{-# LANGUAGE DeriveDataTypeable #-} module Control.Concurrent.STM.Speculation ( specSTM , specSTM' , specOnSTM , specOnSTM' , specBySTM , specBySTM' ) where import Control.Concurrent.STM import Control.Concurrent.Speculation (evaluated) import Control.Exception (Exception, throw, fromException) import Control.Parallel (par) import Data.Typeable (Typeable) import Data.Function (on) import System.IO.Unsafe (unsafePerformIO) newtype Speculation = Speculation Int deriving (Show,Eq,Typeable) instance Exception Speculation -- | @'specSTM' g f a@ evaluates @f g@ while forcing @a@, if @g == a@ then @f g@ is returned. Otherwise the side-effects -- of the current STM transaction are rolled back and @f a@ is evaluated. -- -- If the argument @a@ is already evaluated, we don\'t bother to perform @f g@ at all. -- -- If a good guess at the value of @a@ is available, this is one way to induce parallelism in an otherwise sequential task. -- -- However, if the guess isn\'t available more cheaply than the actual answer then this saves no work, and if the guess is -- wrong, you risk evaluating the function twice. -- -- > specSTM a f a = f $! a -- -- The best-case timeline looks like: -- -- > [------ f g ------] -- > [------- a -------] -- > [--- specSTM g f a ---] -- -- The worst-case timeline looks like: -- -- > [------ f g ------] -- > [------- a -------] -- > [-- rollback --] -- > [------ f a ------] -- > [------------------ spec g f a ------------------------] -- -- Compare these to the timeline of @f $! a@: -- -- > [------- a -------] -- > [------ f a ------] specSTM :: Eq a => a -> (a -> STM b) -> a -> STM b specSTM = specBySTM (==) {-# INLINE specSTM #-} -- | Unlike 'specSTM', 'specSTM'' doesn't check if the argument has already been evaluated. specSTM' :: Eq a => a -> (a -> STM b) -> a -> STM b specSTM' = specBySTM' (==) {-# INLINE specSTM' #-} -- | 'specSTM' using a user defined comparison function specBySTM :: (a -> a -> Bool) -> a -> (a -> STM b) -> a -> STM b specBySTM cmp g f a | evaluated a = f a | otherwise = specBySTM' cmp g f a {-# INLINE specBySTM #-} -- | 'specSTM'' using a user defined comparison function specBySTM' :: (a -> a -> Bool) -> a -> (a -> STM b) -> a -> STM b specBySTM' cmp g f a = a `par` do exn <- freshSpeculation let try = do result <- f g if cmp g a then return result else throw exn try `catchSTM` \e -> case fromException e of Just exn' | exn == exn' -> f a -- rerun with alternative inputs _ -> throw e -- this is somebody else's problem {-# INLINE specBySTM' #-} -- | 'specOnSTM' . 'on' (==)' specOnSTM :: Eq c => (a -> c) -> a -> (a -> STM b) -> a -> STM b specOnSTM = specBySTM . on (==) {-# INLINE specOnSTM #-} -- | 'specOnSTM'' . 'on' (==)' specOnSTM' :: Eq c => (a -> c) -> a -> (a -> STM b) -> a -> STM b specOnSTM' = specBySTM' . on (==) {-# INLINE specOnSTM' #-} speculationSupply :: TVar Int speculationSupply = unsafePerformIO $ newTVarIO 0 {-# NOINLINE speculationSupply #-} freshSpeculation :: STM Speculation freshSpeculation = do n <- readTVar speculationSupply writeTVar speculationSupply $! n + 1 return (Speculation n) {-# INLINE freshSpeculation #-}