;;; extracted  from w3-4.0pre.46/lisp/url.el (last part) 2001-08-17
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;;; How to register a protocol
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(defun url-register-protocol (protocol &optional retrieve expander defport)
  "Register a protocol with the URL retrieval package.
PROTOCOL is the type of protocol being registers (http, nntp, etc),
         and is the first chunk of the URL.  ie: http:// URLs will be
         handled by the protocol registered as 'http'.  PROTOCOL can
         be either a symbol or a string - it is converted to a string,
         and lowercased before being registered.
RETRIEVE (optional) is the function to be called with a url as its
         only argument.  If this argument is omitted, then this looks
         for a function called 'url-PROTOCOL'.  A warning is shown if
         the function is undefined, but the protocol is still
         registered.
EXPANDER (optional) is the function to call to expand a relative link
         of type PROTOCOL.  If omitted, this defaults to
         `url-default-expander'

Any proxy information is read in from environment variables at this
time, so this function should only be called after dumping emacs."
  (let* ((protocol (cond
		    ((stringp protocol) (downcase protocol))
		    ((symbolp protocol) (downcase (symbol-name protocol)))
		    (t nil)))
		     
	 (retrieve (or retrieve (intern (concat "url-" protocol))))
	 (expander (or expander 'url-default-expander))
	 (cur-protocol (assoc protocol url-registered-protocols))
	 (urlobj nil)
	 (cur-proxy (assoc protocol url-proxy-services))
	 (env-proxy (or (getenv (concat protocol "_proxy"))
			(getenv (concat protocol "_PROXY"))
			(getenv (upcase (concat protocol "_PROXY"))))))

    (if (not protocol)
	(error "Invalid data to url-register-protocol."))
    
    (if (not (fboundp retrieve))
	(message "Warning: %s registered, but no function found." protocol))

    ;; Store the default port, if none previously specified and
    ;; defport given
    (if (and defport (not (assoc protocol url-default-ports)))
	(setq url-default-ports (cons (cons protocol defport)
				      url-default-ports)))
    
    ;; Store the appropriate information for later
    (if cur-protocol
	(setcdr cur-protocol (cons retrieve expander))
      (setq url-registered-protocols (cons (cons protocol
						 (cons retrieve expander))
					   url-registered-protocols)))

    ;; Store any proxying information - this will not overwrite an old
    ;; entry, so that people can still set this information in their
    ;; .emacs file
    (cond
     (cur-proxy nil)			; Keep their old settings
     ((null env-proxy) nil)		; No proxy setup
     ;; First check if its something like hostname:port
     ((string-match "^\\([^:]+\\):\\([0-9]+\\)$" env-proxy)
      (setq urlobj (url-generic-parse-url nil)) ; Get a blank object
      (url-set-type urlobj "http")
      (url-set-host urlobj (url-match env-proxy 1))
      (url-set-port urlobj (url-match env-proxy 2)))
     ;; Then check if its a fully specified URL
     ((string-match url-nonrelative-link env-proxy)
      (setq urlobj (url-generic-parse-url env-proxy))
      (url-set-type urlobj "http")
      (url-set-target urlobj nil))
     ;; Finally, fall back on the assumption that its just a hostname
     (t
      (setq urlobj (url-generic-parse-url nil)) ; Get a blank object
      (url-set-type urlobj "http")
      (url-set-host urlobj env-proxy)))

     (if (and (not cur-proxy) urlobj)
	 (progn
	   (setq url-proxy-services
		 (cons (cons protocol (concat (url-host urlobj) ":"
					      (url-port urlobj)))
		       url-proxy-services))
	   (message "Using a proxy for %s..." protocol)))))