2.38.scm

(define nil '())
(define (fold-right op initial sequence)
  (if (null? sequence)
    initial
    (op (car sequence)
        (fold-right op initial (cdr sequence)))))

(define (map p sequence)
  (fold-right (lambda (x y)
                (cons (p x) y))
              nil
              sequence))

(define (fold-left op initial sequence)
  (define (iter result rest)
    (if (null? rest)
      result
      (iter (op result (car rest))
            (cdr rest))))
  (iter initial sequence))