Call the original haddockHook with the updated flags rather than the
haddock command.

-- Define CPP __HADDOCK__ macro when running haddock.
haddockHook' p l h f =
do putStrLn ("f=\"" ++ show f ++ "\"")
     putStrLn ("g=\"" ++ show g ++ "\"")
     haddockHook simpleUserHooks p l h g
where
    -- The Haddock flag to pass a flag to GHC to define the macro.
    define__HADDOCK__ = ("haddock",["--optghc=-D__HADDOCK__"])

    -- Add the flag to the the other flags.
    g = f `mappend` emptyHaddockFlags { haddockProgramArgs = [define__HADDOCK__] }

No change in output.

Regards,
Sean