Extract all hs-source-dirs from a cabal file

Main.hs

module Main (main) where

import System.Environment (getArgs)
import Data.Maybe (maybeToList)
import Data.List.Utils (uniq)
import Distribution.Verbosity (normal)
import Distribution.Package (Dependency)
import Distribution.PackageDescription
import Distribution.PackageDescription.Parse (readPackageDescription)

main :: IO ()
main = do
    args <- getArgs
    packageDescription <- readPackageDescription normal (head args)
    mapM_ putStrLn $ extractHsSourceDirs packageDescription

extractHsSourceDirs :: GenericPackageDescription -> [FilePath]
extractHsSourceDirs d = uniq $ concatMap ($d) extractorFunctions
    where
        extractorFunctions =
            [ getAllHsSourceDirs libBuildInfo       . condLibraryAsList
            , getAllHsSourceDirs buildInfo          . condExecutables
            , getAllHsSourceDirs testBuildInfo      . condTestSuites
            , getAllHsSourceDirs benchmarkBuildInfo . condBenchmarks
            ]
        condLibraryAsList = map (\ x -> (Nothing, x)) . maybeToList . condLibrary

getAllHsSourceDirs :: (a -> BuildInfo) -> [(b, CondTree ConfVar [Dependency] a)] -> [FilePath]
getAllHsSourceDirs f = concatMap (hsSourceDirs . f . condTreeData . snd)

I want to use this program to set the path variable in Vim. I think I need this to get htags work...