You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
41 lines
1.4 KiB
41 lines
1.4 KiB
2 years ago
|
|
||
|
|
Exercise 3.42: Ben Bitdiddle suggests that it’s
|
||
|
|
a waste of time to create a new serialized procedure in response to every
|
||
|
|
withdraw and deposit message. He says that make-account
|
||
|
|
could be changed so that the calls to protected are done outside the
|
||
|
|
dispatch procedure. That is, an account would return the same
|
||
|
|
serialized procedure (which was created at the same time as the account) each
|
||
|
|
time it is asked for a withdrawal procedure.
|
||
|
|
|
||
|
|
|
||
|
|
(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)))
|
||
|
|
(let ((protected-withdraw
|
||
|
|
(protected withdraw))
|
||
|
|
(protected-deposit
|
||
|
|
(protected deposit)))
|
||
|
|
(define (dispatch m)
|
||
|
|
(cond ((eq? m 'withdraw)
|
||
|
|
protected-withdraw)
|
||
|
|
((eq? m 'deposit)
|
||
|
|
protected-deposit)
|
||
|
|
((eq? m 'balance)
|
||
|
|
balance)
|
||
|
|
(else
|
||
|
|
(error "Unknown request:
|
||
|
|
MAKE-ACCOUNT"
|
||
|
|
m))))
|
||
|
|
dispatch)))
|
||
|
|
|
||
|
|
Is this a safe change to make? In particular, is there any difference in what
|
||
|
|
concurrency is allowed by these two versions of make-account?
|