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.send { color:#77bb77; }
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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>Can you point me to some reading? I’m curious what exactly you’re talking about!?</p>
<p>—</p>
<p>I found a negative side effect of sealing <code>T.Type</code>. The problem appears lies in the dynamic <code>is</code> cast.</p>
<p>Currently we could do something like this:</p>
<pre><code class="swift">func isInt<T>(metatype: T.Type) -> Bool {
return metatype is Int.Type
}
</code></pre>
<p>There is no way to express the same type check with <code>Type<T></code> except this:</p>
<pre><code>func isInt<T>(type: Type<T>) -> Bool {
return type == Type<Int>()
// since this check is something where we store/pass a type (name)
// we could implicitly convert `Int` into an instance of `Type<Int>`
//
// The result would look like this:
return type == Int // which is logically equivalent to `metatype is Int.Type`
}
</code></pre>
<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_1468163131612709888" 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 10. Juli 2016 um 16:24:50, L. Mihalkovic (<a href="mailto:laurent.mihalkovic@gmail.com">laurent.mihalkovic@gmail.com</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div dir="auto"><div></div><div>
<title></title>
<br>
<div>
<div>It looks like this is touching on a small subset of the
not-yet-designed reflection API (still tabled for 4.0?). I am
interested to see what balance it strikes between declarations (eg.
being able to reflect extensions declarations), types (eg
reflecting type conformance), and operations (xxx.newInstance(),
xxx.setValue()). The mirror api shows a general direction, maybe
there is a way to squeeze a tiny subset in 3.0 (just like was done
with P&Q that will remain a degenerate case of the upcoming
more general syntax). Maybe just enough for people not to have to
use ObjectIdentifier(:Any.Type).</div>
<div><br>
<div>
<div>Regards</div>
(From<span class="Apple-style-span" style="-webkit-tap-highlight-color: rgba(26, 26, 26, 0.296875); -webkit-composition-fill-color: rgba(175, 192, 227, 0.230469); -webkit-composition-frame-color: rgba(77, 128, 180, 0.230469);"> mobile)</span></div>
</div>
<div><br>
On Jul 10, 2016, at 2:22 PM, Adrian Zubarev via swift-evolution
<<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>>
wrote:<br>
<br></div>
<blockquote type="cite">
<div>
<div class="bloop_markdown">
<p>Hello Chris,</p>
<p>my main concern for this change is the lack of extensibility of
metatypes. We can access the metatype through the
<code>.self</code> postfix, which potentially will be removed in
the future (only the postfix). Through an instance of such a
metatype we can then access the static members and all initializer
for our type.</p>
<ul>
<li><code>SomeType.staticMember</code> equals
<code>metatypeInstance.staticMemeber</code></li>
<li><code>SomeType.init</code> equals
<code>metatypeInstance.init</code></li>
</ul>
<p>It is also possible to extract some more information from the
metatypes with the help of Buildin wrapper functions like
<code>sizeof</code> or <code>strideof</code>. And the last but not
the least we can use metatypes in type checking scenarios.</p>
<p>Last weak I asked the community if there is a need for all
metatypes conform to the <code>Hashable</code> protocol, which
would be really handy. The overall response was positive, even if
only a handful of people responded.</p>
<p>The whole idea of metatypes conforming to <code>Hashable</code>
kept me thinking how this might be solved in the future. The main
problem is that metatypes provide direct access to initializer and
static members of the type and a conformance to
<code>Hashable</code> seems to be impossible without some compiler
magic or a hidden <code>hashValue</code> property. The main
confusion here might be that metatypes can be stored (at least
while runtime) inside variables and pretend to be instances of a
<code>T.Type</code> type.</p>
<p>That said, I’d like to consider of sealing the direct access to
a metatype into a real type for the sake of extensibility and
clarity.</p>
<p>I’m aware that my bikeshedding introduced more boilerplate in
terms of accessing the initializer and static members through a
bottleneck property called <code>sealed</code>. Furthermore there
is one more character to type if we’d go with
<code>Type<T></code> instead of <code>T.Type</code>.</p>
<p>This is why this is a discussion before making any further
decisions. :)</p>
<p>PS: As I showed in my last code example a wrapper type
<code>Type<T></code> can also provide a few more information
about the metatype.</p>
<ul>
<li><code>Type<T>.size</code></li>
<li><code>Type<T>.stride</code></li>
<li><code>Type<T>.alignment</code></li>
</ul>
<p>Furthermore sealing <code>T.Type</code> into
<code>Type<T></code> does solve the problem with the
array/dictionary shorthand syntax.</p>
<p>Only when one would store/pass a type (name) we’d implicitly
convert <code>SomeType</code> to
<code>Type<SomeType></code>.</p>
<p>Declarations do not follow this implicit conversion:</p>
<ul>
<li><code>func foo(_: SomeType)</code> is not equivalent to
<code>func foo(_: Type<SomeType>)</code></li>
<li><code>Array<SomeType>()</code> ==
<code>[SomeType]()</code> but is not equivalent to
<code>[Type<SomeType>]()</code> ==
<code>Array<Type<SomeType>>()</code></li>
</ul>
<p>Here is again my bikeshedding:</p>
<pre><code class="swift">public struct Type<T> : Hashable, CustomStringConvertible, CustomDebugStringConvertible {
// `metatype` is a better name than `sealed`
// The tradeoff is this bottleneck property
public let metatype: T.Type
public let hashValue: Int
public static var size: Int { get }
public static var stride: Int { get }
public static var alignment: Int { get }
public var size: Int { get }
public var stride: Int { get }
public var alignment: Int { get }
public var description: String { get }
public var debugDescription: String { get }
}
public func ==<T, U>(lhs: Type<T>, rhs: Type<U>) -> Bool
</code></pre></div>
<div class="bloop_original_html">
<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_1468144136071841024" 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 10. Juli 2016 um 00:18:02, Chris Lattner
(<a href="mailto:clattner@apple.com">clattner@apple.com</a>)
schrieb:</p>
<blockquote type="cite" class="clean_bq">
<div>
<div 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; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">
<span>Hi Adrian,</span></div>
<div 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; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">
<span><br class=""></span></div>
<div 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; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">
<span>What’s the motivation for this change?</span></div>
<div 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; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">
<span><br class=""></span></div>
<div 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; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">
<span>-Chris</span></div>
</div>
</blockquote>
</div>
<div class="bloop_markdown"></div>
</div>
</blockquote>
<blockquote type="cite">
<div>
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</blockquote>
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