<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><br><br>Sent from my iPad</div><div><br>On May 26, 2016, at 2:49 PM, Austin Zheng via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr">I alway enjoy hearing your ideas.<div><br></div><div>This is quite interesting. It's basically a way to define an ad-hoc interface that a type doesn't need to explicitly declare it conforms to. I know Golang works similarly; if a Go type implements all the requirements of an interface it conforms automatically.</div></div></div></blockquote><blockquote type="cite"><div><div dir="ltr"><div><br></div><div>There are positives and negatives to allowing this sort of ad-hoc interface. </div></div></div></blockquote><div><br></div><div>Agree. It would definitely make the language "feel" a bit more fluid. But it doesn't add any expressive power and could have undesirable consequences.</div><br><blockquote type="cite"><div><div dir="ltr"><div>This would make for a good standalone proposal -- both because it's complex enough to deserve its own discussion, and because if the community is interested someone would have to work through all the implications in order to put together a proposal. It would be quite a big change.</div></div></div></blockquote><div><br></div><div><span style="background-color: rgba(255, 255, 255, 0);">I don't see how this is different from a protocol other than the lack of requirement to declare conformance explicitly. The need to explicitly declare conformance is a design decision that I believe the core team feels pretty strongly about. </span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">That said, it hasn't been debated by the community yet so if someone feels strongly about dropping explicit conformance declarations it might be worth pitching the idea, if for not other reason than to have a discussion about it on the lost.</span></div><div><br></div><blockquote type="cite"><div><div dir="ltr"><div><br></div><div>Best,</div><div>Austin</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Thu, May 26, 2016 at 11:56 AM, 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:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><p>I’d like to throw one idea of mine in the room I couldn’t stop thinking when I read one of Thorsten’s replies on SE–0095 review thread.</p>
<p>This <a href="https://en.wikipedia.org/wiki/Type_system#Existential_types" target="_blank">wiki section</a> explains the existential types where we have something like this:</p>
<blockquote>
<p>"T = ∃X { a: X; f: (X → int); }
This could be implemented in different ways; for example:</p>
<ul>
<li>intT = { a: int; f: (int → int); }</li>
<li>floatT = { a: float; f: (float → int); }</li>
</ul>
</blockquote>
<p>We discussed how we could create existential types with constraints for protocols and classes so far. Such an existential can’t create something like in the example above.</p>
<p>I’m not sure if we need this at all, I’d say it’s a <u><em>nice to have</em></u> idea of mine.</p>
<p>To solve this we could introduce a new scope similar to protocols today but without the need to explicitly conform types to this existential.</p>
<pre><code>// the above example can become
existential T {
associatedtype X
var a: X
func f(_ value: X) -> Int
}
struct A /* no explicit conformance to T needed */ {
var a: Int
init(a: Int) { self.a = a }
func f(_ value: Int) -> Int { return value }
}
let store: T = A() // this could or should work, just because we do have visibility to all constraints from T in A here
// if we had `private var a: Int` in A we wouldn't be able to store A inside `store`
</code></pre>
<p>I din’t though if <code>existential</code> could have potential to replace <code>Any<…></code> completely. Until now I just wanted to solve that particular issue so please don’t judge with me. :)</p>
<p>Just because of associated types we won’t be able to use <code>store</code> in this example, but there might be more trivial examples where one would use such existential type (for only visible portion at compile or dynamically at run-time) without explicit conformance.</p>
<pre><code>struct B {
var x: Int = 42
var y: Double = -100.5
}
struct C: SomeProtocol {
var y: Double = 0.0
var x: Int = 10
}
existential SomeShinyThing {
var x: Int
var y: Double
}
// we should be safe here because the compiler has visibility for
// internal B and C here
let anotherStore: SomeShinyThing = B() /* or */ C()
// otherwise one could use dynamic casts
if let thirdStore = instanceOfCShadowedAsSomeProtocol as? SomeShinyThing { … }
</code></pre>
<p>Feel to tear this idea apart as you want. :D</p><span class="HOEnZb"><font color="#888888">
<p></p></font></span></div><span class="HOEnZb"><font color="#888888"><div><div style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto"><br></div> <br> <div><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div></div><div><p></p></div></font></span></div><br>_______________________________________________<br>
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