How to implement Marker protocol in swift

marker.swift

import UIKit

protocol Paper {}

class APaper: Paper {

    var a: String
    var text: String

    init(a: String, text: String) {
        self.a = a
        self.text = text
    }
}

class BPaper: Paper {

    var b: String
    var text: String

    init(b: String, text: String) {
        self.b = b
        self.text = text
    }
}

struct Box {

    var title: String
    var text: String

    static let none = Box(title: "", text: "")
}

protocol Reader {

    associatedtype Item

    init()

    func wrapping(paper: Item) -> Box
}

struct APaperReader: Reader {
    
    init() { }
    
    func wrapping(paper: APaper) -> Box {
        return Box(title: paper.a, text: paper.text)
    }
}

struct BPaperReader: Reader {
    
    init() { }
    
    func wrapping(paper: BPaper) -> Box {
        return Box(title: paper.b, text: paper.text)
    }
}

func wrapping<T: Reader>(ofType: T.Type, paper: T.Item) -> Box where T.Item == APaper {
    return ofType.init().wrapping(paper: paper)
}

func wrapping<T: Reader>(ofType: T.Type, paper: T.Item) -> Box where T.Item == BPaper {
    return ofType.init().wrapping(paper: paper)
}

let aPaper = APaper(a: "a", text: "ATEST")
let aBox = wrapping(ofType: APaperReader.self, paper: APaper(a: "a", text: "ATEST"))
print(aBox.title, aBox.text)

let bPaper = BPaper(b: "b", text: "BTEST")
let bBox = wrapping(ofType: BPaperReader.self, paper: BPaper(b: "b", text: "BTEST"))
print(bBox.title, bBox.text)