Comments on: SICP section 2.2.4

By: Deokhwan Kim

Deokhwan Kim — Mon, 03 Sep 2007 08:53:29 +0000

The direction of the authors’ y-axis appears to be different from that of your GUI library. I think that the authors have implicitly assumed that y-axis would grow upwards. Their rotate90 is correct under the assumption. MrEd’s y-axis appears to grow downwards.

By: eliben

eliben — Fri, 24 Aug 2007 11:55:54 +0000

Thanks for the tip. For trivial code like the one I’m writing here (just displaying an image, once) my approach works well enough. But surely for any serious code the method you present is the correct one.

By: Robby Findler

Robby Findler — Fri, 24 Aug 2007 11:36:19 +0000

You should probably do something like this (instead of the way you’re setting up drawing above) to setup the canvas:

(define canvas
(new canvas%
[parent frame]
[paint-callback (lamdba (x y) (my-draw))]))

and then you don’t have to put the sleep/yield (or the explicit call to my-draw).

This will make sure your drawing gets redrawn when the window refereshes.