09_classes_and_objects.rkt

#lang racket ; defines the language we are using

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 9. Classes and Objects
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; Create a class fish% (-% is idiomatic for class bindings)
(define fish%
  (class object%
    (init size) ; initialization argument
    (super-new) ; superclass initialization
    ;; Field
    (define current-size size)
    ;; Public methods
    (define/public (get-size)
      current-size)
    (define/public (grow amt)
      (set! current-size (+ amt current-size)))
    (define/public (eat other-fish)
      (grow (send other-fish get-size)))))

;; Create an instance of fish%
(define charlie
  (new fish% [size 10]))

;; Use `send' to call an object's methods
(send charlie get-size) ; => 10
(send charlie grow 6)
(send charlie get-size) ; => 16

;; `fish%' is a plain "first class" value, which can get us mixins
(define (add-color c%)
  (class c%
    (init color)
    (super-new)
    (define my-color color)
    (define/public (get-color) my-color)))
(define colored-fish% (add-color fish%))
(define charlie2 (new colored-fish% [size 10] [color 'red]))
(send charlie2 get-color)
;; or, with no names:
(send (new (add-color fish%) [size 10] [color 'red]) get-color)