从字符串中读取和写入整个文件

以下函数将整个文件读入新字符串并返回：

(defun read-file (infile)
  (with-open-file (instream infile :direction :input :if-does-not-exist nil)
    (when instream 
      (let ((string (make-string (file-length instream))))
        (read-sequence string instream)
        string))))

如果文件不存在，结果是 NIL。

以下函数将字符串写入文件。关键字参数用于指定文件已存在时要执行的操作（默认情况下会导致错误，允许的值是 with-open-file 宏的值）。

(defun write-file (string outfile &key (action-if-exists :error))
   (check-type action-if-exists (member nil :error :new-version :rename :rename-and-delete 
                                        :overwrite :append :supersede))
   (with-open-file (outstream outfile :direction :output :if-exists action-if-exists)
     (write-sequence string outstream)))

在这种情况下，write-sequence 可以用 write-string 代替。