2-78.rkt

#lang racket

(define *op-table* (make-weak-hash))

(define (put op type proc)
  (hash-set! *op-table* (list op type) proc))

(define (get op type)
  (hash-ref *op-table* (list op type) #f))

(define (attach-tag type-tag contents)
  (if (eq? type-tag 'scheme-number)
      contents
      (cons type-tag contents)))

(define (type-tag datum)
  (cond [(number? datum) 'scheme-number]
        [(pair? datum) (car datum)]
        [else (error "Bad tagged datum -- TYPE-TAG" datum)]))

(define (contents datum)
  (cond [(number? datum) datum]
        [(pair? datum) (cdr datum)]
        [else (error "Bad tagged datum -- CONTENTS" datum)]))

(define (apply-generic op . args)
  (let ((type-tags (map type-tag args)))
    (let ((proc (get op type-tags)))
      (if proc
          (apply proc (map contents args))
          (error
           "No method for these types -- APPLY-GENERIC"
           (list op type-tags))))))

(define (install-scheme-number-package)
  (define (tag x)
    (attach-tag 'scheme-number x))
  (put 'add '(scheme-number scheme-number) +)
  (put 'sub '(scheme-number scheme-number) -)
  (put 'mul '(scheme-number scheme-number) *)
  (put 'div '(scheme-number scheme-number) /)
  'done)

(install-scheme-number-package)

(define (add x y) (apply-generic 'add x y))
(define (sub x y) (apply-generic 'sub x y))
(define (mul x y) (apply-generic 'mul x y))
(define (div x y) (apply-generic 'div x y))

(define (make-scheme-number n)
  ((get 'make 'scheme-number) n))