Ben Bitdiddle worries that it would be better to implement the bank account as follows (where the commented line has been changed):
(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))) (define (dispatch m) (cond ((eq? m 'withdraw) (protected withdraw)) ((eq? m 'deposit) (protected deposit)) ((eq? m 'balance) ((protected (lambda () balance)))) ; serialized (else (error "Unknown request -- MAKE-ACCOUNT" m)))) dispatch))
because allowing unserialized access to the bank balance can result in anomalous behavior. Do you agree? Is there any scenario that demonstrates Ben's concern?
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