6 hours ago
my attempt to do the exercises in sicp.
Thursday, March 18, 2010
sicp exercise 3.41
;; Exercise 3.41. Ben Bitdiddle worries that it would be better to implement the bank account as follows (where the commented line has been changed):
(define (make-account balance)
(define (withdraw amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
(define (deposit amount)
(set! balance (+ balance amount))
;; continued on next page
(let ((protected (make-serializer)))
(define (dispatch m)
(cond ((eq? m 'withdraw) (protected withdraw))
((eq? m 'deposit) (protected deposit))
((eq? m 'balance)
((protected (lambda () balance)))) ; serialized
(else (error "Unknown request -- MAKE-ACCOUNT"
;; because allowing unserialized access to the bank balance can result in anomalous behavior. Do you agree? Is there any scenario that demonstrates Ben's concern?
;; The message 'balance only reads the current value, it has no effect on the behavior of 'withdraw or 'deposit.