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54 lines
1.7 KiB
54 lines
1.7 KiB
2 years ago
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Exercise 4.75: Implement for the query language
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a new special form called unique. Unique should succeed if there
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is precisely one item in the data base satisfying a specified query. For
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example,
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(unique (job ?x (computer wizard)))
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should print the one-item stream
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(unique (job (Bitdiddle Ben)
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(computer wizard)))
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since Ben is the only computer wizard, and
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(unique (job ?x (computer programmer)))
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should print the empty stream, since there is more than one computer
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programmer. Moreover,
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(and (job ?x ?j)
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(unique (job ?anyone ?j)))
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should list all the jobs that are filled by only one person, and the people who
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fill them.
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There are two parts to implementing unique. The first is to write a
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procedure that handles this special form, and the second is to make
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qeval dispatch to that procedure. The second part is trivial, since
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qeval does its dispatching in a data-directed way. If your procedure is
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called uniquely-asserted, all you need to do is
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(put 'unique 'qeval uniquely-asserted)
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and qeval will dispatch to this procedure for every query whose
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type (car) is the symbol unique.
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The real problem is to write the procedure uniquely-asserted. This
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should take as input the contents (cdr) of the unique
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query, together with a stream of frames. For each frame in the stream, it
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should use qeval to find the stream of all extensions to the frame that
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satisfy the given query. Any stream that does not have exactly one item in it
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should be eliminated. The remaining streams should be passed back to be
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accumulated into one big stream that is the result of the unique query.
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This is similar to the implementation of the not special form.
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Test your implementation by forming a query that lists all people who supervise
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precisely one person.
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