[swift-evolution] [PROPOSAL]Return subclass type to a protocol where a superclass is defined without the need for associatedtype

[Ross O'Brien]

As of Swift 2.2, if a variable has a closure type of e.g. () -> Shape, a
closure of type () -> Circle would be considered a match.  If a class
implements 'func make() -> Shape', a subclass implementing 'func make() ->
Circle' has to override. However, if a protocol requires a 'func make() ->
Shape', a type implementing 'func make() -> Circle' isn't considered to be
conforming. That does seem strange.

On Mon, Apr 18, 2016 at 12:55 AM, Yogev Sitton via swift-evolution <
swift-evolution at swift.org> wrote:

> In what way?
>
>
>
>
>
> On Sun, Apr 17, 2016 at 3:53 PM -0700, "Howard Lovatt" <
> howard.lovatt at gmail.com> wrote:
>
> I think this should be part of completing generics.
>>
>> On Sunday, 17 April 2016, Yogev Sitton via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> I have a class that is conforming to a protocol with a method that
>>> requires a specific return type.
>>> In case I want to return a subclass of the return type I am forced to
>>> use an associatedtype that feels like a hack.
>>>
>>> As an example:
>>>
>>> *protocol MyShapeProtocol {*
>>> *    func make() -> Shape?*
>>> *}*
>>>
>>> *class Circle : Shape {}*
>>>
>>> *class CircleMaker : MyShapeProtocol{*
>>> *    func make() -> Circle? {*
>>> *        return Circle()*
>>> *    }*
>>> *}*
>>>
>>> This will not work.
>>> For that to work I’ll need to use toe associatedtype “hack”:
>>>
>>> *protocol MyShapeProtocol {*
>>> *    associatedtype ShapeReturnType : Shape*
>>> *    func make() -> ShapeReturnType?*
>>> *}*
>>>
>>> *class Circle : Shape {}*
>>>
>>> *class CircleMaker : MyShapeProtocol{*
>>> *    func make() -> Circle? {*
>>> *        return Circle()*
>>> *    }*
>>> *}*
>>>
>>> Is there a real value in adding the associatedtype line?
>>>
>>
>>
>> --
>> -- Howard.
>>
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160418/c771765b/attachment.html>