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module Cache (Cache,newCache,readCache) where
import Control.Concurrent.MVar
import Data.Map (Map)
import qualified Data.Map as Map
import System.Directory (getModificationTime)
import Data.Time (UTCTime)
import Data.Time.Compat (toUTCTime)
data Cache a = Cache {
cacheLoad :: FilePath -> IO a,
cacheObjects :: MVar (Map FilePath (MVar (Maybe (UTCTime, 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 v <- modifyMVar (cacheObjects c) findEntry
modifyMVar v readObject
where
-- Find the cache entry, inserting a new one if neccessary.
findEntry objs = case Map.lookup file objs of
Just v -> return (objs,v)
Nothing -> do v <- newMVar Nothing
return (Map.insert file v objs, v)
-- Check time stamp, and reload if different than the cache entry
readObject m = do t' <- toUTCTime `fmap` getModificationTime file
x' <- case m of
Just (t,x) | t' == t -> return x
_ -> cacheLoad c file
return (Just (t',x'), x')
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