<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=""><br class=""><div><blockquote type="cite" class=""><div class="">On Sep 12, 2016, at 8:14 AM, Paul Cantrell <<a href="mailto:cantrell@pobox.com" class="">cantrell@pobox.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; -webkit-line-break: after-white-space;" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Aug 11, 2016, at 9:39 PM, Douglas Gregor 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 dir="auto" class=""><div class=""><div style="direction: inherit;" class="">FWIW, I'm planning to write a complete proposal for conditional conformances and will start posting drafts once it is far enough along to be useful.</div></div></div></div></blockquote><div class=""><br class=""></div><div class="">I’m looking forward to that!</div><div class=""><br class=""></div><div class="">I’ll throw a potential use case for this your way, both one of the most obvious and one of the nastiest I’ve encountered, in case it helps refine the proposal and/or clarify its limits:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(102, 139, 73);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures;" class="">// No more runtime exceptions from things buried in</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(102, 139, 73);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> // dictionaries! Wouldn’t it be nice?</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">protocol</span><span style="font-variant-ligatures: no-common-ligatures" class=""> JSONRepresentable { }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(102, 139, 73);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">// So far so good:</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(88, 126, 168);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">String</span><span style="font-variant-ligatures: no-common-ligatures;" class="">: </span><span style="font-variant-ligatures: no-common-ligatures" class="">JSONRepresentable</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> { }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(102, 139, 73);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">// Hmm, extending protocols is out:</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(88, 126, 168);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">Integer</span><span style="font-variant-ligatures: no-common-ligatures;" class="">: </span><span style="font-variant-ligatures: no-common-ligatures" class="">JSONRepresentable</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> { }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(88, 126, 168);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">FloatingPoint</span><span style="font-variant-ligatures: no-common-ligatures;" class="">: </span><span style="font-variant-ligatures: no-common-ligatures" class="">JSONRepresentable</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> { }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(102, 139, 73);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">// I can imagine a universe where this works:</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(88, 126, 168);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">Optional</span><span style="font-variant-ligatures: no-common-ligatures;" class="">: </span><span style="font-variant-ligatures: no-common-ligatures" class="">JSONRepresentable</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">where</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Wrapped: JSONRepresentable { }</span></div></div></div></div></div></blockquote><div><br class=""></div><div>Fine so far.</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=""><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(102, 139, 73);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">// ...but this, oh dear:</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(88, 126, 168);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">Optional</span><span style="font-variant-ligatures: no-common-ligatures;" class="">.</span><span style="font-variant-ligatures: no-common-ligatures" class="">None</span><span style="font-variant-ligatures: no-common-ligatures;" class="">: </span><span style="font-variant-ligatures: no-common-ligatures" class="">JSONRepresentable</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> { }</span></div></div></div></div></div></blockquote><div><br class=""></div><div>This will be ill-formed; Optional.None isn’t a type, it’s part of the value, and allowing the conformance to be present or absent based on a run-time value is a massive complication:</div><div><br class=""></div><div><br class=""></div><div><font face="Menlo" class=""> func f<T: JSONRepresentable>(_: T) { }</font></div><div><font face="Menlo" class=""> func g(stringOpt: String?) { f(stringOpt) } // whether the conformance applies or not depends on the run-time value!</font></div><div><br class=""></div><div><br class=""></div><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=""><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(102, 139, 73);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">// Collection would be better but ... extending protocols again:</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(88, 126, 168);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures;" class="">: </span><span style="font-variant-ligatures: no-common-ligatures" class="">JSONRepresentable</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">where</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Element: JSONRepresentable { }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(88, 126, 168);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">Dictionary</span><span style="font-variant-ligatures: no-common-ligatures;" class="">: </span><span style="font-variant-ligatures: no-common-ligatures" class="">JSONRepresentable</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #323e7d" class="">where</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Key == String, Value: JSONRepresentable { }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div></div><div class="">I imagine this is all a bridge too far for a proposal at this time — and maybe a bridge too far for any conditional protocol conformance ever — but it seems like it needs addressing.</div></div></div></div></blockquote><div><br class=""></div><div>These last two should be fine.</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 class=""><div dir="auto" class=""><div class=""><div style="direction: inherit;" class="">It won't have support for protocols conforming to other protocols, though. </div></div></div></div></blockquote><br class=""></div>Curious. Why not? (I’ll happily wait for the proposal if you explain it there.)</div></div></blockquote><br class=""></div><div>The generics manifesto has a sketch of the reason; I will elaborate in the proposal.</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Doug</div><div><br class=""></div><br class=""></body></html>