;; A Visitor is a structure made up of two functions
;; One for the base case and 
;; one for the inductive case
(define-struct Visitor (fBase fInduct))

;; execute: list-of-any1 Visitor any2 --> any3
;; Executes ("accepts") the visitor on the list
;; returning the result.
;; param is passed to the visitor unmodified.
;; The base case of the Visitor is called on the empty list:
;; Visitor-fBase: list-of-any1 any2 --> any3
;; The inductive case is called on the non-empty list.
;; Visitor-fInduct: list-of-any1 any2 --> any3
(define (execute  a-list visitor param)
  (cond
    [(empty? a-list) ((Visitor-fBase visitor) a-list param)]
    [(cons? a-list) ((Visitor-fInduct visitor) a-list param)]))


#|
;; Visitor template using local
(define listVisitor
  (local 
      [(define (fBase a-list param) 
         ...)
       (define (fInduct a-list param)
         (...(first a-list)...(execute (rest a-list) listVisitor ...)))]
    (make-Visitor fBase fInduct)))
|#

#|
;; Visitor template, using lambda
(define listVisitor
  (make-Visitor
   (lambda (a-list param)
     (...param...))
   (lambda (a-list param)
     (...(first a-list) (execute (rest a-list) listVisitor ...)... param...))))
|#

;; sums a list of numbers
;; param is ignored.
(define sumVisitor
  (local 
      [(define (fBase a-list param) 
         0)
       (define (fInduct a-list param)
         (+ (first a-list) (execute (rest a-list) sumVisitor 0)))]
    (make-Visitor fBase fInduct)))

"sumVisitor test case:"
(= 28 (execute (list 4 2 7 3 5 6 1) sumVisitor 0))

;;reverseVisitor reverses a list
(define reverseVisitor
  (local
      [(define (fBase a-list param)
         empty)
       (define (fInduct a-list param)
         (execute a-list reverseVisitorHelper empty))
       
       ;; reverseVisitorHelper reverses a list
       ;; The reversed list is prepended onto the param.
       (define reverseVisitorHelper
         (local 
             [(define (fBase a-list param) 
                param)
              (define (fInduct a-list param)
                (execute (rest a-list) reverseVisitorHelper (cons (first a-list) param)))]
           (make-Visitor fBase fInduct)))]
    
    (make-Visitor fBase fInduct)))

"reverseVisitor test case:"
(equal? (list 7 6 5 4 3 2 1) (execute (list 1 2 3 4 5 6 7) reverseVisitor 0))


;; prodVisitor multiplies all the elements of a list together.
;; param is ignored.
(define prodVisitor
  (make-Visitor
   (lambda (a-list param)
     1)
   (lambda (a-list param)
     (* (first a-list) (execute (rest a-list) prodVisitor param)))))

"prodList test case:"
(= 5040 (execute (list 1 2 3 4 5 6 7) prodVisitor empty))