hm/Setup.hs

import Distribution.Simple
import System.Process
import System.Environment
import System.Directory

target :: String
target = "src/Hm"

source :: String
source = "hm.cf"

regenerateGrammar :: IO ()
regenerateGrammar = do
  callProcess "bnfc" ["-d", "--text-token", "--functor", source]
  removePathForcibly   target
  renameDirectory "Hm" target

main :: IO ()
main = do
  (a:_) <- getArgs
  if a == "build"
     then do
       b <- doesDirectoryExist target
       if not b
         then regenerateGrammar
         else do
           sourceDate <- getModificationTime source
           targetDate <- getModificationTime target
           if sourceDate >= targetDate
              then regenerateGrammar
              else pure ()
     else pure ()
  defaultMain