Ok, a bit hacking reveals that GLUT on Mac OS X performs differently in GHCi and when compiled. I use the following code: ==== import Graphics.UI.GLUT import Graphics.Rendering.OpenGL main = do getArgsAndInitialize createAWindow "Hello window" mainLoop createAWindow windowName = do createWindow windowName displayCallback $= (clear [ColorBuffer] >> swapBuffers) --idleCallback $= Just (postRedisplay Nothing) passiveMotionCallback $= Just (putStrLn . show) ==== When compiled, it renders a black screen and prints the event when you move mouse over. It's crucial to have swapBuffers in displayCallback, otherwise it will not render. But if run in GHCi, it won't render a thing unless idleCallback line is uncommented. Also, it won't produce any output at mouse over. Another strange thing is with idleCallback set and swapBuffers removed, it still renders black screen in GHCi, but will not render when compiled. I wonder why the event callback is blocked when using GLUT in GHCi? Regards, Paul L