Use HashTable to share strings in tokens when parsing GFC files.

src/GF/Data/SharedString.hs

@@ -0,0 +1,18 @@
+module SharedString (shareString) where
+
+import Data.HashTable as H
+import System.IO.Unsafe (unsafePerformIO)
+
+{-# NOINLINE stringPool #-}
+stringPool :: HashTable String String
+stringPool = unsafePerformIO $ new (==) hashString
+
+{-# NOINLINE shareString #-}
+shareString :: String -> String
+shareString s = unsafePerformIO $ do
+    mv <- H.lookup stringPool s
+    case mv of
+	    Just s' -> return s'
+	    Nothing -> do
+		       H.insert stringPool s s
+		       return s