1 665c255d 2023-08-04 jrmu  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):
 2 665c255d 2023-08-04 jrmu 
 3 665c255d 2023-08-04 jrmu (define (make-account balance)
 4 665c255d 2023-08-04 jrmu   (define (withdraw amount)
 5 665c255d 2023-08-04 jrmu     (if (>= balance amount)
 6 665c255d 2023-08-04 jrmu         (begin (set! balance (- balance amount))
 7 665c255d 2023-08-04 jrmu                balance)
 8 665c255d 2023-08-04 jrmu         "Insufficient funds"))
 9 665c255d 2023-08-04 jrmu   (define (deposit amount)
10 665c255d 2023-08-04 jrmu     (set! balance (+ balance amount))
11 665c255d 2023-08-04 jrmu     balance)
12 665c255d 2023-08-04 jrmu   ;; continued on next page
13 665c255d 2023-08-04 jrmu 
14 665c255d 2023-08-04 jrmu   (let ((protected (make-serializer)))
15 665c255d 2023-08-04 jrmu     (define (dispatch m)
16 665c255d 2023-08-04 jrmu       (cond ((eq? m 'withdraw) (protected withdraw))
17 665c255d 2023-08-04 jrmu             ((eq? m 'deposit) (protected deposit))
18 665c255d 2023-08-04 jrmu             ((eq? m 'balance)
19 665c255d 2023-08-04 jrmu              ((protected (lambda () balance)))) ; serialized
20 665c255d 2023-08-04 jrmu             (else (error "Unknown request -- MAKE-ACCOUNT"
21 665c255d 2023-08-04 jrmu                          m))))
22 665c255d 2023-08-04 jrmu     dispatch))
23 665c255d 2023-08-04 jrmu 
24 665c255d 2023-08-04 jrmu ;; 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?