send と send-generic

racket-class-generic.rkt

#lang racket

(define sprite<%> (interface () draw))
(define draw (generic sprite<%> draw))

(define pix%
  (class* object% (sprite<%>)
    (super-new)
    (define/public (draw)
      'pix-draw)))

(define tex%
  (class* object% (sprite<%>)
    (super-new)
    (define/public (draw)
      'tex-draw)))

;; interface sprice<%>をimplせずにdrawを実装
(define hoge%
  (class object%
    (super-new)
    (define/public (draw)
      'hoge-draw)))

(define ls (list (new tex%) (new pix%) (new hoge%)))

;; OK メソッド名だけでディスパッチ (ダックタイピング)
(map (lambda (obj) (send obj draw))
     ls)
;;=> '(tex-draw pix-draw hoge-draw)


;; Type Error 
;; 同じメソッド名でも、型が違えばエラー
(map (lambda (obj)
       (send-generic obj draw) )
     ls)

;;=> generic:draw: target is not an instance of the generic's interface
;;   target: (object:hoge% ...)
;;   interface name: sprite<%>