Exercise 4.71

Read Exercise 4.71 ~ Solution

(define (simple-query query-pattern frame-stream)
   (lambda (frame)
     (stream-append (find-assertions query-pattern frame)
                    (apply-rules query-pattern frame)))

Using the regular stream-append will evaluate both operands at call time. We’ve already seen the problems that rules can have in particular with rules that cause infinite loops, for example the rule for reverse in Exercise 4.68.
Using stream-append-delayed with the delayed second operand at least displays matching assertions and individual rule results as they are discovered while diving down the potentially infinite rabbit hole of rules. It’s not a 100% satisfactory but it’s better than the alternative.


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