<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>-1 for the fact that if all devs can write working code, fewer can do it in a clear structured fashion that is well designed for extensibility. A couple months ago I even ran into difficulties when trying to extend AlamoFire because some things had not been designed as cleanly as they could have been to make extending it easy. So if the default is now that everything becomes non-extensible but default, it is going to complicate (and partially defeat the purpose of) reusing libraries.<br><div>Regards</div>(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></div><div><br>On Jun 28, 2016, at 2:11 AM, Michael Ilseman via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br><br></div><blockquote type="cite"><div><meta http-equiv="Content-Type" content="text/html charset=utf-8"><div class="">I was also referring to how we present Objective-C classes in Swift. That is, if a Swift user tries to subclass an Objective-C-imported class, then we’d take into account sealed-ness in order to issue an error/warning, etc. If you are also proposing a Clang attribute for this, e.g. ‘swift_sealed’, to import as sealed (meaning issue an error if Swift users try to subclass it), then that should be spelled out as well. I don’t have an opinion on whether this is a good idea yet, just pointing out some more directions to explore. In general it feels like your proposal could use more fleshing out.</div><div class=""><br class=""></div><br class=""><div><blockquote type="cite" class=""><div class="">On Jun 27, 2016, at 5:08 PM, Javier Soto <<a href="mailto:javier.api@gmail.com" class="">javier.api@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">That is a very good point, it should be explicitly mentioned in the proposal. My thought would be that since in the Obj-C runtime it's not possible to guarantee a class won't have subclasses, or that a method is not overriden, Obj-C classes would be imported as open. <br class=""><br class="">On the Swift side, I think today it's possible to declare a "public final @objc class", but you can still inherit from it from Obj-C, right? My hunch would be that that should be disallowed, but perhaps there's a reason why it's allowed today. <br class=""><div class="gmail_quote"><div dir="ltr" class="">On Mon, Jun 27, 2016 at 4:25 PM Michael Ilseman <<a href="mailto:milseman@apple.com" class="">milseman@apple.com</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div class="">Could you elaborate on how we should treat classes imported from Objective-C or CF-style C? That is, do we always annotate them as being “open” because those paradigms permit subclassing anywhere, or do you propose some kind of recommended “sealed” audit, or what?</div><br class=""><div class=""><blockquote type="cite" class=""></blockquote></div></div><div style="word-wrap:break-word" class=""><div class=""><blockquote type="cite" class=""><div class="">On Jun 27, 2016, at 3:40 PM, Javier Soto via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:</div><br class=""></blockquote></div></div><div style="word-wrap:break-word" class=""><div class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class="">Hello!<div class=""><br class=""></div><div class="">I sent this as a <a href="https://github.com/apple/swift-evolution/pull/376" target="_blank" class="">PR</a> on the swift-evolution repo, but we never had any discussion about it on-list, besides <a href="http://thread.gmane.org/gmane.comp.lang.swift.evolution/9702/focus=9708" target="_blank" class="">a long time ago</a>. Here's the first draft of the proposal:</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><h1 style="font-size:2.25em;margin:0px 0px 16px;line-height:1.2;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol"" class="">Sealed classes by default</h1><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol"" class=""><a href="https://github.com/JaviSoto/swift-evolution/blob/a46877afb0302d2b03fa493255f5ced04ccb7f34/proposals/0000-sealed-by-default.md#introduction" style="color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1;background-color:transparent" target="_blank" class=""></a>Introduction</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol";font-size:16px;line-height:25.6px" class="">Introduce a new<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">sealed</code><span class=""> </span>class modifier that makes classes and methods<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">final</code><span class=""> </span>outside of the module they're declared in, but non-<code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">final</code><span class=""> </span>within the module.</p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol"" class=""><a href="https://github.com/JaviSoto/swift-evolution/blob/a46877afb0302d2b03fa493255f5ced04ccb7f34/proposals/0000-sealed-by-default.md#motivation" style="color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1;background-color:transparent" target="_blank" class=""></a>Motivation</h2><ul style="padding-left:2em;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol";font-size:16px;line-height:25.6px" class=""><li class="">It is not uncommon to have a need for a reference type without needing inheritance. Classes must be intentionally designed to be subclassable, carefully deciding which methods are the override entry-points such that the the behavior remains correct and subclasses respect the<span class=""> </span><a href="https://en.wikipedia.org/wiki/Liskov_substitution_principle" style="color:rgb(64,120,192);text-decoration:none;background-color:transparent" target="_blank" class="">Liskov substitution principle</a>.</li><li class="">Defaulting to non-<code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">final</code><span class=""> </span>allows the author of a class to accidentally leave the visible methods open for overrides, even if they didn't carefully consider this possibility.</li><li class="">Requiring that the author of a class mark a class as<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">open</code><span class=""> </span>is akin to requiring symbols to be explicitly<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">public</code>: it ensures that a conscious decision is made regarding whether the ability to subclass a<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">class</code><span class=""> </span>is part of the API.</li></ul><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol"" class=""><a href="https://github.com/JaviSoto/swift-evolution/blob/a46877afb0302d2b03fa493255f5ced04ccb7f34/proposals/0000-sealed-by-default.md#proposed-solution" style="color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1;background-color:transparent" target="_blank" class=""></a>Proposed solution</h2><ul style="padding-left:2em;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol";font-size:16px;line-height:25.6px" class=""><li class="">New<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">sealed</code><span class=""> </span><span style="line-height:25.6px" class="">(</span><em style="line-height:25.6px" class="">actual name pending bike-shedding</em><span style="line-height:25.6px" class="">) </span>class modifier for classes and methods which marks them as only overridable within the module they're declared in.</li><li class=""><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">sealed</code><span class=""> </span>becomes the default for classes and methods.</li><li class="">New<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">open</code><span class=""> </span>(<em class="">actual name pending bike-shedding</em>) class modifier to explicitly mark a class or a method as<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">overridable</code>.</li></ul><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol"" class=""><a href="https://github.com/JaviSoto/swift-evolution/blob/a46877afb0302d2b03fa493255f5ced04ccb7f34/proposals/0000-sealed-by-default.md#detailed-design" style="color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1;background-color:transparent" target="_blank" class=""></a>Detailed design</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol";font-size:16px;line-height:25.6px" class="">Code Examples:</p><div style="margin-bottom:16px;color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol";font-size:16px;line-height:25.6px" class=""><pre style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;margin-top:0px;margin-bottom:0px;font-stretch:normal;line-height:1.45;word-wrap:normal;padding:16px;overflow:auto;border-radius:3px;word-break:normal;background-color:rgb(247,247,247)" class=""><span style="color:rgb(150,152,150)" class="">/// ModuleA:</span>
<span style="color:rgb(150,152,150)" class="">/// This class is `sealed` by default.</span>
<span style="color:rgb(150,152,150)" class="">/// This is equivalent to `sealed class SealedParentClass`</span>
<span style="color:rgb(167,29,93)" class="">class</span> SealedParentClass {
<span style="color:rgb(150,152,150)" class="">/// This method is `sealed` by default`.</span>
<span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">foo</span>()
<span style="color:rgb(150,152,150)" class="">/// This raises a compilation error: a method can't have a "subclassability"</span>
<span style="color:rgb(150,152,150)" class="">/// level higher than that of its class.</span>
open <span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">bar</span>()
<span style="color:rgb(150,152,150)" class="">/// The behavior of `final` methods remains unchanged.</span>
<span style="color:rgb(167,29,93)" class="">final</span> <span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">baz</span>()
}
open <span style="color:rgb(167,29,93)" class="">class</span> OpenParentClass {
<span style="color:rgb(150,152,150)" class="">/// This method is `sealed` by default`.</span>
<span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">foo</span>()
<span style="color:rgb(150,152,150)" class="">/// Overridable methods in an `open` class must be explicitly marked as `open`.</span>
open <span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">bar</span>()
<span style="color:rgb(150,152,150)" class="">/// The behavior of a `final` method remains unchanged.</span>
<span style="color:rgb(167,29,93)" class="">final</span> <span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">baz</span>()
}
<span style="color:rgb(150,152,150)" class="">/// The behavior of `final` classes remains unchanged.</span>
<span style="color:rgb(167,29,93)" class="">final</span> <span style="color:rgb(167,29,93)" class="">class</span> FinalClass { }</pre></div><div style="margin-bottom:16px;color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol";font-size:16px;line-height:25.6px" class=""><pre style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;margin-top:0px;margin-bottom:0px;font-stretch:normal;line-height:1.45;word-wrap:normal;padding:16px;overflow:auto;border-radius:3px;word-break:normal;background-color:rgb(247,247,247)" class=""><span style="color:rgb(150,152,150)" class="">/// ModuleB:</span>
<span style="color:rgb(167,29,93)" class="">import</span> <span style="color:rgb(0,134,179)" class="">ModuleA</span>
<span style="color:rgb(150,152,150)" class="">/// This raises a compilation error: ParentClass is effectively `final` from</span>
<span style="color:rgb(150,152,150)" class="">/// this module's point of view.</span>
<span style="color:rgb(167,29,93)" class="">class</span> SubclassA <span style="color:rgb(167,29,93)" class="">:</span> SealedParentClass { }
<span style="color:rgb(150,152,150)" class="">/// This is allowed since `OpenParentClass` has been marked explicitly `open`</span>
<span style="color:rgb(167,29,93)" class="">class</span> SubclassB <span style="color:rgb(167,29,93)" class="">:</span> OpenParentClass {
<span style="color:rgb(150,152,150)" class="">/// This raises a compilation error: `OpenParentClass.foo` is</span>
<span style="color:rgb(150,152,150)" class="">/// effectively `final` outside of `ModuleA`.</span>
<span style="color:rgb(167,29,93)" class="">override</span> <span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">foo</span>() { }
<span style="color:rgb(150,152,150)" class="">/// This is allowed since `OpenParentClass.bar` is explicitly `open`.</span>
<span style="color:rgb(167,29,93)" class="">override</span> <span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">bar</span>() { }
}</pre></div><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol"" class=""><a href="https://github.com/JaviSoto/swift-evolution/blob/a46877afb0302d2b03fa493255f5ced04ccb7f34/proposals/0000-sealed-by-default.md#impact-on-existing-code" style="color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1;background-color:transparent" target="_blank" class=""></a>Impact on existing code</h2><ul style="padding-left:2em;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol";font-size:16px;line-height:25.6px" class=""><li class="">This would be a backwards-breaking change for all classes and methods that are public and non-final, which code outside of their module has overriden. Those classes/methods would fail to compile. Their superclass would need to be changed to<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">open</code>.</li></ul><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol"" class=""><a href="https://github.com/JaviSoto/swift-evolution/blob/a46877afb0302d2b03fa493255f5ced04ccb7f34/proposals/0000-sealed-by-default.md#alternatives-considered" style="color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1;background-color:transparent" target="_blank" class=""></a>Alternatives considered</h2><ul style="padding-left:2em;margin-top:0px;color:rgb(51,51,51);font-family:"helvetica neue",helvetica,"segoe ui",arial,freesans,sans-serif,"apple color emoji","segoe ui emoji","segoe ui symbol";font-size:16px;line-height:25.6px;margin-bottom:0px" class=""><li class="">Defaulting to<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">final</code><span class=""> </span>instead: This would be comparable to Swift defaulting to<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">private</code>, as opposed to<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">internal</code>. Just like<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">internal</code><span class=""> </span>is a better trade-off,<span class=""> </span><code style="font-family:consolas,"liberation mono",menlo,courier,monospace;font-size:13.6px;padding:0.2em 0px;margin:0px;border-radius:3px;background-color:rgba(0,0,0,0.0392157)" class="">sealed</code><span class=""> </span>by default also makes sure that getting started with Swift, writing code within a module, doesn't require a lot of boilerplate, and fighting against the compiler.</li></ul></div></div><div dir="ltr" class="">-- <br class=""></div><div data-smartmail="gmail_signature" class=""><div dir="ltr" class="">Javier Soto</div></div></div></blockquote></div></div><div style="word-wrap:break-word" class=""><div class=""><blockquote type="cite" class=""><div class="">
_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a><br class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br class=""></div></blockquote></div><br class=""></div></blockquote></div><div dir="ltr" class="">-- <br class=""></div><div data-smartmail="gmail_signature" class=""><div dir="ltr" class="">Javier Soto</div></div>
</div></blockquote></div><br class=""></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>