I was trying to compile GLUT and HOpenGL with profiling on (I added "GhcLibWays = p" in buid.mk). When it goes back to building the profiling versions, it fails with: Graphics/UI/GLUT/Callbacks/Registration.hs:64: Couldn't match `Maybe Window' against `Window' Expected type: Maybe Window Inferred type: Window In the third argument of `maybe', namely `win' In the result of a 'do' expression: maybe (error (func ++ ": no current window")) return win Even though things worked fine for the non profiling build. I don't understand why it would behave differently. I am using version 6.3 of the fptools disribution, because it is at that time that I had to play around to get fragment programs working. Was there known issues with profiling HOpenGL at that time ? Has a lot happened since ? Do I need to do more than set GhcLibWays ? Thank you for your help, Yann Morvan