static func getUsers(completionHandler: @escaping (_ users: [User]?, _ error: Error?) -> Void) {
  let endpointUrl = URL(string: host + listUsersPath)!
  let urlRequest = URLRequest(url: endpointUrl)
  let task = URLSession.shared.dataTask(with: urlRequest) { (data, response, error) in
    guard error == nil else {
      print("error calling GET on \(listUsersPath): \(error!)")
      DispatchQueue.main.async {
        completionHandler(nil, error)
      }
      return
    }

    let usersData = try! JSONSerialization.jsonObject(with: data!, options: []) as! [Any]
    var users = [User]()
    for userData in usersData {
      let userDict = userData as! Dictionary<String, Any>
      let name = userDict["name"] as! String
      let id = userDict["id"] as! Double
      let user = User(id: id, name: name)
      users.append(user)
    }
    DispatchQueue.main.async {
      completionHandler(users, nil)
    }
  }
  task.resume()
}