sicp_2.2.rkt

#lang scheme
(define (length items)
  (if (null? items)
      0
      (+ 1 (length (cdr items)))))

(define (count-leaves x)
  (cond ((null? x) 0)
        ((not (pair? x)) 1)
        (else (+ (count-leaves (car x))
                 (count-leaves (cdr x))))))

(define (map proc items)
  (if (null? items)
      nil
      (cons (proc (car items))
            (map proc (cdr items)))))