;; This file contains excerpts from the textbook Concrete
;; Abstractions: An Introduction to Computer Science Using Scheme, by
;; Max Hailperin, Barbara Kaiser, and Karl Knight, Copyright (c) 1998
;; by Brooks/Cole Publishing Company. This file may not be reproduced
;; or redistributed other than for use with that textbook.

;; Chapter 14: Object-Oriented Programming

;; 14.5  An Application: Adventures in the Imaginary Land of Gack

(define-class
  'scroll
  thing-class
  '()
  '(be-read))

(class/set-method!
 scroll-class 'init
 (lambda (this title)
   (thing^init this title)))

(class/set-method!
 scroll-class 'be-read
 (lambda (this)
   (let ((owner (scroll/owner this))
         (title (scroll/name this)))
     (if (scroll/owned? this)
         (person/say owner
                     (list "I have read" title))
         (display-message (list "No one has" title))))))