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38 lines
1.0 KiB
38 lines
1.0 KiB
2 years ago
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Exercise 3.41: Ben Bitdiddle worries that it
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would be better to implement the bank account as follows (where the commented
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line has been changed):
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(define (make-account balance)
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(define (withdraw amount)
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(if (>= balance amount)
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(begin
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(set! balance
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(- balance amount))
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balance)
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"Insufficient funds"))
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(define (deposit amount)
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(set! balance (+ balance amount))
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balance)
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(let ((protected (make-serializer)))
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(define (dispatch m)
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(cond ((eq? m 'withdraw)
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(protected withdraw))
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((eq? m 'deposit)
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(protected deposit))
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((eq? m 'balance)
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((protected
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(lambda ()
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balance)))) ; serialized
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(else
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(error
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"Unknown request:
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MAKE-ACCOUNT"
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m))))
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dispatch))
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because allowing unserialized access to the bank balance can result in
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anomalous behavior. Do you agree? Is there any scenario that demonstrates
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Ben’s concern?
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