An Implementation Model for Rational Protocol Conformance Behavior

[Douglas_Gregor]

@mattrips

I want to stay focussed on the question you pose about how we think it ought to work: "Is that really the interpretation we want?" I suppose my answer is, I could go either way on the interpretation. But, whatever the interpretation, I think it ought to apply the same way to both your example and my example.

In your example, when Array conforms to P, the Array version of whatAmI() is the most specialized already, so you get the right answer. Add this to your example and it will better parallel @jrose's:

extension Array where Element: P {
  func whatAmI() { print("Array<P>") }
}
extension Int: P {
  func whatAmI() { print("Int") }
}

The output will be:

Array<P>
Array

That's the one that bothers me most. A protocol requirement is meant to be a point of dynamic dispatch, where you jump to a version of the code that's been customized for the concrete type... but we don't actually get all of the customization we want (or expect).

I'm less bothered by @jrose's example because I don't feel like a call to a function that is neither an overridable method in a class or a protocol requirement should allow different behavior with different types at runtime.

I appreciate that we're gathering a corpus of examples and opinions. One needs those to evaluate designs.

Doug