Welcome to MzScheme version 360, Copyright (c) 2004-2006 PLT Scheme Inc.
> (load "ch2support.scm")
> (put  1 1 2)
ok
> (get 1 1)
2
> 
  (define (real-part z) (car z))
> (put 'real-part 'rect real-part)
ok
> 
(define (contents datum)
  (if (pair? datum)
      (cdr datum)
      (error "Bad tagged datum -- CONTENTS" datum)))
> (define rn '(rect . (1 . 2)))
> (contents rn)
(1 . 2)
> ((get 'real-part 'rect) (contents rn))
1
> (put 'real-part 'rect real-part)
ok
> (get 'real-part 'rect)
#<procedure:real-part>
> 
  (define (real-part z) (car z))
  (define (imag-part z) (cdr z))
  (define (make-from-real-imag x y) (cons x y))
> > > 
(define (attach-tag type-tag contents)
  (cons type-tag contents))

(define (type-tag datum)
  (if (pair? datum)
      (car datum)
      (error "Bad tagged datum -- TYPE-TAG" datum)))

> > (define rn '(rect . (1 . 2)))
> (define rn (attach-tag 'rect (make-from-real-imag 1 2)))
> rn
(rect 1 . 2)
> (define rn '(rect . (1 . 2)))
> rn
(rect 1 . 2)
> ((get 'real-part (type-tag rn)) (contents rn))
1
> 
  (define (real-part z) (car z))
> (put 2 3 4)
ok
> done
> (get 'real-part 'rectangular)
#f
> (get 'real-part '(rectangular))
#<procedure:real-part>
> (get 6 7)
#f
>