求 v 友教下怎么 debug lisp 程序

[scalaer]

最近在看 sicp， 发现 lisp 挺有意思的.

刚刚试着写 leetcode 第二题的答案， 求 v 友帮忙看下那里有问题， 顺便教下怎么 debug

(define (extract p)
        (if (null? p)
            0
            (car(p))))

(define (add x y) (
        (+ (extract x)
           (extract y)
        )
))


(define (add2list l1 l2)
    (if (and (null? l1) (null? l2))
        ()
        (cons(
            (add l1 l2)
            (add2list (cdr l1) (cdr l2))))
))
    

用的mit-schema解释器

2 error> (add2list (list 1 2 3) (list 2 3 4))

;The object (4) is not applicable.
;To continue, call RESTART with an option number:
; (RESTART 3) => Specify a procedure to use in its place.
; (RESTART 2) => Return to read-eval-print level 2.
; (RESTART 1) => Return to read-eval-print level 1.

---

输入(debug)

3 error> (debug)

There are 26 subproblems on the stack.

Subproblem level: 0 (this is the lowest subproblem level)
Expression (from stack):
    ('(4))
There is no current environment.
The execution history for this subproblem contains 1 reduction.
You are now in the debugger.  Type q to quit, ? for commands.

[xrlin]

```scheme
(define (extract p)
(if (null? p)
0
(car p)))

(define (add x y)
(+ (extract x)
(extract y)
)
)


(define (add2list l1 l2)
(if (and (null? l1) (null? l2))
()
(cons
(add l1 l2)
(add2list (cdr l1) (cdr l2)))
))```

[xrlin]

括号表达式内会进行求值，scheme 的 callable function 放在括号第一个参数，不像其它语言是括号在方法名后面。

[scalaer]

@xrlin 多谢解答🙏