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40 lines
1.4 KiB
40 lines
1.4 KiB
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Exercise 3.42: Ben Bitdiddle suggests that it’s
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a waste of time to create a new serialized procedure in response to every
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withdraw and deposit message. He says that make-account
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could be changed so that the calls to protected are done outside the
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dispatch procedure. That is, an account would return the same
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serialized procedure (which was created at the same time as the account) each
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time it is asked for a withdrawal procedure.
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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 (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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(let ((protected-withdraw
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(protected withdraw))
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(protected-deposit
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(protected deposit)))
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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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balance)
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(else
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(error "Unknown request:
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MAKE-ACCOUNT"
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m))))
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dispatch)))
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Is this a safe change to make? In particular, is there any difference in what
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concurrency is allowed by these two versions of make-account?
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