[swift-evolution] [swift-evolution-announce] [Review #2] SE-0117: Default classes to be non-subclassable publicly

Leonardo Pessoa me at lmpessoa.com
Mon Jul 18 11:15:10 CDT 2016

I believe sealed by default applied to functions makes the behaviour
consistent with classes and allows for the same behaviour we wanted
with this proposal. It would allows us to create subclassable classes
in which we would be able to internally override a method but not
outside the library and selectively allow other methods to be
overriden. Final won't do it so if this is not the default behaviour,
it will be necessary to introduce the sealed keyword so we can achieve
this. It's inconsistent to have to explicitly open a class and
explicitly seal its methods and vice-versa. It was my assumption that
when we chose sealed by default with the proposal we were talking
about everything and not just classes (I at least was talking

Introducing "dynamic" or some other keyword to mark explicitly methods
that should be overriden is just the same "open" with sealed methods
by default would mean for public methods so it makes no difference to
me. Also having no default will not change that some library
developers will have everything sealed and selectively open. No
default shall also make developers prone to open to think more about
the keyword they'll choose to use, but I'm not fond of no default.

As for the inheritance of openness, I firmly believe it shouldn't. If
a class inherited from an open class is open by default, there will be
no libraries making use of other libraries or we should also introduce
the sealed keyword in order to make the class (or method) sealed
again. This behaviour seems inconsistent to me too.


On 18 July 2016 at 09:07, Károly Lőrentey <swift-evolution at swift.org> wrote:
> On 2016-07-18 09:17:43 +0000, David Hart via swift-evolution said:
>> On 18 Jul 2016, at 11:11, Xiaodi Wu via swift-evolution
>> <swift-evolution at swift.org> wrote:
>> On Mon, Jul 18, 2016 at 3:27 AM, Brent Royal-Gordon via swift-evolution
>> <swift-evolution at swift.org> wrote:
>> > On Jul 17, 2016, at 8:57 PM, L. Mihalkovic via swift-evolution
>> > <swift-evolution at swift.org> wrote:
>> >
>> >> On Jul 17, 2016, at 9:14 PM, Garth Snyder via swift-evolution
>> >> <swift-evolution at swift.org> wrote:
>> >>
>> >> Is there a summary somewhere of the motivation for allowing methods to
>> >> be declared non-overridable within open classes?
>> [...]
>> Garth: I think it's implicit in the reasons to prevent subclassing. The
>> mere fact that a class allows subclassing doesn't necessarily mean that
>> every member in it is designed to be subclassed. Consider
>> `UIViewController`: It's obviously designed to be subclassed, and some
>> methods in it (such as `loadView`) are intended to be overridden, but others
>> (such as `loadViewIfNeeded`) are *not* intended to be overridden.
>> And [if UIViewController were to be written in Swift] there'd be a good
>> reason why `loadViewIfNeeded` and others of its ilk couldn't be final?
>> I don't know UIKit internals, but I could imagine loadViewIfNeeded be
>> overridden internally, if one knows the precise internal workings of
>> UIViewController. That would require open, to allow overriding internally
>> but not externally.
> I thought about this aspect a little more. I think it's fair to say that
> we're breaking new ground for language design here. Classes limiting
> inheritance to a certain set of subclasses are nothing new (I've written &
> used classes doing this in C++, Java and C#), but no language that I know of
> allows limiting overrides of a specific public member in such a way. I think
> we need a convincing rationale for making this esoteric middle ground
> between final and open members the new default.
> The UIKit example above isn't convincing at all. It is already quite easy to
> allow package-internal subclasses to configure the behavior of
> loadViewIfNeeded without such a novel language feature. E.g., the UIKit team
> can simply make loadViewIfNeeded call into a non-final but internal method:
> public open class UIViewController {
>         private var _view: UIView? = nil
>         public final func loadViewIfNeeded() {
>                 internalLoadViewIfNeeded()
>         }
>         internal func internalLoadViewIfNeeded() { // overridable internally
>                 if let view = _view { return }
>                 loadView()
>         }
>         public open func loadView() {
>                 // Load it from a nib or whatevs
>         }
> }
> I see no drawback to this pattern; it is quite clear and simple. Therefore,
> in the interest of keeping the language free of needless complexity, I
> suggest we change the proposal to remove the implicit "sealed" level of
> public member overridability, and support only "open" or "final" class
> members.
> For members, "open" should mean the opposite of "final", with no levels in
> between. Member-level openness should be entirely independent of visibility;
> so it should be possible to say "internal open" to mean an internally
> overridable member that's not at all visible outside the module -- the same
> as today's default.
> (Note that (on platforms with an Objective-C runtime) "dynamic" provides a
> third level of flexibility for class members; I argue that it should imply
> "open". So in order of increasing flexibility, we'd have "final", "open" and
> "dynamic" members. This seems easy enough to describe and understand.)
> I also suggest that for now, we should make neither "final" nor "open" nor
> "dynamic" the default for public members of open classes: we should rather
> require class authors to explicity add one of these qualifiers to all public
> member declarations. This way, we can defer the argument for choosing a
> default to a later (additive) proposal, once we have some experience with
> this setup. Non-public members can safely keep defaulting to "internal
> open", like they do today.
> --
> Károly
> @lorentey
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