sicp-ex-2.88



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meterogan

  
 (define (negate x) (apply-generic 'negate x)) 
  
 ;; add into scheme-number package 
 (put 'negate 'scheme-number 
       (lambda (n) (tag (- n)))) 
  
 ;; add into rational package 
 (put 'negate 'rational 
      (lambda (rat) (make-rational (- (numer rat)) (denom rat)))) 
  
 ;; add into complex package 
 (put 'negate 'complex 
      (lambda (z) (make-from-real-imag (- (real-part z)) 
                                       (- (imag-part z))))) 
  
 ;; add into polynomial package 
 (define (negate-terms termlist) 
   (if (empty-termlist? termlist) 
         the-empty-termlist 
         (let ((t (first-term termlist))) 
           (adjoin-term (make-term (order t) (negate (coeff t))) 
                        (negate-terms (rest-terms termlist)))))) 
 (put 'negate 'polynomial 
          (lambda (poly) (make-polynomial (variable poly) 
                                          (negate-terms (term-list poly))))) 
 (put 'sub '(polynomial polynomial) 
       (lambda (x y) (tag (add-poly x (negate y))))) 
          
 (define (negate-terms termlist) 
    (map  
       (lambda (t)(make-term(order t) 
                            (- (coeff t)))) 
        termlist)) 
  
 ;; I got the same idea as hi-artem to use map procedure, however I think we need to use the generic negate for coeff as well 
  
 (define (negate-poly p) 
   (make-polynomial (variable p) 
                    (map 
                      (lambda (term) 
                        (make-term 
                          (order term) 
                          (negate (coeff term)))) 
                      (term-list p)))) 

I think hi-artem is wrong and meterogan is right. Because MAP only works on lists, it is impossible to achieve Abstract masking if MAP is used.However, there are also errors in meterogan's writing.It should look like this:

 ;; add into complex package 
 (put 'negate 'complex 
      (lambda (z) (make-from-real-imag (negate (real-part z)) 
                                       (negate (imag-part z))))) 
  
 (put 'sub '(polynomial polynomial) 
       (lambda (x y) (tag (add-poly x (contents (negate (tag y)))))))