sicp-ex-5.8

1.

the value in register a is 3.

 (define (label-exist? labels lable-name) 
   (assoc label-name labels)) 
  
 (define (extract-labels text) 
   (if (null? text) 
       (cons '() '()) 
       (let ((result (extract-labels (cdr text)))) 
         (let ((insts (car result)) (labels (cdr result))) 
           (let ((next-inst (car text))) 
             (if (symbol? next-inst) 
                 (if (label-exist? labels next-inst) 
                     (error "the label has existed EXTRACT-LABELS" next-labels) 
                     (cons insts 
                           (cons (make-label-entry next-inst insts) labels))) 
                 (cons (cons (make-instruction next-inst) insts) 
                       labels))))))) 

1. the value of a is 3

 (define (extract-labels text receive) 
   (if (null? text) 
       (receive '() '()) 
       (extract-labels (cdr text) 
                       (lambda (insts labels) 
                         (let ((next-inst (car text))) 
                           (if (sysmbol? next-inst) 
                               (let ((s (assoc next-inst labels))) 
                                 (if s 
                                     (error "Repeated label name" next-inst) 
                                     (receive insts 
                                              (cons (make-label-entry next-inst 
                                                                      insts) 
                                                    labels)))) 
                               (receive (cons (make-instruction next-inst) 
                                              insts) 
                                        labels)))))))