[swift-evolution] Why can't protocols define constants or subtypes?

[Charles Srstka via swift-evolution]

With a class or a struct, I can nest constants and subtypes within the type, 
like so:

class MyWidget {
        static let SomeConstant = “This string is useful to subclasses of 
MyWidget somehow"
        
        enum Type {
                case Cog
                case Sprocket
        }

        enum Error {
                case SomethingWentWrong
                case SomethingElseWentWrong
        }

        var type: Self.Type // This doesn’t work yet, but it should after 
SE-0068 is implemented

        func doSomething() throws // might throw one of the errors above
}

This is nice for namespacing, and it makes the code clear and organized.

However, if I convert this to a protocol-based approach, I have to do this the 
old Objective-C way:

static let MyWidgetSomeConstant = “This string is useful to implementers of 
MyWidget somehow"

enum MyWidgetType {
        case Cog
        case Sprocket
}

enum MyWidgetError {
        case SomethingWentWrong
        case SomethingElseWentWrong
}

protocol MyWidget {
        var type: MyWidgetType

        func doSomething() throws // might throw one of the errors above
}

This is ugly, and pollutes the global namespace with things that are only 
interesting to users and implementers of MyWidget. Is there a reason behind 
this? It seems that it could be useful to define constants and subtypes that 
could be useful for implementers of a protocol.

Charles

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