Solution for exercise 2.33 of SICP:

(define nil '()) (define (square x) (* x x)) (define (accumulate op initial sequence) (if (null? sequence) initial (op (car sequence) (accumulate op initial (cdr sequence))))) (define (map p sequence) (accumulate (lambda (x y) (cons (p x) y)) nil sequence)) (define (append seq1 seq2) (accumulate cons seq2 seq1)) (define (length sequence) (accumulate (lambda (x y) (+ 1 y)) 0 sequence)) ;--------------------tests------------------------- (map square (list 1 2 3)) (append (list 1 2 3) (list 4 5 6)) (length (list 5 6 7 8 9))