You might want to take a look at
http://hackage.haskell.org/package/Adaptivesince it seems really
similar to what you are trying to do. In fact, you
might also want to google 'Functional Reactive Programming'.
-- ryan
On Thu, Feb 24, 2011 at 10:41 PM, Chris Dew
Hello, just like everyone else, I have a question about monads. I've read the tutorials, written one monad myself (not in this email), but I still consider myself a Haskell beginner.
* Does GHC eliminate unneeded MVars during compilation?
I'm expecting that it doesn't, as that would mean optimising away ForkIOs, which would be quite a thing to do. I've included example code below.
* Is there a monad which allows their automatic elimination of MVars (or their creation only when necessary)?
This would be similar to how the IO monad allows you to do purely functional things with a do block, using let.
I've had a go at a lifting function, which wraps a pure function into an IO action which forever reads from one MVar and writes to another. What I'm looking for is some form of Monadic context in which many pure functions, MVar fillers and MVar consumers could be linked together, where only the necessary MVars remain (or were created) at compilation time.
* Would this be a monad, or a monad transformer?
* Can you specialise a monad transformer on a single base (in this case IO) so that you can use forkIO in the bind or return?
Thanks,
Chris.
module Main ( main ) where
import Control.Concurrent (forkIO, MVar, newEmptyMVar, putMVar, takeMVar, ThreadId, threadDelay) import Control.Monad (forever)
stepA :: MVar String -> IO () stepA boxa = forever $ do line <- getLine putMVar boxa line
stepB :: MVar String -> IO () stepB boxb = forever $ do line <- takeMVar boxb putStrLn line
-- This simply wraps a string in brackets. bracket :: String -> String bracket x = "(" ++ x ++ ")"
-- This lifts a function into an action which forever performs the function -- between the two MVars given. lft :: (a -> b) -> MVar a -> MVar b -> IO () lft f c d = forever $ do x <- takeMVar c putMVar d (f x)
-- Just like C's main. main :: IO () main = do box <- newEmptyMVar box2 <- newEmptyMVar forkIO $ stepA box forkIO $ lft bracket box box2 forkIO $ stepB box2 threadDelay 10000000 -- Sleep for at least 10 seconds before exiting.
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