Re: STM experiment

Thanks Dan. I understand, your explanation is clear. I just need to study more Haskell. Im' just a beginner but very enthusiastic learning this "think-different" language (I'm a 12-year experienced C++ programmer). Thanks again. Luca. On Oct 2, 2009, at 6:28 PM, Daniel Peebles wrote:

Hi Luca,

Just in case you weren't aware of it, your example didn't actually contain any STM (beyond the import), just regular Haskell IO-based concurrency.

But the answer to your question is that there's no synchronization on writing to a file descriptor, so both threads are "simultaneously" writing to stdout (hPutStr stdout "...\n" === putStrLn "..." by the way) and result in the interleaved results you see. One solution is to have a thread that effectively owns stdout, and instead of writing to stdout, you write to a Chan (Control.Concurrent.Chan) to talk to the stdout owner, who will then write out your messages. This approach will give you the "Hello world" or "world Hello" output that you were after.

Hope this helps, Dan

On Fri, Oct 2, 2009 at 12:16 PM, Luca Ciciriello wrote:

...

Hi All. I'm very new using Concurrency and STM in Haskell. I'm trying some basic example using STM like this one:

module Main where

import IO import Control.Concurrent import Control.Concurrent.STM

main :: IO () main = do forkIO (hPutStr stdout "Hello") hPutStr stdout " world\n"

Loading this module in GHCi and running main, the result is:

wHoerllldo

On MacOs X 10.5.8 and on WindowsXp

Compiling this module with:

ghc --make Main.hs -o Main

and launcing ./Main the result is just:

Terminal> world

Am I doing something wrong? My expected result was Hello world (or world Hello)

Thanks in advance for any answer.

Luca. _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users