diff --git a/Setup.hs b/Setup.hs
index e7795b5..a7e9cb1 100644
--- a/Setup.hs
+++ b/Setup.hs
@@ -3,16 +3,31 @@ import System.Process
 import System.Environment
 import System.Directory
 
-libdir :: String
-libdir = "src"
+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
-       callProcess "bnfc" ["-d", "--text-token", "--functor", "hm.cf"]
-       removePathForcibly   (libdir <> "/Hm")
-       renameDirectory "Hm" (libdir <> "/Hm")
+       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
diff --git a/hm.cabal b/hm.cabal
index 31c42a5..dc39041 100644
--- a/hm.cabal
+++ b/hm.cabal
@@ -18,6 +18,7 @@ custom-setup
                , Cabal >= 2.4
                , process >= 1
                , directory >= 1
+               , time >= 1
 
 library
   ghc-options:         -Werror=incomplete-patterns -Wall -fno-warn-name-shadowing