Editing WxHaskell/Quick start (section)

==== Painting ====

Let us look at the paint event handler of the panel:

<pre>    -- paint the balls
    paintBalls :: Var [[Point]] -&gt; DC a -&gt; Rect -&gt; IO ()
    paintBalls vballs dc viewArea
      = do balls &lt;- varGet vballs
           set dc [brushColor := red, brushKind := BrushSolid]
           mapM_ (drawBall dc) [p | (p:ps) &lt;- balls]

    drawBall dc pt
      = circle dc pt radius []</pre>
A paint event handler gets two arguments: a device context (<tt>DC</tt>) to draw on and a rectangle that specifies the coordinates of the viewing area. We have supplied the first argument ourselves when setting the event handler, namely the mutable variable that holds the list of all balls.

As said, a single ball is represented as a list of all its future positions. When painting the current balls, we simple extract the head positions of all balls and draw them using <tt>drawBall</tt>. Drawing combinators like <tt>circle</tt> draw using the current ''pen'', ''brush'', and ''font'' of the device context. By default, a brush is transparent so we set it to a solid red brush before drawing the circles. Note that this is an optimization, we could have achieved the same effect by setting it for each circle individually: <tt>circle dc pt radius [brushKind := BrushSolid, brushColor := red]</tt>. You can read more about drawing in the documentation of the [http://hackage.haskell.org/packages/archive/wx/latest/doc/html/Graphics-UI-WX-Draw.html WX.Draw] module.