Use hs-plugins. I'm not sure if this still works on modern ghc, or if it works on my platform (darwin intel), but I managed to get this compiled after "fixing" a use of STArray whose arity apparently changed, but I probably broke it because now I get "Error in array index" all the time. I gather Austin Seipp has been doing some work on this. Is there a newer version that should work? An easy fix for the error?
Which version have you tried to use? plugins-1.2 available on hackage works out of the box with ghc-6.8.2 on both linux and windows, we're using it for happs-hsp. Haven't tried it on Mac though, but I doubt it would contain any arity mismatches there, which leads me to believe that you're using an older one. Cheers, /Niklas