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.

(define (make-from-mag-ang r a)
    (define (dispatch op)
        (cond ((eq? op 'real-part ) (* r (cos a)))
              ((eq? op 'imag-part ) (* r (sin a)))
              ((eq? op 'magnitude ) r)
              ((eq? op 'angle ) a)
              (else (error "Unknown op: MAKE-FROM-MAG-ANG" op)))))