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39 lines
1.0 KiB
39 lines
1.0 KiB
2 years ago
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Exercise 3.10: In the make-withdraw
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procedure, the local variable balance is created as a parameter of
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make-withdraw. We could also create the local state variable
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explicitly, using let, as follows:
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(define (make-withdraw initial-amount)
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(let ((balance initial-amount))
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(lambda (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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Recall from 1.3.2 that let is simply syntactic sugar for a
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procedure call:
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(let ((⟨var⟩ ⟨exp⟩)) ⟨body⟩)
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is interpreted as an alternate syntax for
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((lambda (⟨var⟩) ⟨body⟩) ⟨exp⟩)
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Use the environment model to analyze this alternate version of
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make-withdraw, drawing figures like the ones above to illustrate the
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interactions
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(define W1 (make-withdraw 100))
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(W1 50)
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(define W2 (make-withdraw 100))
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Show that the two versions of make-withdraw create objects with the same
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behavior. How do the environment structures differ for the two versions?
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