<div dir="ltr"><div class="markdown-here-wrapper" style=""><p style="margin:0px 0px 1.2em!important">2016-09-30 22:48 GMT+03:00 Xiaodi Wu via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>>:</p>
<p style="margin:0px 0px 1.2em!important"></p><div class="markdown-here-exclude"><p></p><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Sorry, my question at least has nothing to do with bikeshedding. I'm confused about why the proposal feels it's necessary to have both Type and Subtype. I don't understand Brent's two reasons and was hoping for some elaboration. I've tried to clarify my question in a gist:<div><br></div><div><a href="https://gist.github.com/xwu/0cc2c8d358f1fdf066ba739bcd151167" target="_blank">https://gist.github.com/xwu/<wbr>0cc2c8d358f1fdf066ba739bcd1511<wbr>67</a></div></div></blockquote><p></p></div><p style="margin:0px 0px 1.2em!important"></p>
<p style="margin:0px 0px 1.2em!important">Regardless of syntax, there are <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Metatype<T></code> and <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">ExistentialMetatype<T></code> “under the hood”. I’ll take the liberty to use these names for this post.</p>
<p style="margin:0px 0px 1.2em!important">Purpose and difference between the two kinds of metatypes is easy to understand. If it helps, size of <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Metatype<T></code> is typically <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">0</code> while size of <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">ExistentialMetatype<T></code> is typically <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">8</code>.</p>
<p style="margin:0px 0px 1.2em!important">NOTE: <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Metatype<P></code> can’t be a subtype of <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">ExistentialMetatype<P></code> for protocols <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">P</code>, because protocols do not implement their own static requirements.</p>
<p style="margin:0px 0px 1.2em!important">Apparently, creators of Swift also thought that two kinds of metatypes are too difficult for an average developer. So an attempt was made to unify both under a single name, <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.Type</code>. For “final” types like structs, <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.Type</code> maps onto <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Metatype<T></code>. For classes and protocols, <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.Type</code> maps onto <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">ExistentialMetatype<T></code>.</p>
<p style="margin:0px 0px 1.2em!important">This abstraction turned out to be leaky. I’ll give 3 leaks.</p>
<ol style="margin:1.2em 0px;padding-left:2em">
<li style="margin:0.5em 0px">Because of Note, we could not have ID of a protocol itself inside of any kind of metatype. This feature was needed in real code. The solution was to add <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.Protocol</code> syntax that could only be used for protocols to create a <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Metatype<T></code>. But this solution created a plethora of other inconsistencies and abstraction leaks (which were explored by Adrian). Arguably the most important is (2).</li>
<li style="margin:0.5em 0px">In generic functions accepting <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.Type</code>, passing <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.self</code> creates an instance of <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.Protocol</code>, which is still described in code as <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T.Type</code>. <em>boom!</em></li>
<li style="margin:0.5em 0px">Accepting static metatypes in functions is often what people want. Consider the following basic example:</li>
</ol>
<pre style="font-family:Consolas,Inconsolata,Courier,monospace;line-height:1.2em;margin:1.2em 0px"><code class="hljs language-swift" style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;white-space:pre;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em;color:rgb(51,51,51);background:rgb(248,248,248);display:block!important"><span class="hljs-func"><span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">func</span> <span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">create</span><span class="hljs-generics"><T: Creatable></span><span class="hljs-params">(type: T.<span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">Type</span>)</span></span><span style="font-size:0.85em">
</span></code></pre>
<p style="margin:0px 0px 1.2em!important">Most probably, <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">create</code> does not read contents of <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">type</code> to deal with appropriate subtype. It does not support existentiality, <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">type</code> is here for function specialization. But in case <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">T</code> is subtypable, we still pass around those 8 bytes that are not used by the <code style="font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">create</code>, confusing both compiler and some developers. In 90% cases, when we see such a function, it throws off value of metatype instance.</p>
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