sicp-ex-4.9

  
  
 ;; i only implement while. 
 ;; add this to eval 
  ((while? expr) (evaln (while->combination  expr) env)) 
  
 ;; while expression 
 (define (while? expr) (tagged-list? expr 'while)) 
 (define (while-condition expr) (cadr expr)) 
 (define (while-body expr) (caddr expr)) 
 (define (while->combination expr) 
         (sequence->exp 
                 (list (list 'define  
                                 (list 'while-iter) 
                                         (make-if (while-condition expr)  
                                                          (sequence->exp (list (while-body expr)  
                                                                                        (list 'while-iter))) 
                                                          'true)) 
                           (list 'while-iter))))