Hi, In my CHP library I need to do some exception catching. I want the library to work on GHC 6.8 (with base-3 -- this is the current version in Ubuntu Hardy and Jaunty, for example) and GHC 6.10 (which comes with base-4). But base-3 and base-4 need different code for exception catching (whether it's importing Control.OldException or giving a type to the catch method). Here's what I currently do -- my Haskell file contains this: #if __GLASGOW_HASKELL__ >= 609 import qualified Control.OldException as C #else import qualified Control.Exception as C #endif My cabal file contains this (it used to say just "base,..." but Hackage complained at me the other day when I tried to upload that): Build-Depends: base >= 3 && < 5, ... This works on two machines: one is 6.8+base-3, the other is 6.10+base-3&base-4, where cabal seems to use base-4. However, I have had a bug report (now replicated) which stems from a different 6.10+base-3&base-4 machine where cabal picks base-3 instead. The real problem is that the #if is based on GHC version, but really it should be based on which base-* library is being used. I know the code works with base-3 (use Control.Exception) and base-4 (use Control.OldException) but I can't get the build process right to alter the code based on which base-* library is being used. Can anyone tell me how to fix this? I don't think that changing to always use Control.Exception would fix this, because I need to give a different type for catch in base-3 to base-4, so there's still the incompatibility to be dealt with. Thanks, Neil.