[Haskell-beginners] Learning about channels

Benjamin Edwards edwards.benj at googlemail.com
Tue May 25 05:06:48 EDT 2010

NB: This was posted in fa.haskell  first, I guess it was the wrong forum for
this kind of question as it was left unanswered :)


I'm having a few issues getting some toy programs to work whilst I try
to get a better understanding of how to model processes and channels.
I am just trying to use the real base blocks and failing miserably.
Here is an example (yes this is utterly contrived and sill, but I lack
imagination... sue me):

I want my main thread to do the following:

1. make a channel
2. spawn a thread (producer) that will write a series of lists of
integers to the the channel, then exit.
3. spawn another thread that will read from the channel and sum all of
the input. It should exit when both the channel is empty and and the
producer thread has finished writing to it.
4. Main thread should print the sum.

My current code should uses a trick I have seen else where which is to
have the result of "task" running in the thread put into an MVar. So
my condition for the reading thread exiting is to check if the MVar of
the producer thread is not empty and if the channel is empty. If those
two things are true, exit the thread. Unfortunately if somehow seems
able to to get to a stage where the produce thread has finished and
the channel is empty, but is blocking on a read.

I have the following code, but it always blocks indefinitely on a
read. I am sure there is something obviously deficient with it, but I
can't work out what it is. Any help would be greatly appreciated. Of
course, if I'm doing it all wrong, please tell me that too :)

module Main

import Control.Concurrent
import Control.Concurrent.STM
import Control.Monad (forever)
import Data.Map as M

main :: IO ()
main = do oc <- newChan
          counter <- newTVarIO (0 :: Integer)
          p <- forkJoin $ produce oc [1..1000]
          c <- forkJoin $ loop oc p counter
          takeMVar c >>= print

produce :: Chan [Integer] -> [Integer] -> IO ()
produce ch [] = return ()
produce ch xs = do let (hs,ts) = splitAt 100 xs
                   writeChan ch hs
                   produce ch ts

loop :: Chan [Integer] -> MVar () -> TVar Integer -> IO Integer
loop ch p n = do f <- isEmptyMVar p
                 e <- isEmptyChan ch
                 if e && (not f)
                   then atomically (readTVar n)
                   else do xs <- readChan ch
                           atomically $ do x <- readTVar n
                                           writeTVar n (x + sum xs)
                           loop ch p n
forkJoin :: IO a -> IO (MVar a)
forkJoin task = do mv <- newEmptyMVar
                   forkIO (task >>= putMVar mv)
                   return mv
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