<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="">I really want synthesized equality (etc.), but having seen the previous discussions on this and related topics I am not sure that directly-deriving the `==` function is the right approach. </div><div class=""><br class=""></div><div class="">The main reason I say this is that although this works great for the easy case — all fields equatable, do the obvious thing! — sooner or later people will want to customize it, which would ideally allow someone to say “do the obvious thing for a,b,c but let me handle d and e”, e.g. still get synthesis for the easy parts…but an approach that directly-synthesizes `==` for a type seems like it’ll be difficult to expand to support such customization.</div><div class=""><br class=""></div><div class="">Suppose instead that we had a “magic function” `T#memberwiseEqual(_:_:)` we could invoke like so:</div><div class=""> </div><div class=""><div class=""> // the details are *very* bikesheddable here:</div><div class=""><div class=""> func ==(lhs: Foo, rhs: Foo) -> Bool {</div><div class=""> return Foo#memberwiseEqual(lhs,rhs) // `#` b/c of "compiler magic”</div><div class=""> // ^ compiler error iff any of `Foo`’s members aren’t Equatable</div><div class=""> }</div></div></div><div class=""><br class=""></div><div class="">…which’d expand to the expected “lhs.a == rhs.a && lhs.b == rhs.b && …”. </div><div class=""><br class=""></div><div class="">For trivial equatable synthesis this isn’t as nice as a full automatic derivation, but it seems like an *approach* that’d be much-better positioned for future expansion and enhancement, e.g.:</div><div class=""><br class=""></div><div class=""> extension Foo: Equatable {</div><div class=""><br class=""></div><div class=""> // mock syntax; probably too ambiguous for actual use but i think the idea is clear:</div><div class=""> private static func boringComponentsEqual(lhs: Foo, _ rhs: Foo) -> Bool {</div><div class=""> return Foo(boring,boring2,boringIII)#memberwiseEqual(lhs,rhs)</div><div class=""> }</div><div class=""><br class=""></div><div class=""> }</div><div class=""><br class=""></div><div class=""> func ==(lhs: Foo, rhs: Foo) -> Bool {</div><div class=""> return Foo.boringComponentsEqual(lhs,rhs) && // non-trivial equality logic here</div><div class=""> }</div><div class=""><br class=""></div><div class="">…as opposed to trying to retrofit various “customizations" onto a system that directly synthesizes `==` (without exposing any “internals", so to speak).</div><div class=""><br class=""></div><div class="">You can easily imagine a similar `#casewiseEqual` for enums (it seems likely to be trickier, but not impossible), and again a #memberwiseHash for hashing, and so on.</div><div class=""><br class=""></div><div class="">I think you can summarize the above as “all-in-one derivation is appealing, but I think pragmatically and looking-ahead it’s a better move to expose the 'synthesis mechanism’ itself (leaving it the user to do the 'final assembly’)”.</div><br class=""><div><blockquote type="cite" class=""><div class="">On May 25, 2016, at 1:28 PM, Tony Allevato 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=""><div dir="ltr" class="">I was inspired to put together a draft proposal based on an older discussion in the Universal Equality, Hashability, and Comparability thread <<a href="http://thread.gmane.org/gmane.comp.lang.swift.evolution/8919/" class="">http://thread.gmane.org/gmane.comp.lang.swift.evolution/8919/</a>> that recently got necromanced (thanks Mark Sands!).<div class=""><br class=""></div><div class="">I'm guessing that this would be a significant enough change that it's not possible for the Swift 3 timeline, but it's something that would benefit enough people that I want to make sure the discussion stays alive. If there are enough good feelings about it, I'll move it from my gist into an actual proposal PR.</div></div></div></blockquote></div><br class=""></body></html>