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28 lines
1.2 KiB
28 lines
1.2 KiB
2 years ago
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Exercise 4.19: Ben Bitdiddle, Alyssa P. Hacker,
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and Eva Lu Ator are arguing about the desired result of evaluating the
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expression
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(let ((a 1))
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(define (f x)
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(define b (+ a x))
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(define a 5)
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(+ a b))
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(f 10))
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Ben asserts that the result should be obtained using the sequential rule for
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define: b is defined to be 11, then a is defined to be 5,
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so the result is 16. Alyssa objects that mutual recursion requires the
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simultaneous scope rule for internal procedure definitions, and that it is
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unreasonable to treat procedure names differently from other names. Thus, she
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argues for the mechanism implemented in Exercise 4.16. This would lead
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to a being unassigned at the time that the value for b is to be
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computed. Hence, in Alyssa’s view the procedure should produce an error. Eva
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has a third opinion. She says that if the definitions of a and b
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are truly meant to be simultaneous, then the value 5 for a should be
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used in evaluating b. Hence, in Eva’s view a should be 5,
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b should be 15, and the result should be 20. Which (if any) of these
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viewpoints do you support? Can you devise a way to implement internal
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definitions so that they behave as Eva prefers?230
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