Exercise 3.41 · 习题

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))
balance)
"Insufficient funds"))
(define (deposit amount)
(set! balance (+ balance amount))
balance)
(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"
m))))
dispatch))

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?

练习 3.41：Ben Bitdiddle 担心，最好按如下方式实现银行账户（其中注释行已作修改）：

(define (make-account balance)
(define (withdraw amount)
(if (>= balance amount)
(begin
(set! balance
(- balance amount))
balance)
“Insufficient funds”))
(define (deposit amount)
(set! balance (+ balance amount))
balance)
(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:\nMAKE-ACCOUNT”
m))))
dispatch))

因为允许对银行余额进行非串行化访问可能导致异常行为。你同意吗？是否存在某种情景能证明 Ben 的担忧？