Setup.hs

import Distribution.Simple
import Distribution.Simple.Setup
import Distribution.Simple.LocalBuildInfo
import Distribution.PackageDescription
import Distribution.Version

import Text.Groom

-- This checks the direct dependency of the bytestring package being
-- compiled against to set the bytestring_has_itoa_c and
-- bytestring_has_builder flags accordingly.

main = defaultMainWithHooks simpleUserHooks {
    confHook = \pkg flags -> do
      putStr "pkg:\n\n"
      putStr (groom pkg)
      putStr "\n\n\nflags:\n\n"
      putStr (groom flags)
      if null (configConstraints flags)
       then do
        lbi <- confHook simpleUserHooks pkg flags
        putStr "\n\n\nlbi:\n\n"
        putStr (groom lbi)
        putStr "\n"
        return lbi
       else do
        let bytestring_version =
                case [ versionBranch v
                     | (Dependency pkg ver)  <- configConstraints flags
                     , pkg == PackageName "bytestring"
                     , (Just v) <- [isSpecificVersion ver] ]
                  of
                     [v] -> v
                     vs   -> error ("error detecting bytestring version  "  ++ show vs)
        putStr "\n\n\nbytestring_version:  "
        putStr (groom bytestring_version)


        let has_itoa_c  = ( FlagName "bytestring_has_itoa_c"
                          , bytestring_version >= [0,10]      )

        let has_builder = ( FlagName "bytestring_has_builder"
                          , bytestring_version >= [0,10,4]   )

        let update fs gs =
                 fs ++ [ g | g <- gs, not $ any (\f -> fst f == fst g) fs]

        let flags' = flags { configConfigurationsFlags =
                                update [has_itoa_c, has_builder]
                                       (configConfigurationsFlags flags) }

        putStr "\n\nconfigConfigurationsFlags flags':\n\n"
        putStr (groom (configConfigurationsFlags flags'))
        putStr "\n\n"

        lbi' <- confHook simpleUserHooks pkg flags'
        putStr "\n\nlbi':\n\n"
        putStr (groom lbi')
        putStr "\n"
        return lbi'
  }