HOpenGL postRedisplay in Mac

Hello all,

I am following the tutorial on
http://www.haskell.org/haskellwiki/OpenGLTutorial2 everything works
ok, but the moment I  add "postRedisplay Nothing" to the idle function
nothing is displayed.
I've tried passing the current window to postRedisplay, adding double
buffering (it is supposed to be done internally on mac) but no image.
If I comment out postRedisplay then I can see the image but no animation

I am using Snow Leopard 10.6 and GHC from the 32bit Haskell Platform 2010.2.0.0

Any clue to what's happening?

Thx,
Francisco