Fold Left and Fold Right

collection_operations.scm

; 2.38

(define (fold-right op initial sequence)
  (accumulate op initial 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))

(fold-right / 1 (list 1 2 3)) ;--------> 1 1/2
(fold-left / 1 (list 1 2 3))  ;--------> 1/6

(fold-right list null (list 1 2 3)) ;--> '(1 (2 (3 ())))
(fold-left list null (list 1 2 3))  ;--> '(((() 1) 2) 3)

; 2.39

(define (reverse sequence)
  (fold-right (lambda x y) (append y (list x)) null sequence))

(define (reverse sequence)
  (fold-left (lambda x y) (cons (y x)) null sequence))