3 days ago
my attempt to do the exercises in sicp.
Thursday, March 18, 2010
sicp exercise 3.45
;; Exercise 3.45. Louis Reasoner thinks our bank-account system is unnecessarily complex and error-prone now that deposits and withdrawals aren't automatically serialized. He suggests that make-account-and-serializer should have exported the serializer (for use by such procedures as serialized-exchange) in addition to (rather than instead of) using it to serialize accounts and deposits as make-account did. He proposes to redefine accounts as follows:
(define (make-account-and-serializer balance)
(define (withdraw amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
(define (deposit amount)
(set! balance (+ balance amount))
(let ((balance-serializer (make-serializer)))
(define (dispatch m)
(cond ((eq? m 'withdraw) (balance-serializer withdraw))
((eq? m 'deposit) (balance-serializer deposit))
((eq? m 'balance) balance)
((eq? m 'serializer) balance-serializer)
(else (error "Unknown request -- MAKE-ACCOUNT"
;; Then deposits are handled as with the original make-account:
(define (deposit account amount)
((account 'deposit) amount))
;; Explain what is wrong with Louis's reasoning. In particular, consider what happens when serialized-exchange is called.
;; when serialized-exchange is called, the serializer is invoked twice. once by the serialized-exchange and once more by the (account 'withdraw) invoked from exchange.
;; So the whole process will lock upon itself.