;; define foldl using foldr

;; foldl-original : [X Y -> Y] Y [X] -> Y
(define (foldl-original f acc lst)
  (cond
    [(empty? lst) acc]
    [(cons? lst)
     (foldl f
            (f (first lst) acc)
            (rest lst))]))

;; define foldl using foldr
;; foldl^ : [X Y -> Y] Y [X] -> Y
(define (fold^ f acc ls) ...)

(check-expect (foldl^ + 0 (list 1 2 3 4 5)) 15)
(check-expect (foldl^ cons '() '(a b c)) '(c b a))