需要了解带递归的LISP程序

Question

(defun help(a x)
  (if (null x) nil
    (cons (cons a (car x)) (help a (cdr x)))))

(defun partition(x)
    (if (null x) '(nil)
    (append (help (car x) (partition(cdr x))) (partition(cdr x)))))

这是这样的:(partition '(a b)) -> ((A B) (A) (B) NIL) 我试图了解它是如何工作的.有人可以说明结果是什么吗？我试着按照代码写下来,但是我失败了.

Answer 1

该trace函数允许您在LISP REPL中可视化函数调用.

示例输出 sbcl

* (defun help(a x)
  (if (null x) nil
    (cons (cons a (car x)) (help a (cdr x)))))

HELP
* (defun partition(x)
  (if (null x) '(nil)
    (append (help (car x) (partition(cdr x))) (partition(cdr x)))))

PARTITION
* (trace help)

(HELP)
* (trace partition)

(PARTITION)
* (partition '(a b))
  0: (PARTITION (A B))
    1: (PARTITION (B))
      2: (PARTITION NIL)
      2: PARTITION returned (NIL)
      2: (HELP B (NIL))
        3: (HELP B NIL)
        3: HELP returned NIL
      2: HELP returned ((B))
      2: (PARTITION NIL)
      2: PARTITION returned (NIL)
    1: PARTITION returned ((B) NIL)
    1: (HELP A ((B) NIL))
      2: (HELP A (NIL))
        3: (HELP A NIL)
        3: HELP returned NIL
      2: HELP returned ((A))
    1: HELP returned ((A B) (A))
    1: (PARTITION (B))
      2: (PARTITION NIL)
      2: PARTITION returned (NIL)
      2: (HELP B (NIL))
        3: (HELP B NIL)
        3: HELP returned NIL
      2: HELP returned ((B))
      2: (PARTITION NIL)
      2: PARTITION returned (NIL)
    1: PARTITION returned ((B) NIL)
  0: PARTITION returned ((A B) (A) (B) NIL)
((A B) (A) (B) NIL)

除此之外,我不完全确定如何获得更多帮助.