Ubuntu 11.10 comes with this library libgmp.so.10 and previous versions come with libgmp.so.3. When you make ghc --make app.hs, it dynamically links libgmp.so.10. This won't run on 11.04 or earlier with the error message libgmp.so.10 not found. So, I compiled using ghc -static -optl-static -optl-pthread --make app.hs. (http://stackoverflow.com/questions/7344744/haskell-program-built-on-ubuntu-11-10-doesnt-run-on-ubuntu-10-04 got it running).