[racket] Choosing what class to instantiate at runtime

From: Kieron Hardy (kieron.hardy at gmail.com)
Date: Mon Jun 25 16:25:49 EDT 2012

Hi all,

I'd like to refactor and extend the code below, such that r is either
defined to be an instance of an item-representer% or a
multi-item-representer%, depending on whether param-a is a single item or a

The solution that springs to mind is to choose which class to instantiate
with (if (list? param-a) ,,,) but then I'd have to state the parameter
lists twice.

It seems to me there's got to be a better way in Racket to do this. Thanks
in advance for any suggestions on how to proceed.




#lang racket

(define text-item "hello world")
(define text-item-list (list "alpha bravo" "charlie delta" "echo foxtrot"))

(define x%
  (class object%

(define item-representer%
  (class x%
    (init param-a param-b param-c)
    (field (item param-a))
    (define/public (print)
      (printf "item:~a~n" item))

(define multi-item-representer%
  (class x%
    (init param-a param-b param-c)
    (field (item-list (map (lambda (item) (new item-representer% [param-a
item] [param-b param-b] [param-c param-c])) param-a)))
    (define/public (print)
      (printf "multi-item:~n")
      (map (lambda (item) (send item print)) item-list)

(define r1 (new item-representer% [param-a text-item] [param-b 10] [param-c
(define r2 (new multi-item-representer% [param-a text-item-list] [param-b
20] [param-c "green"]))

(send r1 print)
(send r2 print)

;(define r (some-representer% [param-a x] [param-b 50] [param-c "yellow"]))
;(send r print)
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