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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>Hi Swift community,</p>
<p>I’d like to talk to about current <code>Self</code> keyword. If I’m not totally mistaken then the current <code>Self</code> has a few meanings:</p>
<ol>
<li>Refer to the <em>current</em> type, or refer to the dynamic type for non-final classes inside containing type (SE–0068 - not yet implemented).</li>
<li>For non-final class types use <code>Self</code> as return type on the conforming super type (or <em>return an instance of receiver <code>Self</code></em>).</li>
</ol>
<p>Let me visualize the behaviors quickly in some short code snippet:</p>
<pre><code class="swift">protocol Foo {
func foo(_ f: Self) -> Self
}
class A : Foo {
// forced to use `A` as parameter type and `Self` as return type
func foo(_ f: A) -> Self { return self }
// Returning `A()` would cause an error: Cannot convert return expression of type 'A' to return type 'Self'
func bar() -> A { return A() /* or self */ }
func zoo() -> Self { return /* only */ self }
}
class B : A {
// Both is fine `B` or `Self` as the return type
// If `B` is used you can return a different instance like `B()`
// `Self` does only allow `self` to be used here
override func foo(_ f: A) -> B { return self }
}
struct D : Foo {
// No `Self` allowed here at all
func foo(_ f: D) -> D { return self /* or D() */ }
}
</code></pre>
<p>The behavior of <code>Self</code> is a little magical, because it sometimes refers to the <em>current</em> type it is used in, or it has a contract of using <code>self</code>.</p>
<hr>
<p>I propose of introducing a new keyword called <code>Current</code> to solve a few problems here.</p>
<ol>
<li><p><code>Self</code> on parameter types would be disallowed for protocol members, because conformances to that protocol already disallow that (see <code>A</code> above). Instead one would use <code>Current</code> and get the correct meaning.</p>
<pre><code class="swift">protocol Boo {
func boo(_ b: Current) -> Self
}
procotol Loo {
func loo() -> Current
}
class X : Boo, Loo {
func boo(_ b: X) -> Self { return self }
func loo() -> X { return self /* or X() */ }
}
final class Y : Boo {
func boo(_ b: X) -> Y { return self /* or Y */ }
}
</code></pre></li>
<li><p>Using <code>Self</code> inside the containing type would always mean as one would refer to the dynamic type, like the magical syntax function <code>type(of:)</code> does.</p></li>
<li><p><code>Current</code> can only refer to the current containing type.</p></li>
<li><p>On classes <code>Self</code> has always the contract of returning <code>self</code>.</p></li>
<li><p><code>Self</code> could be discouraged in favor of <code>Current</code> on value types, as a shorthand to refer to the containing type.</p></li>
<li><p>Generics could benefit from <code>Current</code> too:</p>
<pre><code class="swift">extension AReallyLongNonFinalClassName {
func casted<T : Current>() -> T { ... }
}
// `Self` wouldn't here, because it would refer to the dynamic type
</code></pre></li>
</ol>
<p>#1 Would affect a lot of protocols which implies that it would affect ABI.</p>
<hr>
<p>These are the first ideas I had in my mind. We can polish it further if it receives positive and constructive feedback.</p>
<p>Best regards, </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_1483779578617182976" 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>