LISP 动态函数？

Question

我正在尝试自学一些 LISP，虽然我了解其中的大部分内容，但我无法掌握 eval 函数。我知道它已经为我们做了它，而且它不好用（所以我听说），但是我如何制作一个只是添加的功能？

到目前为止，我一直在尝试/思考

(setf input-prompt "Enter addition epression: ")
(setf output-prompt "The value is: ")

(defun prompt-for-input (msg)
  (format t msg))


(defun sum (expression)
  (format t "Summing ~d and ~d.~%" x y)
  (+ x y))


(defun add ()
  (prompt-for-input input-prompt)
  (let ((expression (read)))
       ((sum (expression)))
  (add)))

不太确定在哪里进行此操作，感谢您提供任何帮助。

Answer 1

(setf input-prompt "Enter addition expression: ")
(setf output-prompt "The value is: ")

(defun prompt-for-input (msg)
  (format t msg)
  (finish-output))

(defun sum (expression)
  (let ((x (second expression))
        (y (third expression)))
    (format t "~%Summing ~d and ~d.~%" x y)
    (+ x y)))

(defun add ()
  (prompt-for-input input-prompt)
  (sum (read)))

运行它：

CL-USER > (add)
Enter addition expression: (+ 1 2)
Summing 1 and 2.
3