Thank you very much. That clears everything right up.<div>-Hillary<br><br><div class="gmail_quote">On Wed, Jun 13, 2012 at 4:11 PM, Stephen Bloch <span dir="ltr"><<a href="mailto:bloch@adelphi.edu" target="_blank">bloch@adelphi.edu</a>></span> wrote:<br> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im"><br> On Jun 13, 2012, at 3:16 PM, Hillary Ryan wrote:<br> <br> > Why does this work?<br> > (define canvas (new canvas% [parent window] [paint-callback (lambda (canvas dc) (draw canvas dc))]))<br> ><br> > But this doesn't<br> > (define canvas (new canvas% [parent window] [paint-callback (draw canvas dc)]))<br> ><br> > It seems that in both cases the paint-callback is being set to a 2-arity procedure.<br> <br> </div>No, "(draw canvas dc)" is the RESULT of a 2-arity procedure. "draw" (no parentheses, no arguments) would be a 2-arity procedure, equivalent to "(lambda (canvas dc) (draw canvas dc))".<br> <div class="im"><br> > However, in the second case the window doesn't update unless draw is called from REPL.<br> <br> </div>Yes, that's because you're only calling draw once, at the time you define canvas.<br> <span class="HOEnZb"><font color="#888888"><br> <br> <br> Stephen Bloch<br> <a href="mailto:sbloch@adelphi.edu">sbloch@adelphi.edu</a><br> <br> </font></span></blockquote></div><br></div>