<div dir="ltr">Interesting, it looks like only Joe Groff understands what .Protocol means and considers it important. And a couple others, maybe.<div><br></div><div>Copying my metatypes explanation link:</div><div><div>For everyone who doesn't understand metatypes and our problem, please see</div><div><a href="https://gist.github.com/Anton3/25a66751812f14f76cacc5e382339522">https://gist.github.com/Anton3/25a66751812f14f76cacc5e382339522</a></div></div></div><div class="gmail_extra"><br><div class="gmail_quote">2016-07-22 16:07 GMT+03:00 Adrian Zubarev via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span>:<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"><div><p>Hello everyone,</p>
<p>recently I was pinged on twitter to a conversation between Jacob Bandes-Storch, Joe Groff and Slava Pestov.</p>
<p>Here is the conversation covering a few more facts about the mysterious <code>.Protocol</code>:</p>
<blockquote>
<p><strong>[Jacob Bandes-Storch]</strong> Was hoping to just define <code>type<T>(of x: T) -> T.Type</code> and move <code>impl</code> of <code>emitMetatypeOfValue()</code> there.</p>
<p><strong>[Joe Groff]</strong> Yeah, that won’t do the right thing for existentials compared to <code>x.dynamicType</code>.</p>
<p><strong>[Jacob Bandes-Storch]</strong> When you say “do the right thing” you mean the return type? Shouldn’t <code>type(of x: Proto) -> Proto.Type</code>?</p>
<p><strong>[Slava Pestov]</strong> Right, but if <code>T := P</code>, then <code>T.Type</code> is <code>P.Protocol</code>, not <code>P.Type</code></p>
<p><strong>[Jacob Bandes-Storch]</strong> Mind bending… Suppose <code>T : P</code>, <code>x : P = T()</code>, then <code>type(of: x)</code> returns <code>T</code>, and <code>T: P.Type</code>. What am I missing?</p>
<p><strong>[Joe Groff]</strong> <code>P.Type</code> is really <code>Any<P.Type></code>, and <code>P.Protocol</code> is really <code>(Any<P>).Type</code>. If <code>T == Any<P></code>, <code>T.Type == latter</code></p>
<p><strong>[Joe Groff]</strong> <code>dynamicType</code> is different for existentials: open existential, take opened metatype, erase to <code>e.metatype</code></p>
<p><strong>[Jacob Bandes-Storch]</strong> Now I’m confused by current behavior: <a href="https://gist.github.com/jtbandes/7624fef93eb23010c032b2d1fd3674be#file-dynamictype-swift-L12" target="_blank">GIST</a> … isn’t dynamicType’s job to return <code>T.self</code>?</p>
<p><strong>[Joe Groff]</strong> invokes `foo() on the opened type, then re-closes an existential around the result if Self is involved</p>
<p><strong>[Jacob Bandes-Storch]</strong> But this does what I’d expect: <a href="https://gist.github.com/jtbandes/7624fef93eb23010c032b2d1fd3674be#file-dynamictype-swift-L23" target="_blank">GIST</a></p>
<p><strong>[Joe Groff]</strong> Right, member lookup also implicitly opens an existential.</p>
<p><strong>[Jacob Bandes-Storch]</strong> Point being that there’s no way to do this for generic params?</p>
<p><strong>[Joe Groff]</strong> Not in argument position. Only ‘self’ gets this magic behavior.</p>
<p><strong>[Jacob Bandes-Storch]</strong> Is that a bug or feature? Seems to me dynamicType should always be able to get the actual runtime type.</p>
<p><strong>[Joe Groff]</strong> Consider this situation:</p>
<pre><code>func foo<T>(x:T) -> (T.Type, T.Type) {
return (T.self, x.dynamicType)
}
</code></pre>
<p><strong>[Jacob Bandes-Storch]</strong> Still struggling to see what’s wrong with returning “real” type. What can be subst. for `T that breaks it?</p>
<p><strong>[Joe Groff]</strong> Consider <code>T == P</code> with <code>x.dynamicType</code> trying to produce the existential dynamic type.</p>
<p><strong>[Jacob Bandes-Storch]</strong> I think I’m mostly confused because <code>P.Type</code> vs. <code>P.Protocol</code> is really non-obvious. Has renaming been discussed?</p>
<p><strong>[Joe Groff]</strong> The gotcha mainly comes up if you try to replicate the builtin behavior. Most people don’t try to do that.</p>
<p><strong>[Jacob Bandes-Storch]</strong> Would it make sense to disallow <code>x.dynamicType</code> in such cases? Gotcha would be less confusing if forced to say `T.self.</p>
</blockquote>
<p></p></div><div><span class=""><div style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto"><br></div> <br> <div><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br></span><div><div class="h5"><p>Am 22. Juli 2016 um 14:47:08, Vladimir.S via swift-evolution (<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>) schrieb:</p> <blockquote type="cite"><span><div><div></div><div>Thank you for the article. Very informative.<br><br>So, I believe core team can answer the question : "..why P.Protocol is <br>needed" and if we can remove it without problems.<br><br><br>Small fix - A&B should have ': HasStatic' and closing bracket here:<br><br>protocol HasStatic { static func staticMethod() }<br>struct A { static func staticMethod() -> String { return "A" }<br>struct B { static func staticMethod() -> String { return "B" }<br><br><br>On 22.07.2016 14:40, Anton Zhilin via swift-evolution wrote:<br>> For everyone who doesn't understand metatypes and our problem, please see:<br>><br>> <a href="https://gist.github.com/Anton3/25a66751812f14f76cacc5e382339522" target="_blank">https://gist.github.com/Anton3/25a66751812f14f76cacc5e382339522</a><br>><br>> Read it from beginning to ending and ask if you don't understand something!<br>><br>><br>> _______________________________________________<br>> swift-evolution mailing list<br>> <a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a><br>> <a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br>><br>_______________________________________________<br>swift-evolution mailing list<br><a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a><br><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br></div></div></span></blockquote></div></div></div><div><p></p></div></div><br>_______________________________________________<br>
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<br></blockquote></div><br></div>