members to compute the hashValue (including those from the superclass).</div><div><br></div><div>What do Encodable/Decodable do in these situations? It seems similar solutions there would apply here.</div></div></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div><div>That’s a good question. I don’t recall whether this was addressed explicitly or not.</div></div></div><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_quote"><div><br></div><div>But after writing this all out, I'm inclined to agree that I'd rather see structs/enums make it into Swift 4 even if it meant pushing classes to Swift 4+x.</div></div></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div><div>That is reasonable. </div><div><br></div><div>On the other hand, I think you could come up with straightforward semantics for synthesizing conformance for final classes as well. Final classes with no superclass should be straightforward. Final classes that do have a superclass would be similarly straightforward if we decide to allow this as described in option (2) above regarding hashValue. </div><div><br></div><div>I’m on the fence on this - if we can include final classes using option (2) without jeopardizing getting this in for Swift 4 I would support that. If it’s going to put support for value types in Swift 4 at risk then I would not.</div></div></div><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_quote"><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div dir="ltr"><div><br></div><div><br><div class="gmail_quote"><div dir="ltr">On Fri, May 5, 2017 at 8:16 AM Nevin Brackett-Rozinsky via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Fri, May 5, 2017 at 1:47 AM, Xiaodi Wu via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br></div></div></div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><span class="m_7959734635982325664m_4844033761213030745m_1203078577276602968gmail-">On Fri, May 5, 2017 at 12:41 AM, Brent Royal-Gordon <span dir="ltr"><<a href="mailto:brent@architechies.com" target="_blank">brent@architechies.com</a>></span> wrote:</span><div class="gmail_extra"><div class="gmail_quote"><span class="m_7959734635982325664m_4844033761213030745m_1203078577276602968gmail-"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word"><div>I would think only final classes could participate in this, since a subclassable class would need to allow subclasses to override equality, and you can't override a static `==` operator method.</div></div></blockquote><div><br></div></span><div>I work so rarely with classes that I'm embarrassed to have to ask this question: can classes not satisfy Equatable with a `public class func ==`?</div></div></div></div></blockquote></div></div></div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"></div></blockquote><div><br></div><div>Currently:</div><div><br></div><div><div><font face="monospace, monospace">class C: Equatable {</font></div><div><font face="monospace, monospace"> class func == (lhs: C, rhs: C) -> Bool {</font></div><div><font face="monospace, monospace"> return lhs === rhs</font></div><div><font face="monospace, monospace"> }</font></div><div><font face="monospace, monospace">}</font></div></div><div><br></div><div>Yields an error, “Operator '==' declared in non-final class 'C' must be 'final'”.</div></div></div></div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><br></div><div>Nevin</div></div></div></div>
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