On 12/29/05, Brian Sniffen <brian.sniffen@gmail.com> wrote:

> test_Cubby =
>   do
>     tv <- newTVar 0

You've almost got it!  But "newTVar 0" has type STM Tvar, and you're
trying to use it in the IO monad.  So just say "tv <- atomically
(newTVar 0)" and you're set.  Do notice that you'll see output like
this:

co"nisnusmeer t6 "6
"
"c"oinnssuemret  61""

""cionnsseurmte  95""

""icnosnesrutm e2 "9

since the two threads are interleaved.

--
Brian T. Sniffen
bts@alum.mit.edu    or     brian.sniffen@gmail.com
http://www.evenmere.org/~bts

Thank you all for your help and comments ...

module Main where

import System.Random
import Control.Concurrent
import Control.Concurrent.STM

forever act = act >> forever act

test_Cubby =
do
    tv <- atomically (newTVar 0)
    forkIO (forever $ producer tv) >> (forever $ consumer tv)
where
    producer tv = do r <- randomRIO (1,10)
                     atomically (do { v <- readTVar tv
                                    ; writeTVar tv (v+r)
                                    })
                     print ("insert " ++ show r)
                     threadDelay r
    consumer tv = do r <- randomRIO (1,10)
                     atomically (do { v <- readTVar tv
                                    ; if (v < r) then retry
                                      else writeTVar tv (v-r)
                                    })
                     print ("consume " ++ show r)
                     threadDelay r

I also slow it down a bit to get readable output ... thanks again.
- Quan