<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><span></span></div><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><style>
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.error { color:#AA0000; }</style><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>
<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_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"><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;">Hi Adrian,</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;"><br class=""></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;">What’s the motivation for this change?</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;"><br class=""></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;">-Chris</div></div></span></blockquote></div><div class="bloop_markdown"><p></p></div></div></blockquote><blockquote type="cite"><div><span>_______________________________________________</span><br><span>swift-evolution mailing list</span><br><span><a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a></span><br><span><a href="https://lists.swift.org/mailman/listinfo/swift-evolution">https://lists.swift.org/mailman/listinfo/swift-evolution</a></span><br></div></blockquote></div></body></html>