如何在CLI程序中正确退出循环并避免终端重复提示符

我在CLI程序中为用户提供三个选项，希望菜单保持开启直到用户输入“e”退出。我的实现代码如下：

(format t "~%(E)xit program - (M)ore details - enter (C)ustom data~%")
(loop
  (setq what-to-do-next (read-char))
  (cond ((char-equal what-to-do-next #\m)
         (format t "more details~%"))
        ((char-equal what-to-do-next #\c)
         (format t "custom data~%"))
        ((char-equal what-to-do-next #\e)
         ()))
  (when (char-equal what-to-do-next #\e)
    (return nil)))

程序可以运行，但结束后出现了重复的终端提示符：

(E)xit program - (M)ore details - enter (C)ustom calendar
e
[user@host ~]$ 
[user@host ~]$ 

我尝试添加退出变量的方式仍未解决问题：

(format t "~%(E)xit program - (M)ore details - enter (C)ustom data~%")
(setq exit 0)
(loop
  (setq what-to-do-next (read-char))
  (cond ((char-equal what-to-do-next #\m)
         (format t "more details~%"))
        ((char-equal what-to-do-next #\c)
         (format t "custom data~%"))
        ((char-equal what-to-do-next #\e)
         (setq exit 1)))
  (when (= exit 1)
    (return nil)))

请问我遗漏了什么，该如何避免这种重复提示符的问题？

问题根源是：当你输入e并按下回车时，read-char仅读取了e字符，输入缓冲区里还残留着换行符\n。程序退出后，shell读取到这个残留的换行符，会误以为用户又敲了一次回车，因此输出重复的提示符。

以下是两种可行的解决方式：

方式一：退出前读取残留的换行符 #

在检测到e时，额外读取一次缓冲区里的换行符：

(format t "~%(E)xit program - (M)ore details - enter (C)ustom data~%")
(loop
  (setq what-to-do-next (read-char))
  (cond ((char-equal what-to-do-next #\m)
         (format t "more details~%"))
        ((char-equal what-to-do-next #\c)
         (format t "custom data~%"))
        ((char-equal what-to-do-next #\e)
         (read-char)) ; 读取残留的换行符
        )
  (when (char-equal what-to-do-next #\e)
    (return nil)))

方式二：清空输入缓冲区所有残留字符 #

如果用户可能输入多个字符再回车，这种方式更稳妥，确保缓冲区完全清空：

(format t "~%(E)xit program - (M)ore details - enter (C)ustom data~%")
(loop
  (setq what-to-do-next (read-char))
  (cond ((char-equal what-to-do-next #\m)
         (format t "more details~%"))
        ((char-equal what-to-do-next #\c)
         (format t "custom data~%"))
        ((char-equal what-to-do-next #\e)
         (progn
           ;; 循环读取并丢弃所有剩余缓冲字符
           (loop while (listen) do (read-char))
           (return nil)))
        ;; 忽略无效字符和换行
        (t nil)))

处理后输入缓冲区无残留字符，shell就不会输出重复的提示符了。

内容的提问来源于stack exchange，提问作者dpo