[wxhaskell-users] trouble with event handlers

[Fabian Y. <fa...@cs...>]

hey!
I'm currently writing my first program with wxhaskell, in fact, it's the
first time I'm using haskell to write a program that goes beyond basic
data-structures and sorting algorithms so I hope this question doesn't
turn out to be non-wxhaskell specific.

Basically, what I want to do is react on key-presses just like in the
bouncing balls example or in this tutorial: 
http://www.cs.uu.nl/~daan/download/papers/wxhaskell.pdf.

So I wrote the following:

mainFrame =  do
  cs <- variable [ value := (CSys (Rt start_ul start_lr))]
  f <- frame []
             
  set f[
        layout := space fwidth fheight
       , on paint := drawCoordSys cs fwidth fheight
       , text:= "Coordinate System" 
       , on leftKey := do set cs [value :~ \x -> (csys_shift_x x
(-(fac_x_p2t fwidth x))) ] 
                          repaint f
       ]
                                
        
  return ()
  where
    start_ul = (Pt (-4) 2)
    start_lr = (Pt 2 (-2))
    fwidth = 800
    fheight = 600

When leftKey is pressed, this is supposed to set the mutable variable cs
and repaint f. This doesn't work for some reason. However, if I try the
same with a timer, it works:

t <- timer f [interval := 50
             , on command := do set cs [value :~ \x -> (csys_shift_x x
(-(fac_x_p2t fwidth x))) ] 
                                repaint f
             ]                      


Any ideas?
Thanks,

- Fabian Yamaguchi