[racket] update canvas image

From: kty1104 at gmail.com (kty1104 at gmail.com)
Date: Wed Oct 27 04:54:05 EDT 2010

I am making map editor
one of the editor's ability is select tile image from chip-set(kind of town  
map tile) image
and circle the selected tile
I made a following code but I don't think that this is good way to update  
image and it doesn't working properly as well.
could somebody give me an little advice?

#lang scheme/gui
(require 2htdp/image)
(define pic dc<%>)
(define chipset (make-object bitmap% "town.png" 'png))
(define f (instantiate frame% ("map editor")))
(define mcan%
(class canvas%

(override on-event on-paint)
(define on-paint
(λ () (send (send this get-dc) draw-bitmap chipset 0 0)))
(define on-event (λ (e)
(send msg set-label
(string-append (number->string (floor (/ (send e get-x) 16))) " "
(number->string (floor (/ (send e get-y) 16)))))
(when (send e get-left-down)
(begin (set! chipset (place-image (circle 8 "outline" "orange")
(send e get-x)
(send e get-y)
(send f refresh)));it doesn't seems to good way
(super-instantiate ())))

(define mcan (new mcan% (parent f) (min-width (image-width chipset))  
(min-height (image-height chipset))))

(define msg (instantiate message% ("nothing so far" f)))
(define tile-width-field (instantiate text-field% ("width")  
(init-value "16") (parent f) (stretchable-width #f)))

(define tile-height-field (instantiate text-field% ("height")  
(init-value "16") (parent f) (stretchable-width #f)))

(send f show #t)
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