1 
2 ;; dispatch table
3 
4 (define (make-table)
5   (define (assoc key records)
6     (cond ((null? records) false)
7 	  ((equal? key (caar records)) (car records))
8 	  (else (assoc key (cdr records)))))
9   (let ((local-table (list '*table*)))
10     (define (lookup key-1 key-2)
11       (let ((subtable (assoc key-1 (cdr local-table))))
12 	(if subtable
13 	    (let ((record (assoc key-2 (cdr subtable))))
14 	      (if record
15 		  (cdr record)
16 		  false))
17 	    false)))
18     (define (insert! key-1 key-2 value)
19       (let ((subtable (assoc key-1 (cdr local-table))))
20 	(if subtable
21 	    (let ((record (assoc key-2 (cdr subtable))))
22 	      (if record
23 		  (set-cdr! record value)
24 		  (set-cdr! subtable
25 			    (cons (cons key-2 value)
26 				  (cdr subtable)))))
27 	    (set-cdr! local-table
28 		      (cons (list key-1
29 				  (cons key-2 value))
30 			    (cdr local-table)))))
31       'ok)
32     (define (dispatch m)
33       (cond ((eq? m 'lookup-proc) lookup)
34 	    ((eq? m 'insert-proc!) insert!)
35 	    (else (error "Unknown operation -- TABLE" m))))
36     dispatch))
37 (define operation-table (make-table))
38 (define get (operation-table 'lookup-proc))
39 (define put (operation-table 'insert-proc!))
40 
41 ;; streams/delayed-evaluation
42 
43 (define (memo-proc proc)
44   (let ((already-run? false) (result false))
45     (lambda ()
46       (if already-run?
47 	  result
48 	  (begin (set! already-run? true)
49 		 (set! result (proc))
50 		 result)))))
51 (define-syntax mydelay
52   (rsc-macro-transformer
53    (let ((xfmr
54 	  (lambda (exp)
55 	    `(memo-proc (lambda () ,exp)))))
56      (lambda (e r)
57        (apply xfmr (cdr e))))))
58 (define (myforce delayed-object)
59   (delayed-object))
60 (define-syntax cons-stream
61   (rsc-macro-transformer
62    (let ((xfmr (lambda (x y) `(cons ,x (mydelay ,y)))))
63      (lambda (e r)
64        (apply xfmr (cdr e))))))
65 (define (stream-car s)
66   (car s))
67 (define (stream-cdr s)
68   (myforce (cdr s)))
69 (define stream-null? null?)
70 (define the-empty-stream '())
71 (define (stream-map proc . argstreams)
72   (if (stream-null? (car argstreams))
73       the-empty-stream
74       (cons-stream
75        (apply proc (map stream-car argstreams))
76        (apply stream-map (cons proc (map stream-cdr argstreams))))))
77 (define (display-stream s)
78   (stream-for-each display-line s))
79 (define (stream-for-each proc s)
80   (if (stream-null? s)
81       'done
82       (begin (proc (stream-car s))
83 	     (stream-for-each proc (stream-cdr s)))))
84 (define (display-line x)
85   (newline)
86   (display x))
87 (define (display-streams n . streams)
88   (if (> n 0)
89       (begin (newline) 
90 	     (for-each 
91 	      (lambda (s)
92 		(display (stream-car s))
93 		(newline))
94 	      streams)
95 	     (apply display-streams
96 		    (cons (- n 1) (map stream-cdr streams))))))
97 (define (list->stream list)
98   (if (null? list)
99       the-empty-stream
100       (cons-stream (car list)
101 		   (list->stream (cdr list)))))
102 (define (stream-fold-left op initial sequence)
103   (define (iter result rest)
104     (if (null? rest)
105 	result
106 	(iter (op result (stream-car rest))
107 	      (stream-cdr rest))))
108   (iter initial sequence))
109 
110 ;; query-driver-loop
111 
112 (define input-prompt ";;; Query input:")
113 (define output-prompt ";;; Query results:")
114 (define (query-driver-loop)
115   (prompt-for-input input-prompt)
116   (let ((q (query-syntax-process (read))))
117     (cond ((assertion-to-be-added? q)
118            (add-rule-or-assertion! (add-assertion-body q))
119            (newline)
120            (display "Assertion added to data base.")
121            (query-driver-loop))
122           (else
123            (newline)
124            (display output-prompt)
125            (display-stream
126             (stream-map
127              (lambda (frame)
128                (instantiate q
129                             frame
130                             (lambda (v f)
131                               (contract-question-mark v))))
132              (qeval q (singleton-stream '()))))
133            (query-driver-loop)))))
134 (define (instantiate exp frame unbound-var-handler)
135   (define (copy exp)
136     (cond ((var? exp)
137            (let ((binding (binding-in-frame exp frame)))
138              (if binding
139                  (copy (binding-value binding))
140                  (unbound-var-handler exp frame))))
141           ((pair? exp)
142            (cons (copy (car exp)) (copy (cdr exp))))
143           (else exp)))
144   (copy exp))
145 (define (qeval query frame-stream)
146   (let ((qproc (get (type query) 'qeval)))
147     (if qproc
148         (qproc (contents query) frame-stream)
149         (simple-query query frame-stream))))
150 (define (simple-query query-pattern frame-stream)
151   (stream-flatmap
152    (lambda (frame)
153      (stream-append-delayed
154       (find-assertions query-pattern frame)
155       (delay (apply-rules query-pattern frame))))
156    frame-stream))
157 (define (conjoin conjuncts frame-stream)
158   (if (empty-conjunction? conjuncts)
159       frame-stream
160       (conjoin (rest-conjuncts conjuncts)
161                (qeval (first-conjunct conjuncts)
162                       frame-stream))))
163 (put 'and 'qeval conjoin)
164 (define (disjoin disjuncts frame-stream)
165   (if (empty-disjunction? disjuncts)
166       the-empty-stream
167       (interleave-delayed
168        (qeval (first-disjunct disjuncts) frame-stream)
169        (delay (disjoin (rest-disjuncts disjuncts)
170                        frame-stream)))))
171 (put 'or 'qeval disjoin)
172 (define (negate operands frame-stream)
173   (stream-flatmap
174    (lambda (frame)
175      (if (stream-null? (qeval (negated-query operands)
176                               (singleton-stream frame)))
177          (singleton-stream frame)
178          the-empty-stream))
179    frame-stream))
180 (put 'not 'qeval negate)
181 (define (lisp-value call frame-stream)
182   (stream-flatmap
183    (lambda (frame)
184      (if (execute
185           (instantiate
186            call
187            frame
188            (lambda (v f)
189              (error "Unknown pat var -- LISP-VALUE" v))))
190          (singleton-stream frame)
191          the-empty-stream))
192    frame-stream))
193 (put 'lisp-value 'qeval lisp-value)
194 (define (execute exp)
195   (apply (eval (predicate exp) user-initial-environment)
196          (args exp)))
197 (define (always-true ignore frame-stream) frame-stream)
198 (put 'always-true 'qeval always-true)
199 (define (find-assertions pattern frame)
200   (stream-flatmap (lambda (datum)
201                     (check-an-assertion datum pattern frame))
202                   (fetch-assertions pattern frame)))
203 (define (check-an-assertion assertion query-pat query-frame)
204   (let ((match-result
205          (pattern-match query-pat assertion query-frame)))
206     (if (eq? match-result 'failed)
207         the-empty-stream
208         (singleton-stream match-result))))
209 (define (pattern-match pat dat frame)
210   (cond ((eq? frame 'failed) 'failed)
211         ((equal? pat dat) frame)
212         ((var? pat) (extend-if-consistent pat dat frame))
213         ((and (pair? pat) (pair? dat))
214          (pattern-match (cdr pat)
215                         (cdr dat)
216                         (pattern-match (car pat)
217                                        (car dat)
218                                        frame)))
219         (else 'failed)))
220 (define (extend-if-consistent var dat frame)
221   (let ((binding (binding-in-frame var frame)))
222     (if binding
223         (pattern-match (binding-value binding) dat frame)
224         (extend var dat frame))))
225 (define (apply-rules pattern frame)
226   (stream-flatmap (lambda (rule)
227                     (apply-a-rule rule pattern frame))
228                   (fetch-rules pattern frame)))
229 (define (apply-a-rule rule query-pattern query-frame)
230   (let ((clean-rule (rename-variables-in rule)))
231     (let ((unify-result
232            (unify-match query-pattern
233                         (conclusion clean-rule)
234                         query-frame)))
235       (if (eq? unify-result 'failed)
236           the-empty-stream
237           (qeval (rule-body clean-rule)
238                  (singleton-stream unify-result))))))
239 (define (rename-variables-in rule)
240   (let ((rule-application-id (new-rule-application-id)))
241     (define (tree-walk exp)
242       (cond ((var? exp)
243              (make-new-variable exp rule-application-id))
244             ((pair? exp)
245              (cons (tree-walk (car exp))
246                    (tree-walk (cdr exp))))
247             (else exp)))
248     (tree-walk rule)))
249 (define (unify-match p1 p2 frame)
250   (cond ((eq? frame 'failed) 'failed)
251         ((equal? p1 p2) frame)
252         ((var? p1) (extend-if-possible p1 p2 frame))
253         ((var? p2) (extend-if-possible p2 p1 frame))  ; ***
254         ((and (pair? p1) (pair? p2))
255          (unify-match (cdr p1)
256                       (cdr p2)
257                       (unify-match (car p1)
258                                    (car p2)
259                                    frame)))
260         (else 'failed)))
261 (define (extend-if-possible var val frame)
262   (let ((binding (binding-in-frame var frame)))
263     (cond (binding
264            (unify-match
265             (binding-value binding) val frame))
266           ((var? val)                      ; ***
267            (let ((binding (binding-in-frame val frame)))
268              (if binding
269                  (unify-match
270                   var (binding-value binding) frame)
271                  (extend var val frame))))
272           ((depends-on? val var frame)     ; ***
273            'failed)
274           (else (extend var val frame)))))
275 (define (depends-on? exp var frame)
276   (define (tree-walk e)
277     (cond ((var? e)
278            (if (equal? var e)
279                true
280                (let ((b (binding-in-frame e frame)))
281                  (if b
282                      (tree-walk (binding-value b))
283                      false))))
284           ((pair? e)
285            (or (tree-walk (car e))
286                (tree-walk (cdr e))))
287           (else false)))
288   (tree-walk exp))
289 (define THE-ASSERTIONS the-empty-stream)
290 (define (fetch-assertions pattern frame)
291   (if (use-index? pattern)
292       (get-indexed-assertions pattern)
293       (get-all-assertions)))
294 (define (get-all-assertions) THE-ASSERTIONS)
295 (define (get-indexed-assertions pattern)
296   (get-stream (index-key-of pattern) 'assertion-stream))
297 (define (get-stream key1 key2)
298   (let ((s (get key1 key2)))
299     (if s s the-empty-stream)))
300 (define THE-RULES the-empty-stream)
301 (define (fetch-rules pattern frame)
302   (if (use-index? pattern)
303       (get-indexed-rules pattern)
304       (get-all-rules)))
305 (define (get-all-rules) THE-RULES)
306 (define (get-indexed-rules pattern)
307   (stream-append
308    (get-stream (index-key-of pattern) 'rule-stream)
309    (get-stream '? 'rule-stream)))
310 (define (add-rule-or-assertion! assertion)
311   (if (rule? assertion)
312       (add-rule! assertion)
313       (add-assertion! assertion)))
314 (define (add-assertion! assertion)
315   (store-assertion-in-index assertion)
316   (let ((old-assertions THE-ASSERTIONS))
317     (set! THE-ASSERTIONS
318           (cons-stream assertion old-assertions))
319     'ok))
320 (define (add-rule! rule)
321   (store-rule-in-index rule)
322   (let ((old-rules THE-RULES))
323     (set! THE-RULES (cons-stream rule old-rules))
324     'ok))
325 (define (store-assertion-in-index assertion)
326   (if (indexable? assertion)
327       (let ((key (index-key-of assertion)))
328         (let ((current-assertion-stream
329                (get-stream key 'assertion-stream)))
330           (put key
331                'assertion-stream
332                (cons-stream assertion
333                             current-assertion-stream))))))
334 (define (store-rule-in-index rule)
335   (let ((pattern (conclusion rule)))
336     (if (indexable? pattern)
337         (let ((key (index-key-of pattern)))
338           (let ((current-rule-stream
339                  (get-stream key 'rule-stream)))
340             (put key
341                  'rule-stream
342                  (cons-stream rule
343                               current-rule-stream)))))))
344 (define (indexable? pat)
345   (or (constant-symbol? (car pat))
346       (var? (car pat))))
347 (define (index-key-of pat)
348   (let ((key (car pat)))
349     (if (var? key) '? key)))
350 (define (use-index? pat)
351   (constant-symbol? (car pat)))
352 (define (stream-append s1 s2)
353   (if (stream-null? s1)
354       s2
355       (cons-stream (stream-car s1)
356                    (stream-append (stream-cdr s1) s2))))
357 (define (stream-append-delayed s1 delayed-s2)
358   (if (stream-null? s1)
359       (force delayed-s2)
360       (cons-stream
361        (stream-car s1)
362        (stream-append-delayed (stream-cdr s1) delayed-s2))))
363 (define (interleave-delayed s1 delayed-s2)
364   (if (stream-null? s1)
365       (force delayed-s2)
366       (cons-stream
367        (stream-car s1)
368        (interleave-delayed (force delayed-s2)
369                            (delay (stream-cdr s1))))))
370 (define (stream-flatmap proc s)
371   (flatten-stream (stream-map proc s)))
372 (define (flatten-stream stream)
373   (if (stream-null? stream)
374       the-empty-stream
375       (interleave-delayed
376        (stream-car stream)
377        (delay (flatten-stream (stream-cdr stream))))))
378 (define (singleton-stream x)
379   (cons-stream x the-empty-stream))
380 (define (type exp)
381   (if (pair? exp)
382       (car exp)
383       (error "Unknown expression TYPE" exp)))
384 (define (contents exp)
385   (if (pair? exp)
386       (cdr exp)
387       (error "Unknown expression CONTENTS" exp)))
388 (define (assertion-to-be-added? exp)
389   (eq? (type exp) 'assert!))
390 (define (add-assertion-body exp)
391   (car (contents exp)))
392 (define (empty-conjunction? exps) (null? exps))
393 (define (first-conjunct exps) (car exps))
394 (define (rest-conjuncts exps) (cdr exps))
395 (define (empty-disjunction? exps) (null? exps))
396 (define (first-disjunct exps) (car exps))
397 (define (rest-disjuncts exps) (cdr exps))
398 (define (negated-query exps) (car exps))
399 (define (predicate exps) (car exps))
400 (define (args exps) (cdr exps))
401 (define (rule? statement)
402   (tagged-list? statement 'rule))
403 (define (conclusion rule) (cadr rule))
404 (define (rule-body rule)
405   (if (null? (cddr rule))
406       '(always-true)
407       (caddr rule)))
408 (define (query-syntax-process exp)
409   (map-over-symbols expand-question-mark exp))
410 (define (map-over-symbols proc exp)
411   (cond ((pair? exp)
412          (cons (map-over-symbols proc (car exp))
413                (map-over-symbols proc (cdr exp))))
414         ((symbol? exp) (proc exp))
415         (else exp)))
416 (define (expand-question-mark symbol)
417   (let ((chars (symbol->string symbol)))
418     (if (string=? (substring chars 0 1) "?")
419         (list '?
420               (string->symbol
421                (substring chars 1 (string-length chars))))
422         symbol)))
423 (define (var? exp)
424   (tagged-list? exp '?))
425 (define (constant-symbol? exp) (symbol? exp))
426 (define rule-counter 0)
427 (define (new-rule-application-id)
428   (set! rule-counter (+ 1 rule-counter))
429   rule-counter)
430 (define (make-new-variable var rule-application-id)
431   (cons '? (cons rule-application-id (cdr var))))
432 (define (contract-question-mark variable)
433   (string->symbol
434    (string-append "?" 
435      (if (number? (cadr variable))
436          (string-append (symbol->string (caddr variable))
437                         "-"
438                         (number->string (cadr variable)))
439          (symbol->string (cadr variable))))))
440 (define (make-binding variable value)
441   (cons variable value))
442 (define (binding-variable binding)
443   (car binding))
444 (define (binding-value binding)
445   (cdr binding))
446 (define (binding-in-frame variable frame)
447   (assoc variable frame))
448 (define (extend variable value frame)
449   (cons (make-binding variable value) frame))
450 (define (tagged-list? exp tag)
451   (and (pair? exp) (eq? (car exp) tag)))
452 
453 ;; test procedures
454 
455 (define (eval-queries queries)
456   (if (null? queries)
457       'done
458       (begin (eval-query (car queries))
459 	     (eval-queries (cdr queries)))))
460 (define (eval-query query)
461   (let ((q (query-syntax-process query)))
462     (if (assertion-to-be-added? q)
463 	(add-rule-or-assertion! (add-assertion-body q))
464 	(stream-map
465 	 (lambda (frame)
466 	   (instantiate q
467 	       frame
468 	     (lambda (v f)
469 	       (contract-question-mark v))))
470 	 (qeval q (singleton-stream '()))))))
471 (define (eval-display-query q)
472   (display-stream (eval-query q)))
473 (define (test-case actual expected)
474   (newline)
475   (display "Actual:   ")
476   (display actual)
477   (newline)
478   (display "Expected: ")
479   (display expected)
480   (newline))
481 (define (test-query query . expected)
482   (if (null? expected)
483       (let ((result (eval-query query)))
484 	(if (symbol? result)
485 	    (begin (display "Assertion added") (newline))
486 	    (display-stream (eval-query query))))
487       (let ((list (car expected)))
488 	(let ((result
489 	       (stream-fold-left 
490 		(lambda (x y)
491 		  (and x y))
492 		#t
493 		(stream-map 
494 		 (lambda (e1 e2)
495 		   (equal? e1 e2))
496 		 (eval-query query)
497 		 (list->stream list)))))
498 	  (if result
499 	      (display "Passed -- ")
500 	      (display "Failed! -- "))
501 	  (display query)
502 	  (newline)))))
503 
504       ;; (let ((result (eval-query query)))
505       ;; 	(if (pair? result)
506       ;; 	    (display-stream (eval-query query))
507       ;; 	    result))
508 
509 ;;I suspect that (eval-query query) is 'ok and that we are trying to print it as a steam
510 
511 	;; (display-streams 
512 	;;  (length list)
513 	;;  (eval-query query)
514 	;;  (list->stream list)))))
515 
516 	;; (display-stream
517 	;;  (stream-map 
518 	;;   (lambda (e1 e2)
519 	;;     (equal? e1 e2))
520 	;;   (eval-query query)
521 	;;   (list->stream list))))))
522 ;; test-suite
523 
524 
525 (eval-queries
526 '((assert! (address (Bitdiddle Ben) (Slumerville (Ridge Road) 10)))
527   (assert! (job (Bitdiddle Ben) (computer wizard)))
528   (assert! (salary (Bitdiddle Ben) 60000))
529   (assert! (address (Hacker Alyssa P) (Cambridge (Mass Ave) 78)))
530   (assert! (job (Hacker Alyssa P) (computer programmer)))
531   (assert! (salary (Hacker Alyssa P) 40000))
532   (assert! (supervisor (Hacker Alyssa P) (Bitdiddle Ben)))
533   (assert! (address (Fect Cy D) (Cambridge (Ames Street) 3)))
534   (assert! (job (Fect Cy D) (computer programmer)))
535   (assert! (salary (Fect Cy D) 35000))
536   (assert! (supervisor (Fect Cy D) (Bitdiddle Ben)))
537   (assert! (address (Tweakit Lem E) (Boston (Bay State Road) 22)))
538   (assert! (job (Tweakit Lem E) (computer technician)))
539   (assert! (salary (Tweakit Lem E) 25000))
540   (assert! (supervisor (Tweakit Lem E) (Bitdiddle Ben)))
541   (assert! (address (Reasoner Louis) (Slumerville (Pine Tree Road) 80)))
542   (assert! (job (Reasoner Louis) (computer programmer trainee)))
543   (assert! (salary (Reasoner Louis) 30000))
544   (assert! (supervisor (Reasoner Louis) (Hacker Alyssa P)))
545   (assert! (supervisor (Bitdiddle Ben) (Warbucks Oliver)))
546   (assert! (address (Warbucks Oliver) (Swellesley (Top Heap Road))))
547   (assert! (job (Warbucks Oliver) (administration big wheel)))
548   (assert! (salary (Warbucks Oliver) 150000))
549   (assert! (address (Scrooge Eben) (Weston (Shady Lane) 10)))
550   (assert! (job (Scrooge Eben) (accounting chief accountant)))
551   (assert! (salary (Scrooge Eben) 75000))
552   (assert! (supervisor (Scrooge Eben) (Warbucks Oliver)))
553   (assert! (address (Cratchet Robert) (Allston (N Harvard Street) 16)))
554   (assert! (job (Cratchet Robert) (accounting scrivener)))
555   (assert! (salary (Cratchet Robert) 18000))
556   (assert! (supervisor (Cratchet Robert) (Scrooge Eben)))
557   (assert! (address (Aull DeWitt) (Slumerville (Onion Square) 5)))
558   (assert! (job (Aull DeWitt) (administration secretary)))
559   (assert! (salary (Aull DeWitt) 25000))
560   (assert! (supervisor (Aull DeWitt) (Warbucks Oliver)))
561   (assert! (can-do-job (computer wizard) (computer programmer)))
562   (assert! (can-do-job (computer wizard) (computer technician)))
563   (assert! (can-do-job (computer programmer)
564 		       (computer programmer trainee)))
565   (assert! (can-do-job (administration secretary)
566 		       (administration big wheel)))))
567 
568 (eval-query
569  '(assert! (rule (same ?x ?x))))
570 
571 (newline)
572 (test-query
573  '(supervisor ?employee (Bitdiddle Ben))
574  '((supervisor (tweakit lem e) (bitdiddle ben))
575    (supervisor (fect cy d) (bitdiddle ben))
576    (supervisor (hacker alyssa p) (bitdiddle ben))))
577 (test-query 
578  '(job ?x (accounting . ?title))
579  '((job (cratchet robert) (accounting scrivener))
580    (job (scrooge eben) (accounting chief accountant))))
581 (test-query
582  '(address ?person (Slumerville . ?rest))
583  '((address (aull dewitt) (slumerville (onion square) 5))
584    (address (reasoner louis) (slumerville (pine tree road) 80))
585    (address (bitdiddle ben) (slumerville (ridge road) 10))))
586 (test-query 
587  '(and (supervisor ?x (Bitdiddle Ben))
588        (address ?x ?address))
589  '((and (supervisor (tweakit lem e) (bitdiddle ben)) (address (tweakit lem e) (boston (bay state road) 22)))
590    (and (supervisor (fect cy d) (bitdiddle ben)) (address (fect cy d) (cambridge (ames street) 3)))
591    (and (supervisor (hacker alyssa p) (bitdiddle ben)) (address (hacker alyssa p) (cambridge (mass ave) 78)))))
592 (test-query
593  '(and (salary (Bitdiddle Ben) ?ben-salary)
594        (salary ?x ?x-salary)
595        (lisp-value < ?x-salary ?ben-salary))
596  '((and (salary (bitdiddle ben) 60000) (salary (aull dewitt) 25000) (lisp-value < 25000 60000))
597    (and (salary (bitdiddle ben) 60000) (salary (cratchet robert) 18000) (lisp-value < 18000 60000))
598    (and (salary (bitdiddle ben) 60000) (salary (reasoner louis) 30000) (lisp-value < 30000 60000))
599    (and (salary (bitdiddle ben) 60000) (salary (tweakit lem e) 25000) (lisp-value < 25000 60000))
600    (and (salary (bitdiddle ben) 60000) (salary (fect cy d) 35000) (lisp-value < 35000 60000))
601    (and (salary (bitdiddle ben) 60000) (salary (hacker alyssa p) 40000) (lisp-value < 40000 60000))))
602 (test-query
603  '(and (supervisor ?employee ?supervisor)
604        (job ?supervisor ?job)
605        (not (job ?supervisor (computer . ?title))))
606  '((and (supervisor (aull dewitt) (warbucks oliver)) (job (warbucks oliver) (administration big wheel)) (not (job (warbucks oliver) (computer . ?title))))
607    (and (supervisor (cratchet robert) (scrooge eben)) (job (scrooge eben) (accounting chief accountant)) (not (job (scrooge eben) (computer . ?title))))
608    (and (supervisor (scrooge eben) (warbucks oliver)) (job (warbucks oliver) (administration big wheel)) (not (job (warbucks oliver) (computer . ?title))))
609    (and (supervisor (bitdiddle ben) (warbucks oliver)) (job (warbucks oliver) (administration big wheel)) (not (job (warbucks oliver) (computer . ?title))))))
610 
611 (eval-query
612  '(assert! (rule (can-replace? ?p1 ?p2)
613 		 (and (or (and (job ?p1 ?job)
614 			       (job ?p2 ?job))
615 			  (and (job ?p1 ?j1)
616 			       (job ?p2 ?j2)
617 			       (can-do-job ?j1 ?j2)))
618 		      (not (same ?p1 ?p2))))))
619 (test-query 
620  '(can-replace? ?x (Fect Cy D))
621  '((can-replace? (bitdiddle ben) (fect cy d))
622    (can-replace? (hacker alyssa p) (fect cy d))))
623 (test-query 
624  '(and (salary ?low ?low-salary)
625        (salary ?high ?high-salary)
626        (can-replace? ?low ?high)
627        (lisp-value < ?low-salary ?high-salary))
628  '((and (salary (aull dewitt) 25000) (salary (warbucks oliver) 150000) (can-replace? (aull dewitt) (warbucks oliver)) (lisp-value < 25000 150000))
629    (and (salary (fect cy d) 35000) (salary (hacker alyssa p) 40000) (can-replace? (fect cy d) (hacker alyssa p)) (lisp-value < 35000 40000))))
630 (eval-query
631  '(assert! (rule (big-shot ?bigshot)
632 		 (and (job ?bigshot (?dept . ?job-title))
633 		      (or (not (supervisor ?bigshot ?boss))
634 			  (and (supervisor ?bigshot ?boss)
635 			       (not (job ?boss (?dept . ?boss-title)))))))))
636 (test-query 
637  '(big-shot ?x)
638  '((big-shot (warbucks oliver))
639    (big-shot (scrooge eben))
640    (big-shot (bitdiddle ben))))
641 (eval-queries
642  '((assert! (meeting accounting (Monday 9am)))
643    (assert! (meeting administration (Monday 10am)))
644    (assert! (meeting computer (Wednesday 3pm)))
645    (assert! (meeting administration (Friday 1pm)))
646    (assert! (meeting whole-company (Wednesday 4pm)))))
647 (test-query '(meeting ?div (Friday ?time))
648 	    '((meeting administration (friday 1pm))))
649 (eval-query
650  '(assert! (rule (meeting-time ?person ?day-and-time)
651 		 (or (and (job ?person (?dept . ?title))
652 			  (meeting ?dept ?day-and-time))
653 		     (meeting whole-company ?day-and-time)))))
654 
655 (test-query '(meeting-time (Hacker Alyssa P) (Wednesday ?time))
656 	    '((meeting-time (hacker alyssa p) (wednesday 3pm))
657 (meeting-time (hacker alyssa p) (wednesday 4pm))))
658 
659 (define (name<? name1 name2)
660   (let ((str1 (fold-left 
661 	       (lambda (str sym)
662 		 (string-append str (symbol->string sym)))
663 	       ""
664 	       name1))
665 	(str2 (fold-left 
666 	       (lambda (str sym)
667 		 (string-append str (symbol->string sym)))
668 	       ""
669 	       name2)))
670     (string<? str1 str2)))
671 
672 (eval-query '(assert! (rule (lives-near ?person-1 ?person-2)
673 			    (and (address ?person-1 (?town . ?rest-1))
674 				 (address ?person-2 (?town . ?rest-2))
675 				 (not (same ?person-1 ?person-2))
676 				 (lisp-value name<? ?person-1 ?person-2)))))
677 
678 (test-query '(lives-near ?person-1 ?person-2)
679 	    '((lives-near (aull dewitt) (reasoner louis))
680 	      (lives-near (aull dewitt) (bitdiddle ben))
681 	      (lives-near (fect cy d) (hacker alyssa p))
682 	      (lives-near (bitdiddle ben) (reasoner louis))))
683 (eval-query '(assert! (rule (?x next-to ?y in (?x ?y . ?u)))))
684 (eval-query '(assert! (rule (?x next-to ?y in (?v . ?z))
685 			    (?x next-to ?y in ?z))))
686 (test-query '(?x next-to ?y in (1 (2 3) 4))
687 	    '(((2 3) next-to 4 in (1 (2 3) 4))
688 	      (1 next-to (2 3) in (1 (2 3) 4))))
689 (test-query '(?x next-to 1 in (2 1 3 1))
690 	    '((3 next-to 1 in (2 1 3 1))
691 	      (2 next-to 1 in (2 1 3 1))))
692 (eval-queries
693  '((assert! (rule (last-pair (?x) (?x))))
694    (assert! (rule (last-pair (?x . ?y) (?z))
695 		  (last-pair ?y (?z))))))
696 (test-query '(last-pair (3) ?x)
697 	    '((last-pair (3) (3))))
698 (test-query '(last-pair (1 2 3))
699 	    '((last-pair (1 2 3) (3))))
700 (test-query '(last-pair (2 ?x) (3))
701 	    '((last-pair (2 3) (3))))
702 
703 
704 ;; Exercise 4.63.  The following data base (see Genesis 4) traces the genealogy of the descendants of Ada back to Adam, by way of Cain:
705 
706 (eval-queries
707  '((assert! (son Adam Cain))
708    (assert! (son Cain Enoch))
709    (assert! (son Enoch Irad))
710    (assert! (son Irad Mehujael))
711    (assert! (son Mehujael Methushael))
712    (assert! (son Methushael Lamech))
713    (assert! (wife Lamech Ada))
714    (assert! (son Ada Jabal))
715    (assert! (son Ada Jubal))))
716 
717 ;; Formulate rules such as ``If S is the son of F, and F is the son of G, then S is the grandson of G'' and ``If W is the wife of M, and S is the son of W, then S is the son of M'' (which was supposedly more true in biblical times than today) that will enable the query system to find the grandson of Cain; the sons of Lamech; the grandsons of Methushael. (See exercise 4.69 for some rules to deduce more complicated relationships.) 
718 
719 
720 
721 (eval-queries
722  '((assert! (rule (grandson ?g ?s)
723 		 (and (son ?g ?f)
724 		      (son ?f ?s))))
725    (assert! (rule (son ?f ?s)
726 		  (and (wife ?f ?m)
727 		       (son ?m ?s))))))
728 (test-query 
729  '(grandson Cain ?grandson)
730  '((grandson cain irad)))
731 (test-query 
732  '(son Lamech ?son)
733  '((son lamech jubal)
734    (son lamech jabal)))
735 (test-query 
736  '(grandson Methushael ?grandson)
737  '((grandson methushael jubal)
738    (grandson methushael jabal)))