SICP/ex-4_70-79.scm

(load "util.scm")
(load "misc/sicp-query.scm")

(initialize-data-base microshaft-data-base)

(display "\nex-4.70 let-in-add-assertion\n")

; If we assigned to THE-ASSERTIONS directly then the new stream would reference
; itself and we would get an endless stream of the current assertion.
(display "[answered]\n")

(display "\nex-4.71 - simpler-simple-query\n")

; Using delay for the rule part can help to print at least some results before
; a potential endless loop.

; (eval-query '(rule (married ?x ?y)
;                    (married ?y ?x)))

; (eval-query '(married Minnie ?x))

(display "[answered]\n")

; I got this solution from SICP solutions because I could not think of a good
; example. It would be better to implement the loop-avoidance algorithm than
; relying on delayed evaluation to get some results, in my opinion.

(display "\nex-4.72 - stream-interleave\n")

; If only one of the disjuncts produces a result and the others create an
; endless-loop, interleaving guarantess that we get some results for the valid
; disjuncts.

(display "[answered]\n")


(display "\nex-4.73 - explicit-delay\n")

; The second argument to interleave must be delayed explicitly because
; MIT-Scheme's applicative-order-evaluation would evaluate the complete stream,
; otherwise. That would make the use of streams pointless.

(display "[answered]\n")

(display "\nex-4.74\n")

;(display "\nex-4.75\n")
;(display "\nex-4.76\n")
;(display "\nex-4.77\n")
;(display "\nex-4.78\n")
;(display "\nex-4.79\n")