[swift-evolution] [Pitch] Introducing role keywords to reduce hard-to-find bugs

[Robert Bennett]

I like this proposal a lot and also like Xiaodi’s suggestion that these be made into @ attributes.

> On Jun 14, 2017, at 1:48 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I like that this is minimal impact because it is optional. Since it is optional, I wonder if making it visually distinct from the mandatory keywords such as "override" for classes would be optimal, perhaps by making these @annotations.
> 
> Otherwise the naming is near perfect, IMO. It follows by analogy with `override` and `convenience`. There is one nit here that `override func` reads well in two senses: it's an overriding function with a certain signature _and_ it's overriding a function with that signature. Moreover, "override" can be read as a noun (a nouned verb, but plausibly a noun nonetheless), fitting in better with "convenience" and "default." By contrast, an `extend func` would be an extending function _but_ it's not extending a function, so it may read more awkwardly. It's also definitely not a noun. Might it be possible simply to reuse the keyword `extension` in this context?
> 
> As to the overall design, I like this much better than all previous alternatives, which are amply discussed in your proposal text.
> 
> 
>> On Wed, Jun 14, 2017 at 12:11 PM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
>> Some pals and I have been kicking an idea around about introducing better ways to support the compiler in protocol extensions. We want to eliminate some hard-to-detect bugs. We've been brainstorming on how to do this without affecting backward compatibility and introducing a minimal impact on keywords. 
>> 
>> We'd love to know what you think of our idea, which is to introduce "role" keywords. Roles allow the compiler to automatically check the intended use of a extension member definition against its protocol declarations, and emit errors, warnings, and fixits as needed.  We think it's a pretty straightforward approach that, if adopted, eliminates an entire category of bugs.
>> 
>> The draft proposal is here: https://gist.github.com/erica/14283fe18254489c1498a7069b7760c4
>> 
>> Thanks in advance for your thoughtful feedback,
>> 
>> -- E
>> 
>> 
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