<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><br>Considering how I interpret (i.e. I could be totally wrong) sentiments expressed by the core team on expressiveness, I would wager that duck typing is NOT on their agenda for Swift.</div><div id="AppleMailSignature"><br></div><div><br>On May 26, 2016, at 8:56 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>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">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 class="swift">// 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 class="swift">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>
<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_1464286914046025984" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div></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></body></html>