element-of-set-p: Symbol’s value as variable is void: false

Question

I minic a intersction-set operation from sicp

#+name: case-2.3.3-element-of-set-p
#+BEGIN_SRC elisp :session sicp :results value
(defun element-of-set-p(x set)
  (cond ((null set) false)
        ((equal x (car set)) t)
        (t (element-of-setp x (cdr set)))))
(element-of-set-p 1 (list 1 2 3))
#+END_SRC

#+RESULTS: case-2.3.3-element-of-set-p
: t

and the intersection-set

#+BEGIN_SRC elisp :session sicp :results value
(defun intersection-set (set1 set2)
  (cond ((or (null set1) (null set2))
         '())
        ((element-of-set-p (car set1) set2)
         (cons (car set1)
               (intersection-set (cdr set1)
                                 set2)))
        (t (intresection-set (cdr set1)
                                set2))))
(trace-function #'intersection-set)
(intersection-set  (list 1 2) (list 2 3 4))
#+END_SRC

It report error:

 element-of-set-p: Symbol’s value as variable is void: false

What's the problem?

Answer 1

You have a couple of typos in your code:

• It's not element-of-setp, it's element-of-set-p
• It's not intresection-set, it's intersection-set
• It's not false, it's nil

This should work:

(defun element-of-set-p (x set)
  (cond ((null set) nil)
        ((equal x (car set)) t)
        (t (element-of-set-p x (cdr set)))))

(defun intersection-set (set1 set2)
  (cond ((or (null set1) (null set2))
         '())
        ((element-of-set-p (car set1) set2)
         (cons (car set1)
               (intersection-set (cdr set1) set2)))
        (t (intersection-set (cdr set1) set2))))