SpeakProtocol.swift

protocol SpeakProtocol {
    func speak() -> String
    //    func action() -> String
}

extension SpeakProtocol {
    func speak() -> String {
        print("SpeakProtocol's speak")
        return "I am anything"
    }
    func action() -> String {
        print("SpeakProtocol's action")
        return speak()
    }
}

struct Human: SpeakProtocol {
    func speak() -> String {
        print("Human's speak")
        return "I am human"
    }
    func action() -> String {
        print("Human's action")
        return speak()
    }
}

struct Devil: SpeakProtocol {
    func speak() -> String {
        print("Devil's speak")
        return "I am devil"
    }
}

print("======== human")

let human = Human()
human.speak()
human.action()

print("======== protocol")

let p = Devil()
p.speak()
p.action()

print("======== generic function")

func speakAndAction<T: SpeakProtocol>(speaker: T) -> String {
   return (speaker.action())
}

speakAndAction(human)
speakAndAction(p)