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.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>I replied on your gist directly, but you can also read my reply below.</p>
<hr>
<p>I though I made it crystal clear in my last post what the main problem is.</p>
<p>Again:</p>
<p><code>T.Type</code> serves two jobs at once. </p>
<ol>
<li>It’s a concrete metatype of <code>T</code>.</li>
<li>It’s an existential metatype of <code>T</code> where other metatypes where <code>U</code> is a subtype of <code>T</code> (<code>U : T</code>) are subtypes of this existential metatype.</li>
</ol>
<p>Forget about protocols for a moment:</p>
<pre><code class="swift">struct S { }
let metatype_1: Any.Type = S.self // <~~ (concrete) metatype
// ~~~~~~~~~~~~ ^ existential metatype
let metatype_s: S.Type = S.self // <~~ (concrete) metatype
// ~~~~~~~~~~~~ ^ existential metatype
/*
The relationship looks here like this
(concrete metatype) `T.Type : T.Type` (existential metatype)
OR for the given example: `S.Type : S.Type : Any.Type` (last one is again an existential metatype)
This looks confusing right?
*/
class B { }
class D : B { }
let metatype_b: B.Type = B.self // <~~ (concrete) metatype
// ~~~~~~~~~~~~ ^ existential metatype
metatype_b is D.Type // false
let metatype_d: D.Type = D.self // <~~ (concrete) metatype
// ~~~~~~~~~~~~ ^ existential metatype
let metatype_2: B.Type = metatype_d // Totally fine
let metatype_3: Any.Type = metatype_2 // Okay
metatype_3 is D.Type // true
/*
Relationship:
(existential metatype) `B.Type : Any.Type` (existential metatype)
(concrete metatype) `B.Type : B.Type` (existential metatype)
(existential metatype) `D.Type : B.Type` (existential metatype)
(concrete metatype) `D.Type : D.Type` (existential metatype)
*/
</code></pre>
<p>It should be clear by now that there is this odd <code>T.Type : T.Type</code> relationship. We want to correct this behaviour + solve the problem that raises with protocols with one simple and single design.</p>
<p>Let’s see what happens with protocols:</p>
<pre><code class="swift">protocol P { }
let metatype_p: P.Type = P.self // Error, because the concrete metatype is not a subtype of the existential metatype of P
// Furthermore `P.self` is `P.Protocol`
let metatype_3: Any.Type = P.self // fine <~~ (concrete) metatype
// ~~~~~~~~~~~~ ^ existential metatype
/*
Relationship:
(concrete metatype) `P.Protocol : Any.Type` (existential metatype)
(existential metatype) `P.Type : Any.Type` (existential metatype)
At this time `P.Type : Any.Type` is an existential metatype that exists but it does not have any subtypes!
*/
struct I : P { }
let metatype_i: I.Type = I.self // <~~ (concrete) metatype
// ~~~~~~~~~~~~ ^ existential metatype
let metatype_4: P.Type = metatype_i // fine
// ~~~~~~~~~~~~ ^ existential metatype
metatype_4 is I.Type // true
/*
Relationship:
(existential metatype) `P.Type : Any.Type` (existential metatype)
(existential metatype) `I.Type : P.Type` (existential metatype)
(concrete metatype) `I.Type : I.Type` (existential metatype)
*/
</code></pre>
<p>There is a huge overlap in the current design. I hope this cleared your question here.</p>
<p>Side note: The following function isn’t possible to implement with the current <code>T.Type</code> design because in generic context a protocol will end up <code>T.Protocol</code>.</p>
<pre><code class="swift">func dynamic<T>(subtype: Subtype<Any>, `is` _: Type<T>) -> Bool {
return type is Subtype<T>
}
</code></pre>
<p>The proposed design however solves these problems and the relationship becomes clearer:</p>
<pre><code>(existential metatype) `Subtype<B> : Subtype<Any>` (existential metatype)
(concrete metatype) `Type<B> : Subtype<B>` (existential metatype)
(existential metatype) `Subtype<D> : Subtype<B>` (existential metatype)
(concrete metatype) `Type<D> : Subtype<D>` (existential metatype)
(existential metatype) `Subtype<P> : Subtype<Any>` (existential metatype)
(concrete metatype) `Type<P> : Subtype<Any>` (existential metatype)
(existential metatype) `Subtype<P> : Subtype<Any>` (existential metatype)
(existential metatype) `Subtype<I> : Subtype<P>` (existential metatype)
(concrete metatype) `Type<I> : Subtype<I>` (existential metatype)
</code></pre>
<p>The only way to work with <code>Subtype<T></code> is by using <code>subtype(of:)</code> function of by manually shadowing a concrete metatype <code>Type<T></code>.</p>
<p>The only way to instantiate a concrete metatype is done with <code>T.self</code>.</p>
<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_1475268416089321216" 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 30. September 2016 um 21:48:39, Xiaodi Wu (<a href="mailto:xiaodi.wu@gmail.com">xiaodi.wu@gmail.com</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div><div></div><div>
<title></title>
<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">https://gist.github.com/xwu/0cc2c8d358f1fdf066ba739bcd151167</a></div>
<div><br>
<div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Fri, Sep 30, 2016 at 2:09 PM, Adrian
Zubarev via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div style="word-wrap:break-word">
<div class="gmail-m_6144360559732206633bloop_markdown">
<p>About the proposed names:</p>
<p>To be crystal clear we could use more descriptive names for our
two types. Today <code>T.Type</code> is referred as *metatype* and
serving two different purposes at once.</p>
<ol>
<li>
<p>It’s a concrete type; we call it <code>Type<T></code> or
other suggested names looked like <code>ExactType<T></code>,
<code>StaticType<T></code> etc.</p>
</li>
<li>
<p><code>T.Type</code> is also the <em>base type</em> for all
subtypes of <code>T</code>.</p>
</li>
</ol>
<p>Protocols has one exception here.</p>
<p>1.1. The concrete type for protocols is not <code>T.Type</code>
but <code>T.Protocol</code>.</p>
<p>2.1. <code>T.Protocol</code> has only one supertype, which is
the existential (#2) <code>Any.Type</code> type.</p>
<p>Our proposal slices this behaviour into two different types,
where you only can create a *concrete type*
<code>Type<T></code> with <code>T.self</code> or shadow a
concrete type behind <code>Subtype<U></code> with
<code>subtype(of:)</code> function.</p>
<p>To be precise the correct names should be:</p>
<ul>
<li><code>Metatype<T></code> for the concrete type (#1).</li>
<li><code>ExistentialMetatype<T></code> for the existential
type (#2).</li>
</ul>
<p>But we felt that we should adopt the existing name from
<code>T.Type</code> and use the short form for the *concrete type*
<code>Type<T></code>.</p>
<hr>
<p>Brent already showed in multiple examples but the question seems
to come up over and over about the correct name of the current
<code>type(of:)</code> function.</p>
<p>Imagine this scenario:</p>
<pre><code class="gmail-m_6144360559732206633swift">protocol P {}
struct A : P {}
let proto: P = A()
let any: Any = proto
// the old behaviour looked like this
// *concrete* `A.Type` is hidden behind the existential `Any.Type`
let anyMetatype: Any.Type = any.dynamicType
anyMetatype is P.Type //=> true `P.Type` is the existential type here
anyMetatype is A.Type //=> true
let aMetatype = anyMetatype as! A.Type // Okay
// today `type(of:)` does the same trick
// After this proposal:
// subtype<T>(of instance: T) -> Subtype<T>
// The function will extract `Type<A>` for `any` but shadow it behind `Subtype<Any>`
let anyMetatype: `Subtype<Any>` = subtype(of: any)
// The correct relationship look like this:
// Subtype<P> : Subtype<Any>
// Subtype<A> : Subtype<P>
// Type<A> : Subtype<A>
anyMetatype is Subtype<P> //=> true
anyMetatype is Subtype<A> //=> true
anyMetatype is Type<A> //=> true
anyMetatype is Type<P> //=> false
anyMetatype is Type<Any> //=> false
let aMetatype_1 = anyMetatype as! Subtype<A> // Okay
let aMetatype_2 = anyMetatype as! Type<A> // Okay
</code></pre>
<p><code>subtype(of:)</code> function extracts the *concrete type*
from the given instance but shadows it behind the *existential
type* equal to the type of the given instance.</p>
<p><code>subtype(of: T)</code> returns a existential metatype
instance <code>Subtype<T></code> where in reality it’s a
concrete metatype <code>Type<U></code> with the relationship
like <code>U : T</code>.</p>
<p>This is exact the same behaviour as the old
<code>.dynamicType</code> had.</p>
<p>I hope that cleared some raising questions.</p>
</div>
<div class="gmail-m_6144360559732206633bloop_original_html">
<div id="gmail-m_6144360559732206633bloop_customfont" style="font-family:helvetica,arial;font-size:13px;color:rgb(0,0,0);margin:0px">
<span class="gmail-"><br></span></div>
<span class="gmail-"><br></span>
<div id="gmail-m_6144360559732206633bloop_sign_1475260675378149888" class="gmail-m_6144360559732206633bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">
<span class="gmail-">-- <br>
Adrian Zubarev<br>
Sent with Airmail</span></div>
</div>
<span class="gmail-"><br></span>
<p class="gmail-m_6144360559732206633airmail_on"><span class="gmail-">Am 30. September 2016 um 09:00:53, Goffredo Marocchi via
swift-evolution (<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>) schrieb:</span></p>
<blockquote type="cite" class="gmail-m_6144360559732206633clean_bq">
<div><span class="gmail-"><span style="color:rgb(0,0,0);font-family:"helvetica neue",helvetica;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);float:none;display:inline">
Calling it SuperTypeOf<T> and SubTypeOf<T> would make
it less confusing as that is how I read it in my mind in your last
example.</span></span></div>
</blockquote>
</div>
<div class="gmail-m_6144360559732206633bloop_markdown"></div>
</div>
<br>
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<br></blockquote>
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