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{-# OPTIONS_GHC -funbox-strict-fields #-}
-- A heap reference with transparent indirection. I'm not convinced
-- that this does anything different than a dumb IORef. The IORefRef
-- module implements the same interface but simply delegates to IORef.
-- Currently there does not seem to be an asymptotic difference.
module IndirRef
( Ref, new, read, write, link )
where
import Prelude hiding (read)
import Data.IORef
data RefData a
= Concrete a
| Indirect !(Ref a)
newtype Ref a = Ref (IORef (RefData a))
deriving (Eq)
new :: a -> IO (Ref a)
new = fmap Ref . newIORef . Concrete
squashRead :: Ref a -> IO (a, Ref a)
squashRead ref@(Ref ioref) = do
dat <- readIORef ioref
case dat of
Concrete x -> return (x, ref)
Indirect ref -> do
(x,ref') <- squashRead ref
writeIORef ioref (Indirect ref')
return (x,ref')
read :: Ref a -> IO a
read = fmap fst . squashRead
write :: Ref a -> a -> IO ()
write (Ref ioref) = writeIORef ioref . Concrete
link :: Ref a -> Ref a -> IO ()
link (Ref old) = writeIORef old . Indirect
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