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36 lines
1.5 KiB
36 lines
1.5 KiB
2 years ago
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Exercise 4.23: Alyssa P. Hacker doesn’t
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understand why analyze-sequence needs to be so complicated. All the
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other analysis procedures are straightforward transformations of the
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corresponding evaluation procedures (or eval clauses) in
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4.1.1. She expected analyze-sequence to look like this:
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(define (analyze-sequence exps)
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(define (execute-sequence procs env)
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(cond ((null? (cdr procs))
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((car procs) env))
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(else ((car procs) env)
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(execute-sequence
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(cdr procs) env))))
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(let ((procs (map analyze exps)))
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(if (null? procs)
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(error "Empty sequence:
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ANALYZE"))
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(lambda (env)
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(execute-sequence procs env))))
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Eva Lu Ator explains to Alyssa that the version in the text does more of the
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work of evaluating a sequence at analysis time. Alyssa’s sequence-execution
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procedure, rather than having the calls to the individual execution procedures
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built in, loops through the procedures in order to call them: In effect,
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although the individual expressions in the sequence have been analyzed, the
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sequence itself has not been.
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Compare the two versions of analyze-sequence. For example, consider the
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common case (typical of procedure bodies) where the sequence has just one
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expression. What work will the execution procedure produced by Alyssa’s
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program do? What about the execution procedure produced by the program in the
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text above? How do the two versions compare for a sequence with two
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expressions?
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