(fastcgi) replace DataRef with a more general Cache type, which can hold several PGF grammars.

src/server/Cache.hs

@@ -0,0 +1,36 @@
+module Cache (Cache,newCache,readCache) where
+
+import Control.Concurrent
+import Data.Map (Map)
+import qualified Data.Map as Map 
+import System.Directory (getModificationTime)
+import System.Time (ClockTime)
+
+data Cache a = Cache {
+      cacheLoad :: FilePath -> IO a,
+      cacheObjects :: MVar (Map FilePath (MVar (ClockTime, a)))
+    }
+
+newCache :: (FilePath -> IO a) -> IO (Cache a)
+newCache load = 
+    do objs <- newMVar Map.empty
+       return $ Cache { cacheLoad = load, cacheObjects = objs }
+
+readCache :: Cache a -> FilePath -> IO a
+readCache c file = 
+    do t' <- getModificationTime file
+       objs <- takeMVar (cacheObjects c)
+       case Map.lookup file objs of
+         -- object is in cache
+         Just v  -> do (t,x) <- takeMVar v
+                       putMVar (cacheObjects c) objs
+                       -- check timestamp
+                       x' <- if t == t' then return x else cacheLoad c file
+                       putMVar v (t',x')
+                       return x'
+         -- first time this object is requested
+         Nothing -> do v <- newEmptyMVar
+                       putMVar (cacheObjects c) (Map.insert file v objs)
+                       x' <- cacheLoad c file
+                       putMVar v (t',x')
+                       return x'