Type Registry

bob_and_alice.scm

#lang racket

(define (register h method tag function)
  (hash-set! h (list tag method) function))

(define (lookup h key)
  (hash-ref h key))

(define h (make-hash))
(register h  'alice-box 'width alice-width)
(register h  'bob-box 'width bob-width)

(define (width box)
  ((lookup h (list 'width (car box))) box))

(define (height box)
  ((lookup h (list 'height (car box))) box))

(define b (make-bob-box 1 2 3 4))
(width b) ;--> 3