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35 lines
1.9 KiB
35 lines
1.9 KiB
2 years ago
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Exercise 5.30: Our evaluator currently catches
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and signals only two kinds of errors—unknown expression types and unknown
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procedure types. Other errors will take us out of the evaluator
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read-eval-print loop. When we run the evaluator using the register-machine
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simulator, these errors are caught by the underlying Scheme system. This is
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analogous to the computer crashing when a user program makes an
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error.317 It is a
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large project to make a real error system work, but it is well worth the effort
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to understand what is involved here.
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Errors that occur in the evaluation process, such as an attempt to access an
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unbound variable, could be caught by changing the lookup operation to make it
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return a distinguished condition code, which cannot be a possible value of any
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user variable. The evaluator can test for this condition code and then do what
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is necessary to go to signal-error. Find all of the places in the
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evaluator where such a change is necessary and fix them. This is lots of work.
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Much worse is the problem of handling errors that are signaled by applying
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primitive procedures, such as an attempt to divide by zero or an attempt to
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extract the car of a symbol. In a professionally written high-quality
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system, each primitive application is checked for safety as part of the
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primitive. For example, every call to car could first check that the
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argument is a pair. If the argument is not a pair, the application would
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return a distinguished condition code to the evaluator, which would then report
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the failure. We could arrange for this in our register-machine simulator by
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making each primitive procedure check for applicability and returning an
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appropriate distinguished condition code on failure. Then the
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primitive-apply code in the evaluator can check for the condition code
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and go to signal-error if necessary. Build this structure and make it
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work. This is a major project.
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