Swift 5 Brand New Result Type: Write Cleaner API Code

 struct Course: Decodable {
    let id: Int
    let name: String
    let imageUrl: String
    let number_of_lessons: Int
  }

  func start() {

    fetchCoursesJSON { (res) in
      switch res {
      case .success(let courses):
        courses.forEach({ (course) in
          print(course.name)
        })
      case .failure(let err):
        print("failed", err)
      }
    }
  }

  func fetchCoursesJSON(completion: @escaping (Result<[Course], Error>) -> ()) {
    let urlString = ""
    guard let url = URL(string: urlString) else { return }

    URLSession.shared.dataTask(with: url) { (data, resp, err) in

      if let err = err {
        completion(.failure(err))
        return
      }

      // successful
      do {
        let courses = try JSONDecoder().decode([Course].self, from: data!)
        completion(.success(courses))
      } catch let jsonError {
        completion(.failure(jsonError))
      }

    }.resume()

  }