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Subject: Re: [PHP-DEV] [DISCUSSION] Class Constant Enums?
In-Reply-To: <79817594-f9a9-4f02-b48a-4bf39e4372e6@app.fastmail.com>
Date: Fri, 16 Aug 2024 12:46:43 -0400
Cc: php internals <internals@lists.php.net>
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Message-ID: <E249EB4D-8052-4122-9009-15AEB9A57C49@zort.net>
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To: Larry Garfield <larry@garfieldtech.com>
From: jbafford@zort.net (John Bafford)

On Aug 16, 2024, at 10:17, Larry Garfield <larry@garfieldtech.com> =
wrote:
>=20
> I would have to research to see if other languages did this, but one =
option would be to allow an inner class to extend an outer class even if =
it's final, which would essentially give us sealed classes for free:
>=20
> final class Outer
> {
>  class InnerA extends Outer {}
>  class InnerB extends Outer {}
>  class InnerC extends Outer {}
> }
>=20
> // But this is still not OK:
> class Sibling extends Outer {}
>=20
> Note: I have no idea how difficult/complicated this would be, but I =
would be in favor of exploring it.
>=20
> --Larry Garfield


Swift allows nested classes to extend their parent. It does not allow =
nested classes to extend a final parent. Visibility modifiers can be =
applied and work as expected. Basically, types also function as the =
equivalent of a namespace. More broadly, any type can be nested in any =
other type, so you could certainly do something weird like:

```
class A {
	enum E {
		class C: A {}
	=09
		case a(A)
		case c(C)
	}
}

let e =3D A.E.c(A.E.C())
```

However, you could implement your sealed class example by having a =
private constructor:

```
class Parent {
	final class Child : Parent {
		override init() { //within lexical scope of Parent, so =
can see Parent's private members
			super.init()
		}
	}
=09
	private init() {}
}

let parent =3D Parent() //'Parent' initializer is inaccessible due to =
'private' protection level
let child =3D Parent.Child() //ok
```

Swift also has a fileprivate visibility, so the same could be =
accomplished with a fileprivate init on Parent with a non-nested Child =
in the same file.

-John=