<html><head><style>
body {
font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
padding:1em;
margin:auto;
background:#fefefe;
}
h1, h2, h3, h4, h5, h6 {
font-weight: bold;
}
h1 {
color: #000000;
font-size: 28pt;
}
h2 {
border-bottom: 1px solid #CCCCCC;
color: #000000;
font-size: 24px;
}
h3 {
font-size: 18px;
}
h4 {
font-size: 16px;
}
h5 {
font-size: 14px;
}
h6 {
color: #777777;
background-color: inherit;
font-size: 14px;
}
hr {
height: 0.2em;
border: 0;
color: #CCCCCC;
background-color: #CCCCCC;
display: inherit;
}
p, blockquote, ul, ol, dl, li, table, pre {
margin: 15px 0;
}
a, a:visited {
color: #4183C4;
background-color: inherit;
text-decoration: none;
}
#message {
border-radius: 6px;
border: 1px solid #ccc;
display:block;
width:100%;
height:60px;
margin:6px 0px;
}
button, #ws {
font-size: 12 pt;
padding: 4px 6px;
border-radius: 5px;
border: 1px solid #bbb;
background-color: #eee;
}
code, pre, #ws, #message {
font-family: Monaco;
font-size: 10pt;
border-radius: 3px;
background-color: #F8F8F8;
color: inherit;
}
code {
border: 1px solid #EAEAEA;
margin: 0 2px;
padding: 0 5px;
}
pre {
border: 1px solid #CCCCCC;
overflow: auto;
padding: 4px 8px;
}
pre > code {
border: 0;
margin: 0;
padding: 0;
}
#ws { background-color: #f8f8f8; }
.bloop_markdown table {
border-collapse: collapse;
font-family: Helvetica, arial, freesans, clean, sans-serif;
color: rgb(51, 51, 51);
font-size: 15px; line-height: 25px;
padding: 0; }
.bloop_markdown table tr {
border-top: 1px solid #cccccc;
background-color: white;
margin: 0;
padding: 0; }
.bloop_markdown table tr:nth-child(2n) {
background-color: #f8f8f8; }
.bloop_markdown table tr th {
font-weight: bold;
border: 1px solid #cccccc;
margin: 0;
padding: 6px 13px; }
.bloop_markdown table tr td {
border: 1px solid #cccccc;
margin: 0;
padding: 6px 13px; }
.bloop_markdown table tr th :first-child, table tr td :first-child {
margin-top: 0; }
.bloop_markdown table tr th :last-child, table tr td :last-child {
margin-bottom: 0; }
.bloop_markdown blockquote{
border-left: 4px solid #dddddd;
padding: 0 15px;
color: #777777; }
blockquote > :first-child {
margin-top: 0; }
blockquote > :last-child {
margin-bottom: 0; }
code, pre, #ws, #message {
word-break: normal;
word-wrap: normal;
}
hr {
display: inherit;
}
.bloop_markdown :first-child {
-webkit-margin-before: 0;
}
code, pre, #ws, #message {
font-family: Menlo, Consolas, Liberation Mono, Courier, monospace;
}
.send { color:#77bb77; }
.server { color:#7799bb; }
.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>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></body></html>