Created
April 4, 2019 19:27
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STRefs that carry an extra tag to enable hashing, comparison &c. Safe, I think.
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{-# LANGUAGE RankNTypes #-} | |
{-# LANGUAGE RoleAnnotations #-} | |
module IxSTRef | |
( IxSTRef, ixSTRefId, ixSTRef | |
, IxCounter, withIxCounter, newIxSTRef | |
) where | |
import Control.Monad.ST | |
import Data.Function | |
import Data.STRef | |
import Data.Word | |
import Data.Hashable | |
type role IxSTRef nominal _ _ | |
data IxSTRef i s a = IxSTRef Word (STRef s a) | |
deriving Eq | |
ixSTRefId :: IxSTRef i s a -> Word | |
ixSTRefId (IxSTRef i _) = i | |
ixSTRef :: IxSTRef i s a -> STRef s a | |
ixSTRef (IxSTRef _ r) = r | |
-- Imagine read, write, modify &c implemented in terms | |
-- of ixSTRef + the corresponding function from Data.STRef | |
instance Ord (IxSTRef i s a) where | |
compare = compare `on` ixSTRefId | |
instance Hashable (IxSTRef i s a) where | |
hashWithSalt = hashUsing ixSTRefId | |
type role IxCounter nominal _ | |
newtype IxCounter i s = IxCounter (STRef s Word) | |
-- Note: this is the same trick runST uses to keep STRefs et al. from escaping | |
-- Here we use it to keep the counter from escaping, so you can't juggle counters | |
-- unsafely. | |
withIxCounter :: (forall i. IxCounter i s -> ST s a) -> ST s a | |
withIxCounter inner = newSTRef 0 >>= inner . IxCounter | |
newIxSTRef :: IxCounter i s -> a -> ST s (IxSTRef i s a) | |
newIxSTRef (IxCounter cr) v = do | |
i <- readSTRef cr | |
modifySTRef' cr (+1) | |
IxSTRef i <$> newSTRef v | |
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