LISP動態函數？

Question

我試圖教自己一些LISP，雖然我理解它的大部分，但我很難理解eval函數。我知道它已經爲我們做了，它不好用（所以我聽到），但我怎麼才能做出一個剛添加的功能？

到目前爲止，我試圖/想

(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))) 

不是真的知道該去哪裏這一點，任何幫助表示讚賞。

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))) 

運行：

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