Hello all, I have been exposed to a problem that have happened to others too, and since I have found a (scary) solution with the help of Duncan Coutts, I'm now sharing it with you. The reason I wanted to pass specific CPP flags to haddock was to allow the documentation of the full module to get built, even when only parts of the module would end up getting compiled because of restrictions on the build platform. In this mail, Duncan Coutts gives a very helpful bit of code to override the haddockHook and pass additional options to haddock : http://www.haskell.org/pipermail/haskell-cafe/2008-December/051785.html In my case, I needed something very similar, except that I needed to pass those additional options to hsc2hs which my code uses. This worked fine as long as the cabal commands sequence was ["configure","haddock"] and not ["configure","build","haddock"]. The reason for this is that in the latter case, the library wasn't being processed again, but output of previous runs of hsc2hs were reused. So I worked around this with an ugly hack that removes the .hs files before running haddock, in order to force re-processing! Here's the final code I ended up with : import Distribution.Simple import Distribution.Simple.LocalBuildInfo (LocalBuildInfo(withPrograms), buildDir) import Distribution.Simple.Program (userSpecifyArgs) import System.Directory import System.FilePath -- Define __HADDOCK__ when building documentation. main = defaultMainWithHooks simpleUserHooks { haddockHook = \pkg lbi h f -> do let progs = userSpecifyArgs "hsc2hs" ["-D__HADDOCK__"] (withPrograms lbi) removePreProcessedFiles (buildDir lbi) haddockHook simpleUserHooks pkg lbi { withPrograms = progs } h f } -- Horrible hack to force re-processing of the .hsc file. Otherwise -- the __HADDOCK__ macro doesn't end up being defined. removePreProcessedFiles :: FilePath -> IO () removePreProcessedFiles dir = removeFile (dir "System/BSD/Sysctl.hs") `catch` \_ -> return () Cheers, Maxime