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)) balance) "Insufficient funds")) (define (deposit amount) (set! balance (+ balance amount)) balance) (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" m)))) dispatch))
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.
Nobody's finished this exercise yet. You'll be the first!
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