On Tue, Mar 28, 2006 at 10:25:04AM +0100, Simon Marlow wrote:

On 28 March 2006 00:24, Ross Paterson wrote:

...

How about STM (minus retry/orElse) and TVars as the portable interface? They're trivial for a single-threaded implementation, and provide a comfortable interface for everyone.

It just occurred to me that STM isn't completely trivial in a single-threaded implementation, because exceptions have to abort a transaction in progress.

Almost trivial, though: import Prelude hiding (catch) import Control.Exception import Data.IORef -- The reference contains a rollback action to be executed on exceptions newtype STM a = STM (IORef (IO ()) -> IO a) unSTM (STM f) = f instance Functor STM where fmap f (STM m) = STM (fmap f . m) instance Monad STM where return x = STM (const (return x)) STM m >>= k = STM $ \ r -> do x <- m r unSTM (k x) r atomically :: STM a -> IO a atomically (STM m) = do r <- newIORef (return ()) m r `catch` \ ex -> do rollback <- readIORef r rollback throw ex catchSTM :: STM a -> (Exception -> STM a) -> STM a catchSTM (STM m) h = STM $ \ r -> m r `catch` \ ex -> unSTM (h ex) r newtype TVar a = TVar (IORef a) newTVar :: a -> STM (TVar a) newTVar a = STM $ const $ do ref <- newIORef a return (TVar ref) readTVar :: TVar a -> STM a readTVar (TVar ref) = STM (const (readIORef ref)) writeTVar :: TVar a -> a -> STM () writeTVar (TVar ref) a = STM $ \ r -> do oldval <- readIORef ref modifyIORef r (writeIORef ref oldval >>) writeIORef ref a