Question

我正在尝试使用CLISP阅读一系列网页（如果存在），但我不明白open-http如何跳过不存在的网页。我有以下内容：

(dolist (word '(a b c))
  (with-open-stream (stream (ext:open-http
                              (format nil
                                      "https://en.wikipedia.org/wiki/~a.html"
                                      word)
                              :if-does-not-exist nil))
    (when stream
      (print word))))

我想简单地跳过一个网页，如果它不存在，但CLISP似乎挂起并返回“无效参数”错误。任何人都可以解释这个论点：if-does-not-exist是否有效和/或提供如何使用open-http的示例。谢谢！

Answer 1

它对我有用：

(with-open-stream (stream (ext:open-http
                           "http://stackoverflow.com/questions/234242424242"
                           :if-does-not-exist nil))
(format t "~&Stream: ~A~%" stream))

输出：

;; connecting to "http://stackoverflow.com/questions/234242424242"...connected...HTTP/1.1 404 Not Found
;; HTML source of Page not found
Stream: NIL
NIL

获得连接有延迟，但是有效。

如果页面确实存在：

[7]> (with-open-stream (stream (ext:open-http
                                "http://stackoverflow.com/questions/36003343/clisp-open-http-example"
                                :if-does-not-exist nil))
       (format t "~&Stream: ~A~%" stream))
;; connecting to "http://stackoverflow.com/questions/36003343/clisp-open-http-example"...connected...HTTP/1.1 200 OK
Stream: #<IO INPUT-BUFFERED SOCKET-STREAM CHARACTER stackoverflow.com:80>
NIL

使用Wikipedia我无法使其工作，因为Wikipedia.org将其重定向到HTTPS并且EXT:OPEN-HTTP既不能直接处理HTTPS，也不能处理重定向：

此处如果直接使用HTTPS：

[10]> (with-open-stream (stream (ext:open-http
                                 "https://en.wikipedia.org/wiki/Common_Lisp"
                                 :if-does-not-exist nil))
        (format t "~&Stream: ~A~%" stream))

*** - OPEN-HTTP: "https://en.wikipedia.org/wiki/Common_Lisp" is not an HTTP URL
The following restarts are available:
ABORT          :R1      Abort main loop
Break 1 [11]> :r1

如果＆＃34; https＆＃34;被＆＃34; http＆＃34;取代，CLISP没有构建一个合适的地址：

[12]> (with-open-stream (stream (ext:open-http
                                 "http://en.wikipedia.org/wiki/Common_Lisp"
                                 :if-does-not-exist nil))
        (format t "~&Stream: ~A~%" stream))
;; connecting to "http://en.wikipedia.org/wiki/Common_Lisp"...connected...HTTP/1.1 301 TLS Redirect --> "https://en.wikipedia.org/wiki/Common_Lisp"
;; connecting to "http://en.wikipedia.orghttps://en.wikipedia.org/wiki/Common_Lisp"...
*** - PARSE-INTEGER: substring "" does not have integer syntax at position 0
The following restarts are available:
ABORT          :R1      Abort main loop
Break 1 [13]>

CLISP open-http示例

1 个答案: