<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 Sep 8, 2017, at 12:46 AM, Haravikk 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; -webkit-line-break: after-white-space;" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 7 Sep 2017, at 22:02, Itai Ferber <<a href="mailto:iferber@apple.com" class="">iferber@apple.com</a>> wrote:</div><div class=""><div style="font-family: Helvetica; font-size: 12px; 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-stroke-width: 0px;" class=""><br class=""><div class=""><pre class=" language-swift" style="-webkit-print-color-adjust: exact; margin-top: 0.5em; margin-bottom: 0.5em; background-color: rgb(245, 242, 240); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 1.5; overflow: auto; padding: 1em; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; text-shadow: white 0px 1px; font-family: 'SF Mono', Menlo, Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; white-space: pre-wrap; word-break: normal; word-wrap: normal; tab-size: 4; -webkit-hyphens: none;"><code class=" language-swift" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-image: none; text-shadow: white 0px 1px; font-family: 'SF Mono', Menlo, Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; word-spacing: normal; word-break: normal; word-wrap: normal; line-height: 1.5; tab-size: 4; -webkit-hyphens: none;">protocol <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Fooable</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Equatable</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{ // Equatable is just a simple example</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">var</span> myFoo<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Int</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">get</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">extension</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Fooable</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">static</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">func</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">==</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span><span class="token number" style="-webkit-print-color-adjust: exact; color: rgb(153, 0, 85);">_</span> lhs<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">Self</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> <span class="token number" style="-webkit-print-color-adjust: exact; color: rgb(153, 0, 85);">_</span> rhs<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">Self</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">-</span><span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">></span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Bool</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">return</span> lhs<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span>myFoo <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">==</span> rhs<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">.</span>myFoo
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">struct</span> X <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Fooable</span> <span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">{</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">let</span> myFoo<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">Int</span>
<span class="token keyword" style="-webkit-print-color-adjust: exact; color: rgb(0, 119, 170);">let</span> myName<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token builtin" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">String</span>
<span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: rgb(112, 128, 144);">// Whoops, forgot to give an implementation of ==</span>
<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">}</span>
<span class="token function" style="-webkit-print-color-adjust: exact; color: rgb(221, 74, 104);">print</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span><span class="token function" style="-webkit-print-color-adjust: exact; color: rgb(221, 74, 104);">X</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>myFoo<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token number" style="-webkit-print-color-adjust: exact; color: rgb(153, 0, 85);">42</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> myName<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token string" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">"Alice"</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token operator" style="-webkit-print-color-adjust: exact; color: rgb(166, 127, 89); background-color: rgba(255, 255, 255, 0.498039);">==</span> <span class="token function" style="-webkit-print-color-adjust: exact; color: rgb(221, 74, 104);">X</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">(</span>myFoo<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token number" style="-webkit-print-color-adjust: exact; color: rgb(153, 0, 85);">42</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">,</span> myName<span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">:</span> <span class="token string" style="-webkit-print-color-adjust: exact; color: rgb(102, 153, 0);">"Bob"</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span><span class="token punctuation" style="-webkit-print-color-adjust: exact; color: rgb(153, 153, 153);">)</span> <span class="token comment" spellcheck="true" style="-webkit-print-color-adjust: exact; color: rgb(112, 128, 144);">// true</span></code></pre><div class="">This property is<span class="Apple-converted-space"> </span><i class="">necessary</i>, but not<span class="Apple-converted-space"> </span><i class="">sufficient</i><span class="Apple-converted-space"> </span>to provide a correct implementation. A default implementation might be able to<span class="Apple-converted-space"> </span><i class="">assume</i> something about the types that it defines, but it does not necessarily know enough.</div></div></div></div></blockquote><div class=""><br class=""></div><div class="">Sorry but that's a bit of a contrived example; in this case the protocol should <b class="">not</b> implement the equality operator if more information may be required to define equality. It should only be implemented if the protocol is absolutely clear that .myFoo is the only part of a Fooable that can or should be compared as equatable, e.g- if a Fooable is a database record and .myFoo is a primary key, the data could differ but it would still be a reference to the same record.</div><div class=""><br class=""></div><div class="">To be clear, I'm not arguing that someone can't create a regular default implementation that also makes flawed assumptions, but that synthesised/reflective implementations <b class="">by their very nature have to</b>, as they cannot under every circumstance guarantee correctness when using parts of a concrete type that they know nothing about.</div></div></div></div></blockquote><div>You can’t argue this both ways:</div><div><ul class="MailOutline"><li class="">If you’re arguing this on principle, that in order for synthesized implementations to be correct, they <i class="">must</i> be able to — <i class="">under every circumstance</i> — guarantee correctness, then you have to apply the same reasoning to default protocol implementations. Given a default protocol implementation, it is possible to come up with a (no matter how contrived) case where the default implementation is wrong. Since you’re arguing this <i class="">on principle</i>, you cannot reject contrived examples.</li><li class="">If you are arguing this <i class="">in practice</i>, then you’re going to have to back up your argument with evidence that synthesized examples are more often wrong than default implementations. You can’t declare that synthesized implementations are <i class="">by nature</i> incorrect but allow default implementations to slide because <i class="">in practice</i>, many implementations are allowable. There’s a reason why synthesis passed code review and was accepted: in the majority of cases, synthesis was deemed to be beneficial, and would provide correct behavior. If you are willing to say that yes, sometimes default implementations are wrong but overall they’re correct, you’re going to have to provide hard evidence to back up the opposite case for synthesized implementations. You stated in a previous email that "<span style="font-family: SFHello-Regular;" class="">A synthesised/reflective implementation however may return a result that is simply incorrect, because it is based on assumptions made by the protocol developer, with no input from the developer of the concrete type. In this case the developer must override it in to provide </span><b class="" style="font-family: SFHello-Regular;">correct</b><font face="SFHello-Regular" class=""> behaviour." — if you can back this up with evidence (say, taking a survey of a large number of model types and see if in the majority of cases synthesized implementation would be incorrect) to provide a compelling argument, then this is something that we should in that case reconsider.</font></li></ul></div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><blockquote type="cite" class=""><div style="font-family: Helvetica; font-size: 12px; 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-stroke-width: 0px;" class=""><blockquote type="cite" class=""><span style="font-family: SFHello-Regular;" class="">Reflective/synthesised default implementations</span> <b class="" style="font-family: SFHello-Regular;">must by their very nature</b><span style="font-family: SFHello-Regular;" class=""> make assumptions about a concrete type that are</span> <b class="" style="font-family: SFHello-Regular;">not</b> <span style="font-family: SFHello-Regular;" class="">cannot be guaranteed to be correct. The properties and methods they may end up interacting with</span><b class="" style="font-family: SFHello-Regular;">may have nothing at all to do with the protocol</b><span style="font-family: SFHello-Regular;" class="">. Equatable remains by far the simplest example; just because a developer has used equatable properties does</span> <b class="" style="font-family: SFHello-Regular;">not</b><span style="font-family: SFHello-Regular;" class=""> guarantee that all of them should be compared during a check for equality.</span><br class=""></blockquote><div class="">In the same way that you might consider synthesized conformances to overreach into a type and touch things which are not related to a protocol, default implementations can be considered<span class="Apple-converted-space"> </span><i class="">underreach</i> in that they don’t know anything about properties which are necessary for providing a correct implementation.</div></div></blockquote><div class=""><br class=""></div><div class="">If more information is necessary to provide a correct implementation, then a default implementation shouldn't be provided. This is what unimplemented properties and methods are for; either getting the developer to provide the missing information, or getting them to implement the correct behaviour.</div></div></div></div></blockquote><div>I agree, but you can’t selectively argue this.</div><br class=""><blockquote type="cite" class=""><div 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=""></div></blockquote></div><br class=""></body></html>