sicp exercise 3.1

;; Exercise 3.1.  An accumulator is a procedure that is called repeatedly with a single numeric argument and accumulates its arguments into a sum. Each time it is called, it returns the currently accumulated sum. Write a procedure make-accumulator that generates accumulators, each maintaining an independent sum. The input to make-accumulator should specify the initial value of the sum; for example

;; (define A (make-accumulator 5))
;; (A 10)
;; 15
;; (A 10)
;; 25

(define (make-accumulator sum)
  (lambda(arg)
    (begin
      (set! sum (+ sum arg))
      sum)))

(define A (make-accumulator 5))

(display (A 10)) (newline)
(display (A 10)) (newline)