91 lines
2.3 KiB
Scheme
91 lines
2.3 KiB
Scheme
(load "util.scm")
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(load "misc/sicp-query.scm")
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(initialize-data-base microshaft-data-base)
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(display "\nex-4.70 let-in-add-assertion\n")
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; If we assigned to THE-ASSERTIONS directly then the new stream would reference
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; itself and we would get an endless stream of the current assertion.
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(display "[answered]\n")
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(display "\nex-4.71 - simpler-simple-query\n")
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; Using delay for the rule part can help to print at least some results before
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; a potential endless loop.
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; (eval-query '(rule (married ?x ?y)
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; (married ?y ?x)))
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; (eval-query '(married Minnie ?x))
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(display "[answered]\n")
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; I got this solution from SICP solutions because I could not think of a good
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; example. It would be better to implement the loop-avoidance algorithm than
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; relying on delayed evaluation to get some results, in my opinion.
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(display "\nex-4.72 - stream-interleave\n")
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; If only one of the disjuncts produces a result and the others create an
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; endless-loop, interleaving guarantess that we get some results for the valid
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; disjuncts.
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(display "[answered]\n")
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(display "\nex-4.73 - explicit-delay\n")
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; The second argument to interleave must be delayed explicitly because
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; MIT-Scheme's applicative-order-evaluation would evaluate the complete stream,
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; otherwise. That would make the use of streams pointless.
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(display "[answered]\n")
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(display "\nex-4.74 - simple-stream-flatmap\n")
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(define (simple-stream-flatmap proc s)
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(simple-flatten (stream-map proc s)))
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(define (stream-not-null? x)
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(not (stream-null? x)))
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(define (simple-flatten stream)
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(stream-map stream-car
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(stream-filter stream-not-null? stream)))
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(eval-query
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'(and (salary (Bitdiddle Ben) ?ben-amount)
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(salary ?person ?amount)
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(lisp-value > ?amount ?ben-amount)))
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(newline)
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; The query system's behavior does not change because interleaving does not
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; have an effect for empty and singleton streams.
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(display "\nex-4.75 - unique\n")
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(eval-query
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'(unique (job ?x (computer wizard))))
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(newline)
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(eval-query
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'(unique (job ?x (computer programmer))))
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(newline)
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(eval-query
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'(and (job ?x ?j) (unique (job ?anyone ?j))))
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(newline)
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(eval-query
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'(and (job ?p ?j) (unique (supervisor ?s ?p))))
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(newline)
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(display "\nex-4.76\n")
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(display "\nex-4.77\n")
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;(display "\nex-4.78\n")
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;(display "\nex-4.79\n")
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