1 (define (make-from-real-imag x y)
2   (define (dispatch op)
3     (cond ((eq? op 'real-part) x)
4 	  ((eq? op 'imag-part) y)
5 	  ((eq? op 'magnitude)
6 	   (sqrt (+ (square x) (square y))))
7 	  ((eq? op 'angle) (atan y x))
8 	  (else (error "Unknown op -- MAKE-FROM-REAL-IMAG" op))))
9   dispatch)
10 
11 (define (apply-generic op arg) (arg op))
12 
13 ;; Exercise 2.75.  Implement the constructor make-from-mag-ang in message-passing style. This procedure should be analogous to the make-from-real-imag procedure given above. 
14 
15 (define (make-from-mag-ang r a)
16   (define (dispatch op)
17     (cond ((eq? op 'real-part) (* r (cos a)))
18 	  ((eq? op 'imag-part) (* r (sin a)))
19 	  ((eq? op 'magnitude) r)
20 	  ((eq? op 'angle) a)
21 	  (else (error "Unknown op -- MAKE-FROM-REAL-IMAG" op))))
22   dispatch)
23 
24