On Jan 22, 2008 3:49 PM, Yitzchak Gale
In a library, you have a function that starts up an external system, runs a calculation, then shuts down the external system. Like this:
bracketSystem :: MonadIO m => m a -> m a bracketSystem x = do startUpSystem ret <- x shutDownSystem return ret
Now you would really like to wrap that in bracket to make sure that "shutDownSystem" is called even when an IO exception is thrown. But unfortunately, bracket is currently not available for MonadIO, nor is there any way to emulate it AFIK. (This is a "maybe" for HaskellPrime: http://hackage.haskell.org/trac/haskell-prime/ticket/110)
Following is what I've been using to solve that problem. I can add it to that HaskellPrime ticket if people think it's useful. ============== module IO1 where import Control.Monad.State import Control.Monad.Error import Control.Exception import System.IO class MonadIO m => MonadIO1 m where liftIO1 :: (forall b . IO b -> IO b) -> m a -> m a instance MonadIO1 IO where liftIO1 = id instance MonadIO1 m => MonadIO1 (StateT s m) where liftIO1 f = mapStateT (liftIO1 f) instance (Error e, MonadIO1 m) => MonadIO1 (ErrorT e m) where liftIO1 f = mapErrorT (liftIO1 f) -- and so on for ReaderT, ListT, etc. block1, unblock1 :: MonadIO1 m => m a -> m a block1 = liftIO1 block unblock1 = liftIO1 unblock bracket1 :: MonadIO1 m => m a -> (a -> IO b) -> (a -> m c) -> m c bracket1 before after thing = block1 $ do a <- before r <- liftIO1 (handle (\e -> do {after a; throw e})) (unblock1 (thing a)) liftIO (after a) return r -- example: bracket file operations in an arbitrary monad withFile1 :: MonadIO1 m => FilePath -> IOMode -> (Handle -> m a) -> m a withFile1 name mode = bracket1 (liftIO (openFile name mode)) hClose ============== Note that in bracket1, the "after" action must run in IO. In practice, that hasn't been a problem for me. In fact, since the "after" clause might run in response to an asynchronous exception, I don't see how it could be sequenced with an arbitrary monad, anyway. Best wishes, -Judah