ex-3.45.md

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))
               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.

With Louis's change, we can't make complex operations such as serialized-exchange which requires to get and set account balances.

For example, serialized-exchange consists of two steps; (1) calculate the difference of two account balances; then (2) transfer the difference between the two accounts. These two steps must not be interleaved, so that serialized-exchange must acquire the rights for exclusive access to the two accounts before executing the two steps.

If each account's withdraw and deposit are automatically serialized, serialized-exchange can't modify each account's balance. Though withdraw and deposit requires the rights for exclusive access, the rights are already acquired by serialized-exchange. withdraw and deposit in serialized-exchange wait until the execution of the current serialized-exchange is finished. In other words, they waits forever.