sicp exercise 3.13

; Exercise 3.13.  Consider the following make-cycle procedure, which uses the last-pair procedure defined in exercise 3.12:
;
; (define (make-cycle x)
;   (set-cdr! (last-pair x) x)
;   x)
;
; Draw a box-and-pointer diagram that shows the structure z created by
;
; (define z (make-cycle (list 'a 'b 'c)))
;
; What happens if we try to compute (last-pair z)?

(define (last-pair x)
  (if (null? (cdr x))
      x
      (last-pair (cdr x))))

(define (make-cycle x)
  (set-cdr! (last-pair x) x)
  x)

(define z (make-cycle (list 'a 'b 'c)))

(display z) (newline)

(display (last-pair z)) (newline)
;infinite loop