sicp-ex-5.30

  
  
 ;; if the variable is unbounded, cons 'unbound with it;  
 ;; otherwise, cons 'bounded with it. then we can use 
 ;; car to judge it's bounded or not. 
 (define (lookup-variable-value var env) 
   (define (env-loop env) 
     (define (scan vars vals) 
       (cond ((null? vars) 
              (env-loop (enclosing-environment env))) 
             ((eq? var (car vars)) 
              (cons 'bounded (car vals)))              ;; **** 
             (else (scan (cdr vars) (cdr vals))))) 
     (if (eq? env the-empty-environment) 
             (cons 'unbounded '())                        ;; *** 
         (let ((frame (first-frame env))) 
           (scan (frame-variables frame) 
                 (frame-values frame))))) 
   (env-loop env)) 
 (define (bound-variable? var) 
  (and (pair? var) (eq? (car var) 'bounded))) 
 (define (extract-variable-value var) 
  (cdr var)) 
  
 ev-variable 
   (assign val (op lookup-variable-value) (reg exp) (reg env)) 
   (test (op bound-variable?) (reg val)) 
   (branch (label bound-variable)) 
   (assign val (const unbounded-variable-error)) 
   (goto (label signal-error))  
 bound-variable 
   (assign val (op extract-variable-value) (reg val)) 
   (goto (reg continue))