[swift-evolution] [RFC] #Self

[Matthew Johnson]

Sent from my iPad

> On May 10, 2016, at 5:46 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> On Tue, May 10, 2016 at 5:24 PM, Hooman Mehr via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>>>> On May 10, 2016, at 2:49 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>>> That said, I’m not sure I understand the concrete use-cases.  When is this concept important?  When is “Self” not good enough?
>>> 
>>> The only case where there is new functionality is when this is used in a protocol requirement.  I gave an example earlier today.  
>> 
>> This functionality is the key: Ability of an open (non-final) class to conform to a protocol that lets it return an instance of the conforming type (itself). Self does not work for that and we can’t change its behavior (or can we?) So one solution seems to be Matt’s proposal. This functionality is important for me and an example use case is class clusters. For the client code it is sealed and acts just like a final class, but internally it may return a subclass that is an implementation detail. We should be able to do this.
> 
> Help me understand this. Maybe an example will help. Why is it a problem to return the subclass instead of the base class?
>  

The problem is that there is no way to guarantee that all subclasses of a non-final class provide the necessary override.  See the NSURL example I posted earlier today.

>> 
>> Hooman
>> 
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