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Speed up the Eq instance for TArray #37

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Speed up the Eq instance for TArray #37

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15 changes: 13 additions & 2 deletions Control/Concurrent/STM/TArray.hs
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Expand Up @@ -25,7 +25,7 @@ module Control.Concurrent.STM.TArray (
import Data.Array (Array, bounds)
import Data.Array.Base (listArray, arrEleBottom, unsafeAt, MArray(..),
IArray(numElements))
import Data.Ix (rangeSize)
import Data.Ix (Ix (rangeSize))
import Data.Typeable (Typeable)
import Control.Concurrent.STM.TVar (TVar, newTVar, readTVar, writeTVar)
#ifdef __GLASGOW_HASKELL__
Expand All @@ -41,7 +41,18 @@ import Control.Sequential.STM (STM)
-- but it may be replaced by a more efficient implementation in the future
-- (the interface will remain the same, however).
--
newtype TArray i e = TArray (Array i (TVar e)) deriving (Eq, Typeable)
newtype TArray i e = TArray (Array i (TVar e)) deriving (Typeable)

-- There are no provisions for moving/copying TVars between TArrays.
-- Therefore, two TArrays are equal if and only if they are both empty or are
-- actually the same array in memory. We have no safe operations for checking
-- that directly (though in practice we could use `unsafeCoerce#` with
-- `sameMutableArray#`). So instead we take a quick look at the array sizes and
-- then decide based on the first TVar of each.
instance Ix i => Eq (TArray i e) where
TArray t1 == TArray t2
= numElements t1 == numElements t2
&& (numElements t1 == 0 || unsafeAt t1 0 == unsafeAt t2 0)

instance MArray TArray e STM where
getBounds (TArray a) = return (bounds a)
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