;; ---------------------------------------------------------------------
;; redefining + to keep a running total of all additions
;; ---------------------------------------------------------------------

#lang racket
(require "display-line.rkt")
(provide add)

;; ---------------------------------------------------------------------
;; This first draft uses several ideas we have learned to implement
;; the basic idea: a closure with set!, variable arity, and apply.
;; ---------------------------------------------------------------------

(define add-v1
  (let ((sum 0))
    (lambda args
      (set! sum (+ sum (apply + args)))
      (display-line "... running total:" sum)
      (apply + args))))

;; ---------------------------------------------------------------------
;; add-v1 adds the numbers twice.  Let's add them once, store
;; the result in a local variable, and use the *variable* twice.
;; ---------------------------------------------------------------------

(define add
  (let ((sum 0))
    (lambda args
      (let ((result (apply + args)))
        (set! sum (+ sum result))
        (display-line "... running total:" sum)
        result))))

;; -------------------------------------------------------------------------
;; Now, we provide add and let the client require it in and define as +.
;; -------------------------------------------------------------------------


;; -------------------------------------------------------------------------
;; Here is an alternative way to export the function that renames the
;; function on the way out of the module:
;;    (provide (rename-out (add +))))
;;
;; The client can (require "add-with-memory.rkt") as usual, with no define.
;; -------------------------------------------------------------------------