[swift-evolution] [RFC] #Self

Matthew Johnson matthew at anandabits.com
Tue May 10 14:04:42 CDT 2016

> On May 10, 2016, at 1:38 PM, Vladimir.S <svabox at gmail.com> wrote:
> On 10.05.2016 20:50, Matthew Johnson wrote:
>> No, the whole point is that D.f() returns C because C is the requirement
>> of 'f' is declared to return #Self which is C where the protocol
>> conformance is declared and implemented.  If you want a covariant
>> requirement you would use Self as the return type, not #Self.
> I just followed your example with NSURL.. Probably I don't understand the point, but you said you want to conform a non-final class to protocol with method -> #Self.
> >---------------------<
> protocol A { static func createWithString(s: String) -> Self }
> extension NSURL: A { // cannot conform because NSURL is non-final }
> If we could define a protocol requirement that didn't covary (using
> #Self or whatever) we would be able to write the desired conformance.
> >---------------------<
> And I don't understand how do you want to achieve the target, as even if we 'invent' #Self, this (as I understand) can't work as class is not final. Just like with 'simple' Self - class must be final to conform. Thank you for clarification.

No, class would not need to be final to conform to a requirement returning #Self.  The difference is that with Self covaries and #Self does not.  This means you do not need to guarantee that all subclasses override a requirement that returns #Self, while you do need to guarantee that all subclasses override a requirement that returns Self.  

The requirement to guarantee that all subclasses provide this override is the reason you cannot declare conformance in the case of requirements returning Self.  Since this isn’t necessary for #Self (will probably have a different name) the class does not need to be final in order to conform.


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