forked from GitHub/gf-core
35 lines
1.3 KiB
Haskell
35 lines
1.3 KiB
Haskell
module Cache (Cache,newCache,readCache) where
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import Control.Concurrent.MVar
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import Data.Map (Map)
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import qualified Data.Map as Map
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import System.Directory (getModificationTime)
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import System.Time (ClockTime)
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data Cache a = Cache {
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cacheLoad :: FilePath -> IO a,
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cacheObjects :: MVar (Map FilePath (MVar (Maybe (ClockTime, a))))
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}
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newCache :: (FilePath -> IO a) -> IO (Cache a)
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newCache load =
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do objs <- newMVar Map.empty
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return $ Cache { cacheLoad = load, cacheObjects = objs }
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readCache :: Cache a -> FilePath -> IO a
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readCache c file =
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do v <- modifyMVar (cacheObjects c) findEntry
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modifyMVar v readObject
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where
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-- Find the cache entry, inserting a new one if neccessary.
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findEntry objs = case Map.lookup file objs of
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Just v -> return (objs,v)
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Nothing -> do v <- newMVar Nothing
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return (Map.insert file v objs, v)
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-- Check time stamp, and reload if different than the cache entry
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readObject m = do t' <- getModificationTime file
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x' <- case m of
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Just (t,x) | t' == t -> return x
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_ -> cacheLoad c file
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return (Just (t',x'), x')
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