<div dir="ltr"><br><br><div class="gmail_quote"><div dir="ltr">On Wed, Aug 9, 2017 at 4:59 PM Michael Ilseman 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;line-break:after-white-space"><div><br><blockquote type="cite"><div>On Aug 9, 2017, at 4:09 PM, Chris Lattner <<a href="mailto:clattner@nondot.org" target="_blank">clattner@nondot.org</a>> wrote:</div><br class="m_-5263372723672438488Apple-interchange-newline"><div><div>Hello Swift community,<br><br>The review of SE-0185 - "Synthesizing Equatable and Hashable conformance" begins now and runs through August 15, 2017. The proposal is available here:<br><a href="https://github.com/apple/swift-evolution/blob/master/proposals/0185-synthesize-equatable-hashable.md" target="_blank">https://github.com/apple/swift-evolution/blob/master/proposals/0185-synthesize-equatable-hashable.md</a><br><br><br>Reviews are an important part of the Swift evolution process. All reviews should be sent to the swift-evolution mailing list at:<br><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br><br>or, if you would like to keep your feedback private, directly to the review manager. When replying, please try to keep the proposal link at the top of the message:<br><br>What goes into a review?<br><br>The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift. When writing your review, here are some questions you might want to answer in your review:<br><br><span class="m_-5263372723672438488Apple-tab-span" style="white-space:pre-wrap"> </span>• What is your evaluation of the proposal?<br></div></div></blockquote><div><br></div><div><br></div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div>+1, it addresses one of the biggest pain points that cause people to reach for metaprogramming facilities.</div><div><br></div><div>I have one clarifying point and tweak. The proposal states:</div><div><span style="color:rgb(36,41,46);font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:16px;background-color:rgb(255,255,255)"><br></span></div><div><blockquote type="cite"><span style="color:rgb(36,41,46);font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:16px;background-color:rgb(255,255,255)">A </span><code style="box-sizing:border-box;font-family:SFMono-Regular,Consolas,"Liberation Mono",Menlo,Courier,monospace;font-size:13.600000381469727px;padding:0.2em 0px;margin:0px;background-color:rgba(27,31,35,0.0470588);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;color:rgb(36,41,46)">struct T: Hashable</code><span style="color:rgb(36,41,46);font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:16px;background-color:rgb(255,255,255)"> that satisfies the conditions above will receive a synthesized implementation of </span><code style="box-sizing:border-box;font-family:SFMono-Regular,Consolas,"Liberation Mono",Menlo,Courier,monospace;font-size:13.600000381469727px;padding:0.2em 0px;margin:0px;background-color:rgba(27,31,35,0.0470588);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;color:rgb(36,41,46)">var hashValue: Int { get }</code><span style="color:rgb(36,41,46);font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:16px;background-color:rgb(255,255,255)"> that uses an unspecified hash function</span><span style="box-sizing:border-box;line-height:0;vertical-align:baseline;color:rgb(36,41,46);font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol"">†</span><span style="color:rgb(36,41,46);font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:16px;background-color:rgb(255,255,255)"> to compute the hash value by incorporating the hash values of the fields as its terms, in definition order.</span></blockquote></div><div><br></div><div>This means that if the hash-combine operation is not commutative, then hashValues are not stable modulo member reordering. Was this scenario explicitly thought through? I don’t think it’s likely to be an issue in practice, but it's an artifact of implicit and ordered member-wise synthesis. It’s not obvious that declaration order is clearly the best order, e.g. declaration order is not necessarily memory-layout order. It might be better to leave the precise order unspecified, so long as it's guaranteed consistent during the execution of a program.</div></div></blockquote><div><br></div><div>The stdlib documentation for hashValue states "<i>Hash values are not guaranteed to be equal across different executions of your program. Do not save hash values to use during a future execution.</i>" So, we don't need to provide member-reordering stability since the only way to reorder members is to recompile the code (at this time).</div><div><br></div><div>That being said, we can certainly change the wording from "in definition order" to "in an arbitrary, implementation-defined order" without changing the behavior of the proposal and keeping the possibility open for future optimizations.</div><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;line-break:after-white-space"><div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><br><blockquote type="cite"><div><div><span class="m_-5263372723672438488Apple-tab-span" style="white-space:pre-wrap"> </span>• Is the problem being addressed significant enough to warrant a change to Swift?<br></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><div>Yes</div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><br><blockquote type="cite"><div><div><span class="m_-5263372723672438488Apple-tab-span" style="white-space:pre-wrap"> </span>• Does this proposal fit well with the feel and direction of Swift?<br></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><div>It's a reasonably scoped chunk of functionality for the biggest pain points, so yes.</div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><br><blockquote type="cite"><div><div><span class="m_-5263372723672438488Apple-tab-span" style="white-space:pre-wrap"> </span>• If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?<br></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><div>Haskell and Rust both have “deriving" constructs for generating these kinds of things, and that simplifies a lot of code.</div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><br><blockquote type="cite"><div><div><span class="m_-5263372723672438488Apple-tab-span" style="white-space:pre-wrap"> </span>• How much effort did you put into your review? A glance, a quick reading, or an in-depth study?<br><br></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><div>Thought about it a bit; read it quickly.</div><br><blockquote type="cite"><div><div></div></div></blockquote></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><blockquote type="cite"><div><div>More information about the Swift evolution process is available at:<br><a href="https://github.com/apple/swift-evolution/blob/master/process.md" target="_blank">https://github.com/apple/swift-evolution/blob/master/process.md</a><br><br><br>Thank you,<br><br>Chris Lattner<br>Review Manager<br><br><br>_______________________________________________<br>swift-evolution-announce mailing list<br><a href="mailto:swift-evolution-announce@swift.org" target="_blank">swift-evolution-announce@swift.org</a><br><a href="https://lists.swift.org/mailman/listinfo/swift-evolution-announce" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution-announce</a><br></div></div></blockquote></div></div>_______________________________________________<br>
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