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40 lines
1.4 KiB
40 lines
1.4 KiB
2 years ago
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Exercise 4.79: When we implemented the Lisp
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evaluator in 4.1, we saw how to use local environments to avoid
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name conflicts between the parameters of procedures. For example, in
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evaluating
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(define (square x)
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(* x x))
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(define (sum-of-squares x y)
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(+ (square x) (square y)))
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(sum-of-squares 3 4)
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there is no confusion between the x in square and the x in
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sum-of-squares, because we evaluate the body of each procedure in an
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environment that is specially constructed to contain bindings for the local
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variables. In the query system, we used a different strategy to avoid name
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conflicts in applying rules. Each time we apply a rule we rename the variables
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with new names that are guaranteed to be unique. The analogous strategy for
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the Lisp evaluator would be to do away with local environments and simply
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rename the variables in the body of a procedure each time we apply the
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procedure.
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Implement for the query language a rule-application method that uses
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environments rather than renaming. See if you can build on your environment
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structure to create constructs in the query language for dealing with large
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systems, such as the rule analog of block-structured procedures. Can you
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relate any of this to the problem of making deductions in a context (e.g., “If
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I supposed that
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P
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were true, then I would be able to deduce
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A
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and
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B
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.”) as a method of problem solving? (This problem is open-ended. A good
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answer is probably worth a Ph.D.)
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