;; 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
  'thing
  named-object-class
  '(owner)
  '(owned?
    owner
    become-unowned
    become-owned-by))

(class/set-method!
 thing-class 'init
 (lambda (this name)
   (named-object^init this name)
   (thing/set-owner! this 'no-one)))

(class/set-method!
 thing-class 'owned?
 (lambda (this)
   (not (equal? (thing/owner this)
                'no-one))))

(class/set-method!
 thing-class 'owner
 (lambda (this)
   (thing/get-owner this)))

(class/set-method!
 thing-class 'become-unowned
 (lambda (this)
   (thing/set-owner! this 'no-one)))

(class/set-method!
 thing-class 'become-owned-by
 (lambda (this person)
   (thing/set-owner! this person)))