为什么在 Lisp 中读取文件时 read-line 会运行两次？
这是用 lisp 实现“cat”命令的代码，如书中所述 ANSI Common Lisp，第 122 页。
(defun pseudo-cat (file)
(with-open-file (str file :direction :input)
(do ((line (read-line str nil 'eof)
(read-line str nil 'eof)))
((eql line 'eof))
(format t "~A~%" line))))
为什么 read-line 函数运行两次？我尝试只用一行读取来运行它，但 Lisp 无法完成代码。
This is the code to implement the 'cat' command with lisp, as is explained in the book ANSI Common Lisp, page 122.
(defun pseudo-cat (file)
(with-open-file (str file :direction :input)
(do ((line (read-line str nil 'eof)
(read-line str nil 'eof)))
((eql line 'eof))
(format t "~A~%" line))))
Why is the read-line function run twice? I tried to run it with only one read-line, but the Lisp couldn't finish the code.
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DO变量的语法为：变量、初始化形式、更新形式。在这种情况下，初始化形式与更新形式相同。但 DO 中没有这种情况的简写，所以你必须写两次。
The syntax of DO variables is: variable, initialization form, update form. In this case, the initialization form is the same as the update form. But there is no shorthand for that case in DO, so you have to write it out twice.
您需要阅读 DO 的语法： http://www.lispworks.com/ Documentation/HyperSpec/Body/m_do_do.htm
第一个 READ-LINE 形式是 init 形式，第二个是 step 形式。因此，在第一次迭代中，变量被设置为 init-form 的结果。在接下来的迭代中，变量被设置为步骤形式的值。
You need to read the syntax of DO: http://www.lispworks.com/documentation/HyperSpec/Body/m_do_do.htm
The first READ-LINE form is the init-form and the second is the step-form. So in the first iteration the variable is set to the result of the init-form. In the next iterations the variable is set to the value of the step-form.
您可以使用
(listen file)来测试是否可以读取该文件。这是我的打印文件功能
You can use
(listen file)for test if you can read from the file.This is my print-file function