Types as Dispatch Procedures

dispatch.scm

(define (make-bob-box x y width height)
  (define (dispatch message)
    (cond ((eq? message 'width) width)
          ((eq? message 'height) height)
          ((eq? message 'type) 'bob-box)
          (else (error "Bad method"))))
  dispatch)

(define (make-alice-box x1 y1 x2 y2)
  (define (dispatch message)
    (cond ((eq? message 'width) (abs(- x2 x1)))
          ((eq? message 'height) (abs(- y2 y1)))
          ((eq? message 'type) 'alice-box)
          (else (error "Bad method"))))
  dispatch)

(define a (make-alice-box 1 2 3 4))
(define b (make-bob-box 1 2 3 4))

(b 'width) ;---> 3