(element? 8 (integers-from-to 1 15)) ⇒ #t
(element? 20 (integers-from-to 1 15))
Error: car expects argument of type <pair>; given ()
Q: What's the problem?
A: Forgot to deal with the empty list (the base case of the recursion).
Q: Can an item be an element of the empty list?
A: No.
(define element?
(lambda (item lst)
(cond ((null? lst) #f)
((equal? item (car lst)) #t)
(else (element? item (cdr lst))))))