sicp-ex-4.77



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poly

  
  
  
 (define (filter? exp) 
   (or (list-value? exp) 
       (not? exp))) 
  
 (define (conjoin conjuncts frame-stream) 
   (conjoin-mix conjuncts '() frame-stream)) 
  
 (define (conjoin-mix conjs delayed-conjs frame-stream) 
   (if (null? conjs) 
       (if (null? delayed-conjs) 
           frame-stream          ; conjoin finish if both of conjuncts are empty 
           the-empty-stream)     ; no result return cause filters with unbound vars exist 
       (let ((first (first-conjunct conjs)) 
             (rest (rest-conjuncts conjs))) 
         (if (filter? first) 
             (let ((check-result 
                    (conjoin-check first delayed-conjs frame-stream))) 
               (conjoin-mix rest 
                            (car check-result) 
                            (cdr check-result))) 
             (let ((new-frame-stream (qeval first frame-stream))) 
               (let ((delayed-result 
                      (conjoin-delayed delayed-conjs '() new-frame-stream))) 
                 (conjoin-mix rest (car delayed-result) (cdr delayed-result)))))))) 
  
 (define (conjoin-delayed delayed-conjs rest-conjs frame-stream) 
   ; evaluate those conjuncts in delayed-conjs if there are 
   ; enough bindings for them. 
   (if (null? delayed-conjs) 
       (cons rest-conjs frame-stream) 
       (let ((check-result 
              (conjoin-check (first-conjunct delayed-conjs) 
                             rest-conjs frame-stream))) 
         (conjoin-delayed (cdr delayed-conjs) 
                          (car check-result) 
                          (cdr check-result))))) 
  
 (define (conjoin-check target conjs frame-stream) 
   ; Check if there are any unbound vars in target. 
   ; Delay it if there are unbound vars, or just evaluate it. 
   (if (has-unbound-var? (contents target) (stream-car frame-stream)) 
       (cons (cons target conjs) frame-stream) 
       (cons conjs (qeval target frame-stream)))) 
  
 (define (has-unbound-var? exp frame) 
   (define (tree-walk exp) 
     (cond ((var? exp) 
            (let ((binding (binding-in-frame exp frame))) 
              (if binding 
                  (tree-walk (binding-value binding)) 
                  true))) 
           ((pair? exp) 
            (or (tree-walk (car exp)) (tree-walk (cdr exp)))) 
           (else false))) 
   (tree-walk exp)) 

unique is also sort of filter, but won't work with this because the contents of unique usually contain at least one variable that is not relevant with other variables.