[swift-evolution] Keyword for protocol conformance

[Charles Srstka]

> On Aug 22, 2016, at 5:41 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> There's been agreement even from the core team that the quality of diagnostics when conforming to a protocol is sub-par.
> 
> The modified rule you propose has also been suggested before. The reason it doesn't help is that (1) if a method signature is mismatched accidentally due to a typo, you get a compilation error already because your type doesn't conform to the protocol (it's the quality of the error message that needs improvement);

You don’t get any error at all if there’s a default value.

protocol P {
	func doSomething()
}

extension P {
	func doSomething() { print(“Do This Thing”) }
}

struct S: P {
	func doSomthing() { print(“Do This Instead”) } // Whoops, doesn’t get called. And we don’t find out until mysterious behavior occurs at runtime.
}

If there were a way to tell the compiler that the function was meant to satisfy a protocol, we could prevent the mistake above from occurring.

> (2) otherwise, if your type fulfills all protocol requirements but also implements an additional method unnecessary for conformance, what is the harm that is being prevented by a compiler error?

The fact that implementing protocol requirements that have default values is, in effect, stringly typed.

Charles

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