emacs 设置代理 toggle-url-proxy: Symbol’s value as variable is void: url-proxy-services

Emacs-general
1 
(defun toggle-url-proxy ()
  "Toggle proxy for the url.el library."
  (interactive)
  (cond
   (url-proxy-services
    (message "Turn off URL proxy")
    (setq url-proxy-services nil))
   (t
    (message "Turn on URL proxy")
    (setq url-proxy-services
          '(("http" . "localhost:7890")
            ("https" . "localhost:7890")
            ("no_proxy" . "0.0.0.0"))))))
(toggle-url-proxy)

在论坛上找的配置, 但是报错了. toggle-url-proxy: Symbol’s value as variable is void: url-proxy-services

2 
(require 'url)