Say I'm writing a library to send / receive a given request / response.

I had the thought of doing something like:

protocol Request {
associatedtype ResponseType: Response
}

protocol Response {}

With this, you can do things like:

func request<T: Request>(_ r: T) -> T.ResponseType { }

It ends up being quite nice because you keep a strong association between the Request and Response types (and can do tricks w/ Codable).

I can't help but feeling there's a way to make this nicer… any ideas?