<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Aug 16, 2016 at 6:59 PM, Karl <span dir="ltr">&lt;<a href="mailto:razielim@gmail.com" target="_blank">razielim@gmail.com</a>&gt;</span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><br><div><blockquote type="cite"><div><div class="h5"><div>On 17 Aug 2016, at 00:34, Charles Srstka via swift-evolution &lt;<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>&gt; wrote:</div><br></div></div><div><div style="word-wrap:break-word"><div><div class="h5"><blockquote type="cite" style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">On Aug 16, 2016, at 5:30 PM, Xiaodi Wu &lt;<a href="mailto:xiaodi.wu@gmail.com" target="_blank">xiaodi.wu@gmail.com</a>&gt; wrote:<br></blockquote><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><blockquote type="cite"><br><div><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;float:none;display:inline!important">On Tue, Aug 16, 2016 at 5:19 PM, Charles Srstka via swift-evolution<span> </span></span><span dir="ltr" style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">&lt;<a href="mailto:swift-evolution@swift.org" target="_blank">swift-<wbr>evolution@swift.org</a>&gt;</span><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;float:none;display:inline!important"><span> </span></span><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;float:none;display:inline!important">wrote:</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><blockquote class="gmail_quote" style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div style="word-wrap:break-word"><span><blockquote type="cite">On Aug 16, 2016, at 5:13 PM, David Sweeris &lt;<a href="mailto:davesweeris@mac.com" target="_blank">davesweeris@mac.com</a>&gt; wrote:<br></blockquote><div><blockquote type="cite"><br><div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">Any proposal that expands the power of generic programming gets an almost automatic +1 from me.</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><br></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">I can&#39;t think of any circumstances in which I wouldn&#39;t want to use &quot;:==&quot; instead of &quot;:&quot;. Are there any downsides to expanding &quot;:&quot; to mean what &quot;:==&quot; does?</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><br></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">Incidentally, I kinda thought things either already worked like this, or would work like this after generics were &quot;completed&quot;, but I can&#39;t tell you why I thought that.</div></div></blockquote></div><br></span><div>Me neither, but the last time I proposed that, people stated that there were some cases where this could not work. No concrete examples were given, but I assume it probably has something to do with associated type wackiness. :== seems like a workable compromise to me.</div></div></blockquote><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><br></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">If an existential of a protocol P doesn&#39;t conform to itself, what can you do inside the body of a generic function that has a generic constraint specified with `:==`? In other words, what would we know about what&#39;s in common between such an existential of a protocol and types that conform to the protocol?</div></div></blockquote></div><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">My proposal is that in such cases, using :== would lead to a compiler error.</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><br></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">Charles</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><br></div></div></div><span class=""><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;float:none;display:inline!important">______________________________<wbr>_________________</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;float:none;display:inline!important">swift-evolution mailing list</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><a href="mailto:swift-evolution@swift.org" style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">swift-evolution@swift.org</a><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">https://lists.swift.org/<wbr>mailman/listinfo/swift-<wbr>evolution</a></span></div></div></blockquote></div><br><div>I think the point is that existentials not conforming to their protocols is the underlying reason this doesn’t work already.</div><div><br></div><div>From the previous thread:</div><div><blockquote type="cite"><pre style="white-space:pre-wrap;background-color:rgb(255,255,255)">There are a couple of reasons this is the case.  IIRC in some cases it actually isn&#39;t possible for the existential to conform to the protocol in a sound way.  And even when it is possible, I believe it has been said that it is more difficult to implement than you might think.  Hopefully the situation will improve in the future but I&#39;m not aware of any specific plans at the moment.<br></pre></blockquote></div><div>It seems like a reasonably straightforward axiom. I would be interested to learn more about those edge-cases.</div></div></blockquote><div><br></div><div>Hmm, re-reading that makes me worry about this proposal at a practical level. IIUC, this is something that is pretty much desired across the board, and if we could have it for `:` it&#39;d be best.</div><div>But, it sounds like the reason `:` doesn&#39;t work that way isn&#39;t that the core team has a different opinion, but rather that it&#39;s very difficult to implement. And, IIUC, the situation is that no protocol existential currently conforms to itself, not that some do and some don&#39;t. The implementation work wouldn&#39;t be any easier if we called the operator `:==`...<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"><span class="HOEnZb"><font color="#888888"><div>Karl</div></font></span></div></blockquote></div><br></div></div>