hm/Setup.hs

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

libdir :: String
libdir = "src"

main :: IO ()
main = do
  (a:_) <- getArgs
  if a == "build"
     then do
       callProcess "bnfc" ["-d", "--text-token", "--functor", "hm.cf"]
       removePathForcibly   (libdir <> "/Hm")
       renameDirectory "Hm" (libdir <> "/Hm")
     else pure ()
  defaultMain