#lang sicp

;; Copied/useful defns
(define (square x) (* x x))
(define (echo x) (display x) (newline) x)
(define (accumulate op initial sequence)
  (if (null? sequence)
      initial
      (op (car sequence)
          (accumulate op
                      initial
                      (cdr sequence)))))

;; Problem
(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) (+ y 1)) 0 sequence))

(map square (list 1 2 3 4))
(append (list 1 2 3 4) (list 5 6 7 8))
(length (list 1 2 3 4 5 6 7 8))