For some reason all traffic from this beginners list stopped reaching me after the 13th of December. Thanks for the response, Antoine. I've tried to install more recent versions of GHC (6.10 and 6.12) but I cannot get either of them to work at all. GHC 6.8 generates code that segfaults and the GHC 6.10 from Debian testing cannot compile anything for me. So I thought I'd try GHC 6.12. That isn't in any repo so I decided to build it from source. I uninstalled all other GHCs to give it a fresh start. The GHC page says "Stop, install the Haskell Platform" but the Haskell Platform says you must install GHC first, so I've got a nice circular dependency right from the start! So I tried installing GHC 6.12 from source but that requires GHC to be installed. So I installed GHC 6.8 (the one that segfaults) using apt again from Debian. That seems to have built a GHC 6.12 that I can run from the command line but it cannot compile that program because it doesn't have parallel stuff. So I thought I'd install the Haskell Platform. Unfortunately, the Haskell Platform configure script gives the nonsensical error that I must "upgrade" from GHC 6.12 down to GHC 6.10. I was getting pretty run down by this point so I just told it to sod off using the --enable-unsupported-ghc-version command line option. The Haskell Platform's configure script then completed but building it failed with: [21 of 21] Compiling Graphics.UI.GLUT ( Graphics/UI/GLUT.hs, dist/build/Graphics/UI/GLUT.p_o ) Registering GLUT-2.1.1.2... Setup: GLUT-2.1.1.2: dependency "OpenGL-2.2.1.1-182b091280ce0de861295bc592bae77c" doesn't exist (use --force to override) Error: Building the GLUT-2.1.1.2 package failed make: *** [build.stamp] Error 2 I have glut and use it all the time from OCaml without a hitch so I've no idea what the problem is here. Oh well, I just discovered that installing GHC 6.12 has at least fixed GHC 6.10 so it can now compile and run that mergesort. Oh FFS, spoke too soon: $ time ./mergesort +RTS -N8 Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. real 0m38.801s user 4m0.851s sys 0m1.308s -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e