Swift weird protocols

December 10, 2017

What’s the difference between interface and abstract class? I have heard this question on almost every interview, when last time I have looked for a job. In Swift, protocols are very close to Java/C# interfaces, and the can’t contain implementations.

protocol IListener {
    
    var id : Int { get set }
    
    mutating func setId(newId : Int) {
        self.id = newId
    }
}

Here Xcode throws a error:
error: protocol methods may not have bodies

But, as it turns out, it is possible to make protocol extensions, and that extension can have implementation:

protocol IListener {
    
    var id : Int { get set }
}

extension IListener {
    
    mutating func setId(newId : Int) {
        self.id = newId
    }
}

I have no idea, why it is so.
Another strange protocol limitation is that, if it use self reference, it can be used only as a generic constraint. I.e., compilator deniedto compile this code:

protocol IListener : Hashable {

}

/* here some IListener implementation */

_ = Set<IListener> // error!

This is some strange things, that clearly demonstrated, that Swift is still in development mode. If extension protocol hack can be explained as some philosophy/architecture principle, but why protocol, that supposed to support hashing function, cannot be used as a container templat, is not clear for me.