Question

在此站点：http://www.gigamonkeys.com/book/practical-a-simple-database.html，用户输入功能如下所示：

(defun prompt-read (prompt)
    (format *query-io* "~%~a: " prompt)  
    (force-output *query-io*)
    (read-line *query-io*))

与以下更简单的形式相比，上述功能是否有任何主要优点：

(defun prompt-read2 (prompt)
    (format t "~%~a: " prompt)
    (setf answer (read-line)))

建议始终始终使用force-output和*query-io*吗？

Answer 1

设置这样的全局变量的答案是不好的。你应该只是回答答案，让调用者用它做你想做的事。如果你使用特殊（〜全局）变量，你应该在名称（*ANSWER*而不是ANSWER）周围添加星号。

需要

FORCE-OUTPUT以确保用户在必须回答之前实际看到提示。如果我在终端中使用SBCL运行第二个版本，程序就会冻结等待输入而不说任何内容。

应该使用

*QUERY-IO*来查询用户的内容，因为某些环境可能希望以不同于其他输出的方式处理。例如，有人可能会为您的程序编写一个GUI包装器，将查询转换为图形对话框。或者他们可能希望将其作为脚本的一部分运行，从字符串提供输入。

(defun prompt-read (prompt)
  (format *query-io* "~%~a: " prompt)  
  (force-output *query-io*)
  (read-line *query-io*))

(defun hello ()
  (format t "~&Hello ~a!~%" (prompt-read "What's your name")))

(defmacro with-input ((input) &body body)
  `(let ((*query-io* (make-two-way-stream (make-string-input-stream ,input)
                                          (make-string-output-stream))))
     ,@body))

(defun test ()
  (with-input ("jkiiski")
    (hello))
  (with-input ("rnso")
    (hello)))
(test)
; Hello jkiiski!
; Hello rnso!

修改

使用SBCL灰色流的更复杂的示例。

(defclass foo-stream (sb-gray:fundamental-character-input-stream) ((output-input-script :initarg :script :accessor foo-stream-script) (output-stream :initarg :out :accessor foo-stream-out) (current-input :initform nil :accessor foo-stream-current-input))) (defmethod sb-gray:stream-read-char ((stream foo-stream)) (with-accessors ((input foo-stream-current-input) (out foo-stream-out) (script foo-stream-script)) stream (when (or (null input) (not (listen input))) (let ((output (string-trim '(#\space #\newline) (get-output-stream-string out)))) (setf input (make-string-input-stream (format nil "~a~%" (cdr (assoc output script :test #'string=))))))) (read-char input))) (defun prompt-read (prompt) (format *query-io* "~%~a: " prompt) (force-output *query-io*) (read-line *query-io*)) (defun hello () (format t "~&Hello ~a!~%" (prompt-read "What's your name")) (format t "~&I'm ~a too!" (prompt-read "How are you")) (format t "~&~a~%" (if (string-equal (prompt-read "Do you want to delete all your files") "yes") "Deleting all files... (not really)" "Not deleting anything."))) (defmacro with-input-script ((script) &body body) (let ((out-sym (gensym "out"))) `(let* ((,out-sym (make-string-output-stream)) (*query-io* (make-two-way-stream (make-instance 'foo-stream :out ,out-sym :script ,script) ,out-sym))) ,@body))) (defun test () (with-input-script ('(("What's your name:" . "jkiiski") ("How are you:" . "great") ("Do you want to delete all your files:" . "No"))) (hello)) (with-input-script ('(("What's your name:" . "Foo Bar") ("How are you:" . "fine") ("Do you want to delete all your files:" . "Yes"))) (hello))) (test) ; Hello jkiiski! ; I'm great too! ; Not deleting anything. ; Hello Foo Bar! ; I'm fine too! ; Deleting all files... (not really)

Answer 2

是的，你的代码很容易，但第一个更清楚你在做什么：

* query-io *是一个全局变量（由于全局变量的*命名约定，你可以告诉它）包含输入流连接到终端。 prompt-read的返回值将是最后一个表单的值，对READ-LINE的调用，返回它读取的字符串（没有尾随换行符。）

这就是他们所说的* query-io *

关于你可以放在那里的溪流如下：

大多数其他I / O函数也接受T和NIL作为流指示符但具有不同的含义：作为流指示符，T表示双向流* TERMINAL-IO *，而NIL指定 * STANDARD-OUTPUT *作为输出流，* STANDARD-INPUT *作为输入流

在这种情况下，它似乎只指向* standard-input *而不指向双向流t

Lisp中的简单vs复杂用户输入函数

2 个答案: