;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-intermediate-reader.ss" "lang")((modname rev-accu) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))
;; A LON is either empty or (cons Number LON). 

;; LON -> LON 
;; reverse the order of the numbers on the list 

; (check-expect (rev (list 1 2 3)) (list 3 2 1))

(define (rev l0)
  (local (;; LON LON -> LON 
          ;; so-far: the reverse of the list between l0 and l
          ;; l0 = '(1 2 3 4)
          ;; l  = '(3 4)
          ;; so-far = '(2 1)
          (define (rev-with-accu l so-far)
            (cond    
              [(empty? l) so-far]
              [else (rev-with-accu (rest l) (cons (first l) so-far))])))
    (rev-with-accu l0 empty)))

#| stress tests 

(define (main n)
  (throw-away (time (rev (make-list n 0)))))
(define (throw-away x) 10)

(main 10)
 |#

(rev '(1 2 3))