;; rev-list-using-accum : (listof X) -> (listof X)
;; reverses a list
;; uses accumulator style
(define (rev-list-using-accum l)
  {local ; accum is the reverse of all the elements of l
         ; just before l-tail
      [(define (rev-accum l-tail accum)
         (cond
           [(empty? l-tail) accum]
           [else (rev-accum (rest l-tail) (cons (first l-tail) accum))]))]
    (rev-accum l empty)})

;; example/test
(equal?
 (rev-list-using-accum (list 'a 1 5 'help 25))
 (list 25 'help 5 1 'a))

#|
For comparison, here's another version not using an accumulator but
conforming to the standard structural recursion template for lists
|#

;; rev-list : (listof X) -> (listof X)
;; reverses a list
(define (rev-list l)
  (cond
    [(empty? l) empty]
    [else (append (rev-list (rest l)) (list (first l)))]))