[swift-evolution] Keyword for protocol conformance

[Charles Srstka]

> On Aug 25, 2016, at 10:24 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> Yes, and it's covered in those previous threads. In brief, not all retroactive modeling involves extending a type:
> 
> * A vendor supplies you with a closed-source library with a struct S that conforms to standard library protocol P.
> 
> * Protocol P requires a method named foo(), so struct S has its own implementation of foo().
> 
> * You extend protocol P by adding a new default implementation of foo().
> 
> This cannot be done if a keyword is required for overriding a default implementation. There is also nowhere for you to append any sort of "retro" keyword anywhere, because no part of your own code extends S in any way.
> 
> Please, please take the time to study the previous threads; we should not be re-playing existing discussions four or five times on this list.

How is this a problem, though? The implementation of foo() in S will not break, because it’s already been compiled in. The change will only affect code that is subsequently compiled with your extension of P in place. And it’ll be doing its job; making sure that your code to implement the protocol is there deliberately, and preventing you from accidentally mistyping the method name and thus silently failing to override the default implementation.

Charles

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