In short, it doesn't work :-( . OpenGL (at least on MacOS) keeps track of the "current context" on a per-thread basis. The GLUT library sets the current context and calls back to the program. With GHC 5.03 compiled with --enable-threaded-rts, the callback gets executed in a different thread. There is no OpenGL context set up for that thread, so the first OpenGL call crashes. This might also be a problem for other state-based interfaces that use thread-local state. Has anyone already thought about how to solve this problem? I'm thinking about adding hooks to the RTS (in grabCapability, releaseCapability and scheduleThread_) which would be used for setting up the correct thread-local state whenever Haskell execution "switches" to a different OS thread. Those hook routines would have to be written in C and would be platform-specific most of the time. It's not a nice solution, but it's the only one I can think of at this time. Unless someone comes up with a better idea _quickly_, I'll try it out and then report how ugly it really is... :-) Cheers, Wolfgang