Thank you for all your valuable replies! The last days I experimented, mostly with "cabal install world"; although this terminated regularly, my broken packages were not repaired. I guess this was because many of them are not "user" (local), which seems to be a prerequisite according to Daniel Fischer's detailed advice here: http://stackoverflow.com/questions/7961604/fixing-issues-noted-by-ghc-pkg-ch... I therefore decided to reinstall GHC and all the packages completely, what should not be too hard in my case. Unfortunately, it seems that there is no Haskell platform available yet which is compatible with ghc-7.6.2, the current ghc version in the Ubuntu 13.04 repo. So I shall wait for the upcoming release of the Haskell platform in May. I guess I should study Albert Lai's explanations in http://www.vex.net/~trebla/haskell/sicp.xhtml about cabalized packages in order to avoid similar flops in the future. (I just don't find this as exciting as learning about functors, monads, combinators ...) Thanks again for your suggestions! Johannes