SICP 問題 2.75
(define (make-from-mag-ang r a) (define (dispatch op) (cond ((eq? op 'magnitude) r) ((eq? op 'angle) a) ((eq? op 'real-part) (* r (cos a))) ((eq? op 'imag-part) (* r (sin a))) (else (error "Unknown op -- MAKE-FROM-MAG-ANG" op)))) dispatch)