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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>Here is a part of a draft I put together in case we should go for a review on this.</p>
<p>Here is the formatted version if you cannot read it here: <a href="https://gist.github.com/DevAndArtist/a8d11011a376d47d0afc941017e64c75">https://gist.github.com/DevAndArtist/a8d11011a376d47d0afc941017e64c75</a></p>
<h2 id="proposedsolution">Proposed solution</h2>
<p>I propose to seal <code>T.Type</code> into a standalone type <code>Type<T></code> and disallow <code>.Type</code> in public declaration usage.</p>
<h4 id="behaviorchanges:">Behavior Changes:</h4>
<ul>
<li><p>Make a distinction between internal and public <code>.self</code>.</p></li>
<li><p>Rename internal version of <code>.self</code> to <code>.metatype</code> or revision SE–0090 for Swift 3 and leave <code>.self</code> internally.</p></li>
<li><p>Make public <code>.self</code> (if not dropped for Swift 3) construct an instance of<code>Type<T></code> instead a metatype <code>T.Type</code>.</p></li>
<li><p>Update all codebase to use <code>Type<T></code> instead of <code>T.Type</code>.</p>
<p>Small example:</p>
<pre><code class="diff">- public func sizeof<T>(_: T.Type) -> Int
+ public func sizeof<T>(_: Type<T>) -> Int
// Current implementation
- return Int(Builtin.sizeof(T.self))
// Possible internal implementation
+ return Int(Builtin.sizeof(T.metatype))
// Possible implementation with `Type<T>`
+ return Int(Builtin.sizeof(Type<T>.metatype))
}
</code></pre></li>
</ul>
<h4 id="pros:">Pros:</h4>
<ul>
<li>Removing ambiguity in array/dictionary shorthand syntax.</li>
<li>Extensibility around the metatype:
<ul>
<li>Conformance to <code>Hashable</code> protocol.</li>
<li>Direct access to metatype related information like <code>size</code>, <code>stride</code> or <code>aligment</code>:
<ul>
<li><code>Type<T>.size</code> equivalent to current <code>sizeof(T.self)</code></li>
<li><code>Type<T>.stride</code> equivalent to current <code>strideof(T.self)</code></li>
<li><code>Type<T>.alignment</code> equivalent to current <code>alignof(T.self)</code></li>
</ul></li>
</ul></li>
<li>A step to remove public <code>.self</code> magic.</li>
<li>Less magical types floating around the codebase.</li>
<li>Better future maintenance.</li>
</ul>
<h4 id="cons:">Cons:</h4>
<ul>
<li><p>A distinction between internal and public <code>.self</code> must be introduced.</p></li>
<li><p>Dynamic types must be tweaked.</p>
<ul>
<li><p>Currently it’s possible to check the Type like this:</p>
<pre><code class="swift">//
func isInt<T>(metatype: T.Type) -> Bool {
return metatype is Int.Type
}
</code></pre></li>
<li><p><code>is</code> cast must be tweaked after this proposal:</p>
<pre><code class="swift">func isInt<T>(type: Type<T>) -> Bool {
return type == Type<Int>()
// Since we store/pass a type (name) in this check,
// we can safely convert `Int` into an instance of `Type<Int>`
//
// The result would look like this:
return type == Int
// This is logically equivalent to `metatype is Int.Type`
}
</code></pre></li>
</ul></li>
<li><code>Type<T></code> adds more code noise for declarations.
<ul>
<li><code>T.Type</code> = 6 characters</li>
<li><code>Type<T></code> = 7 characters</li>
</ul></li>
<li><code>Type<T></code> adds a bottleneck (static) property to access the metatype:
<ul>
<li>Direct access today <code>T.self</code> (in future <code>T</code>)</li>
<li>Metatype access after this proposal:
<ul>
<li>Direct access through static member: <code>Type<T>.metatype</code></li>
<li>Access through an initialized instance: <code>typeInstance.metatype</code></li>
</ul></li>
</ul></li>
</ul>
<h2 id="detaileddesign">Detailed Design</h2>
<h4 id="rules:">Rules:</h4>
<ol>
<li>Whenever a type (name) is passed or stored, it will be implicitly converted to <code>Type<SomeType></code> (no need for <code>.self</code> - <strong>SE–0090</strong>).</li>
<li>Declarations do not follow the rule #1.</li>
</ol>
<p>These rules would imply the following behavior and potentially solve the ambiguity in array/dictionary shorthand syntax:</p>
<pre><code class="swift">// Type: Type<T>
let variable1 = T
// Type: T
let variable2 = T.init()
// Type: T
let variable3 = T()
// Return-type of static member from T
let variable4 = T.staticMember
// Return-type of an Array<T> static member
let variable6 = [T].staticMember
// Return-type of a Dictionary<T, U> static member
let variable7 = [T: U].staticMember
// Type: Array<T>
let array1 = [T]()
// Type: Array<T>
let array2: [T] = []
// Type: Array<Type<T>>
let array3: [Type<T>] = []
// Type: Array<Type<T>>
let array4 = [Type<T>]()
// Type: Dictionary<T, U>
let dictionary1 = [T: U]()
// Type: Dictionary<T, U>
let dictionary2: [T: U] = [:]
// Type: Dictionary<Type<T>, U>
let dictionary3: [Type<T>: U] = [:]
// Type: Dictionary<Type<T>, U>
let dictionary4 = [Type<T>: U]()
</code></pre>
<h4 id="possibleimplementationdesign:">Possible Implementation Design:</h4>
<pre><code class="swift">///
public struct Type<T> : Hashable, CustomStringConvertible, CustomDebugStringConvertible {
/// Creates an instance of `Type<T>`.
public init() {
// Possible new internal access to the metatype
self.metatype = T.metatype
// Same hash calculation like in `ObjectIdentifier`
let rawPointerMetatype = unsafeBitCast(T.metatype, to: Builtin.RawPointer.metatype)
self.hashValue = Int(Builtin.ptrtoint_Word(rawPointerMetatype))
// Iff the value of `size`, `stride` and `alignment` properties cannot
// be changed due runtime, we could move the calculation directly to
// the initializer and make these properties to constants!
}
///
public let metatype: T.Type
/// The Type<T>'s hash value.
///
/// Hash values are not guaranteed to be equal across different executions of
/// your program. Do not save hash values to use during a future execution.
public let hashValue: Int
/// Returns the contiguous memory footprint of `T`.
///
/// Does not include any dynamically-allocated or "remote" storage.
/// In particular, when `T` is a class type, is the
/// same regardless of how many stored properties `T` has.
public var size: Int {
return sizeof(self)
// OR:
return Int(Builtin.sizeof(T.metatype))
}
/// Returns the least possible interval between distinct instances of
/// `T` in memory. The result is always positive.
public var stride: Int {
return strideof(self)
// OR:
return Int(Builtin.strideof_nonzero(T.metatype))
}
/// Returns the minimum memory alignment of `T`.
public var alignment: Int {
return alignof(self)
// OR:
return Int(Builtin.alignof(T.metatype))
}
/// A textual representation of `self`.
public var description: String {
return "Type<\(self.metatype)>"
}
/// A textual representation of `self`, suitable for debugging.
public var debugDescription: String {
return "<" + self.description
+ " metatype: \(self.metatype)"
+ " size: \(self.size)"
+ " stride: \(self.stride)"
+ " alignment: \(self.alignment)>"
}
}
/// Static members for access metatype specific information
/// without constructing an instance of `Type<T>`:
///
/// - `Type<T>.metatype`
/// - `Type<T>.size`
/// - `Type<T>.stride`
/// - `Type<T>.alignment`
public extension Type {
///
public static var metatype: T.Type {
return T.metatype
}
/// Returns the contiguous memory footprint of `T`.
///
/// Does not include any dynamically-allocated or "remote" storage.
/// In particular, when `T` is a class type, is the
/// same regardless of how many stored properties `T` has.
public static var size: Int {
return sizeof(self)
// OR:
return Int(Builtin.sizeof(T.metatype))
}
/// Returns the least possible interval between distinct instances of
/// `T` in memory. The result is always positive.
public static var stride: Int {
return strideof(self)
// OR:
return Int(Builtin.strideof_nonzero(T.metatype))
}
/// Returns the minimum memory alignment of `T`.
public static var alignment: Int {
return alignof(self)
// OR:
return Int(Builtin.alignof(T.metatype))
}
}
public func ==<T, U>(lhs: Type<T>, rhs: Type<U>) -> Bool {
return lhs.hashValue == rhs.hashValue
}
</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_1468401707444188160" 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 18:03:17, Adrian Zubarev (<a href="mailto:adrian.zubarev@devandartist.com">adrian.zubarev@devandartist.com</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div></div><div>
<title></title>
<div class="bloop_markdown">
<p>Just to be crystal clear, here is my vision.</p>
<p>Currently Swift is heading in this direction:</p>
<pre><code class="swift">SomeType.staticMember == metatypeInstance.staticMember == SomeType.self.staticMember
SomeType.init == metatypeInstance.init == SomeType.self.init
SomeType.self -> SomeType.Type (metatype)
SomeType -> SomeType.Type (removed .self)
SomeType.self is SomeType.Type == true
func foo<T>(metatype: T.Type) -> T {
// lets pretend T conforms to an initializable protocol
return metatype.init()
}
</code></pre>
<p>With some tweaking and a little tradeoff for
<code>Type<T></code> we’d get this model:</p>
<pre><code class="swift">SomeType.staticMember == Type<SomeType>.metatype.staticMember
SomeType.init == Type<SomeType>.metatype.init
SomeType -> Type<SomeType>() (initialized instance; removed .self)
SomeType == Type<SomeType>() == true
SomeType is Type<SomeType>() == true
func foo<T>(type: Type<T>) -> T {
print(type.size)
print(type.debugDescription)
print(type.hashValue)
// lets pretend T conforms to an initializable protocol
return type.metatype.init()
}
</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_1468165621603607040" 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 17:28:35, Adrian Zubarev
(<a href="mailto:adrian.zubarev@devandartist.com">adrian.zubarev@devandartist.com</a>)
schrieb:</p>
<blockquote type="cite" class="clean_bq">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">
<div>
<div class="bloop_markdown">
<p><span>Can you point me to some reading? I’m curious what exactly
you’re talking about!?</span></p>
<p><span>—</span></p>
<p><span>I found a negative side effect of sealing
<code>T.Type</code>. The problem appears lies in the dynamic
<code>is</code> cast.</span></p>
<p><span>Currently we could do something like this:</span></p>
<pre><span><code class="swift">func isInt<T>(metatype: T.Type) -> Bool {
return metatype is Int.Type
}
</code></span></pre>
<p><span>There is no way to express the same type check with
<code>Type<T></code> except this:</span></p>
<pre><span><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></span></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;">
<span><br></span></div>
<span><br></span>
<div id="bloop_sign_1468163131612709888" class="bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">
<span>-- <br>
Adrian Zubarev<br>
Sent with Airmail</span></div>
</div>
<span><br></span>
<p class="airmail_on"><span>Am 10. Juli 2016 um 16:24:50, L.
Mihalkovic (<a href="mailto:laurent.mihalkovic@gmail.com">laurent.mihalkovic@gmail.com</a>)
schrieb:</span></p>
<blockquote type="cite" class="clean_bq">
<div dir="auto">
<div><span><span><br></span></span>
<div>
<div><span>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).</span></div>
<div><span><br></span>
<div>
<div><span>Regards</span></div>
<span>(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></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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