<html><head><style>
body {
font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
padding:1em;
margin:auto;
background:#fefefe;
}
h1, h2, h3, h4, h5, h6 {
font-weight: bold;
}
h1 {
color: #000000;
font-size: 28pt;
}
h2 {
border-bottom: 1px solid #CCCCCC;
color: #000000;
font-size: 24px;
}
h3 {
font-size: 18px;
}
h4 {
font-size: 16px;
}
h5 {
font-size: 14px;
}
h6 {
color: #777777;
background-color: inherit;
font-size: 14px;
}
hr {
height: 0.2em;
border: 0;
color: #CCCCCC;
background-color: #CCCCCC;
display: inherit;
}
p, blockquote, ul, ol, dl, li, table, pre {
margin: 15px 0;
}
a, a:visited {
color: #4183C4;
background-color: inherit;
text-decoration: none;
}
#message {
border-radius: 6px;
border: 1px solid #ccc;
display:block;
width:100%;
height:60px;
margin:6px 0px;
}
button, #ws {
font-size: 12 pt;
padding: 4px 6px;
border-radius: 5px;
border: 1px solid #bbb;
background-color: #eee;
}
code, pre, #ws, #message {
font-family: Monaco;
font-size: 10pt;
border-radius: 3px;
background-color: #F8F8F8;
color: inherit;
}
code {
border: 1px solid #EAEAEA;
margin: 0 2px;
padding: 0 5px;
}
pre {
border: 1px solid #CCCCCC;
overflow: auto;
padding: 4px 8px;
}
pre > code {
border: 0;
margin: 0;
padding: 0;
}
#ws { background-color: #f8f8f8; }
.bloop_markdown table {
border-collapse: collapse;
font-family: Helvetica, arial, freesans, clean, sans-serif;
color: rgb(51, 51, 51);
font-size: 15px; line-height: 25px;
padding: 0; }
.bloop_markdown table tr {
border-top: 1px solid #cccccc;
background-color: white;
margin: 0;
padding: 0; }
.bloop_markdown table tr:nth-child(2n) {
background-color: #f8f8f8; }
.bloop_markdown table tr th {
font-weight: bold;
border: 1px solid #cccccc;
margin: 0;
padding: 6px 13px; }
.bloop_markdown table tr td {
border: 1px solid #cccccc;
margin: 0;
padding: 6px 13px; }
.bloop_markdown table tr th :first-child, table tr td :first-child {
margin-top: 0; }
.bloop_markdown table tr th :last-child, table tr td :last-child {
margin-bottom: 0; }
.bloop_markdown blockquote{
border-left: 4px solid #dddddd;
padding: 0 15px;
color: #777777; }
blockquote > :first-child {
margin-top: 0; }
blockquote > :last-child {
margin-bottom: 0; }
code, pre, #ws, #message {
word-break: normal;
word-wrap: normal;
}
hr {
display: inherit;
}
.bloop_markdown :first-child {
-webkit-margin-before: 0;
}
code, pre, #ws, #message {
font-family: Menlo, Consolas, Liberation Mono, Courier, monospace;
}
.send { color:#77bb77; }
.server { color:#7799bb; }
.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="bloop_markdown"><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">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">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 class="swift">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 class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div> <br> <div id="bloop_sign_1469191693554679808" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 22. Juli 2016 um 14:47:08, Vladimir.S via swift-evolution (<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><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>> https://gist.github.com/Anton3/25a66751812f14f76cacc5e382339522<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>> swift-evolution@swift.org<br>> https://lists.swift.org/mailman/listinfo/swift-evolution<br>><br>_______________________________________________<br>swift-evolution mailing list<br>swift-evolution@swift.org<br>https://lists.swift.org/mailman/listinfo/swift-evolution<br></div></div></span></blockquote></div><div class="bloop_markdown"><p></p></div></body></html>