Generic protocols.swift

// Siesta’s Resource class represents the current “best information available”
// about a RESTful resource. Other parts of the code can observe changes to
// that state (started loading, received new data, received error, etc):

public protocol ResourceObserver
    {
    func resourceChanged(_ resource: Resource, event: ResourceEvent)
    // (plus other stuff I’m ignoring)
    }

class HappyControllerDoodad: ResourceObserver
    {
    func resourceChanged(_ resource: Resource, event: ResourceEvent)
        { … }
    }

// In a future version of Siesta, I’d like to add a type param to Resource
// that describes what kind of content/model it holds, say for example
// Resource<Article> vs Resource<Comment>. With generic protocols, I imagine
// one could do this:

class HappyControllerDoodad: ResourceObserver<Article>, ResourceObserver<Comment>
    {
    func resourceChanged(_ resource: Resource<Article>, event: ResourceEvent)
        { /* update article view */ }

    func resourceChanged(_ resource: Resource<Comment>, event: ResourceEvent)
        { /* update comment view */ }
    }

// Associated types prevent this kind of multiple conformance, of course.
// With generalized existentials, this alternative would still be possible:

class HappyControllerDoodad: ResourceObserver<Article>, ResourceObserver<Comment>
    {
    func resourceChanged(_ resource: Resource, event: ResourceEvent)
        {
        switch resource
            {
            case is Resource<Article>:
                // update article view
            case is Resource<Comment>:
                // update article view
            default:
                fatalError("What is this madness?!")
            }
        }
    }

// …but that's markedly inferior to my eyes.
//
// HOWEVER
//
// The reason I haven’t already done this whole Resource<T> thing is that
// without generalized existentials it's … well, not technically impossible,
// but just too stupidly awkward in practice. I tried it, and it’s not pretty.
//
// So even though I'd like generic protocols, I want generalized existentials
// far, far, far more.