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?