構造体にjsonメソッドとjsonプロパティを拡張すると便利？？？？

import UIKit

protocol Json : Codable {
    func json(key: String) -> String!
    var json:String! { get }

}

extension Json {
    func json(key: String) -> String! {
        let encoder = JSONEncoder()
        encoder.dateEncodingStrategy = .iso8601
        encoder.outputFormatting = .prettyPrinted
        guard let data = try? encoder.encode([key : self]) else {
            return nil
        }
        return String(data: data, encoding: .utf8)!
    }
}

extension Json {
    var json: String! {
        let encoder = JSONEncoder()
        encoder.dateEncodingStrategy = .iso8601
        encoder.outputFormatting = .prettyPrinted
        guard let data = try? encoder.encode(self) else {
            return nil
        }
        return String(data: data, encoding: .utf8)!
    }
}

struct User: Json {
    let name: String
    let age: Int
}

struct Admin: Json {
    let user: User
    let permission: Int
}

let user = User(name: "田中", age: 9)
let admin = Admin(user: user, permission: 5)

print(admin.json(key: "top") ?? "失敗")
print(admin.json ?? "top")