Since the file handle is closed after you read the contents, you need to open a new one before writing to it. On Thu, Apr 9, 2009 at 12:20 PM, emmanuel.delaborde < emmanuel.delaborde@cimex.com> wrote:

From: Quentin Moser Subject: Re: [Haskell-beginners] STM and IO

Your problem has nothing to do with lazyness; Haskell simply kills all other threads when the main thread returns from main. You have to somehow wait for them to complete in main or they won't have time to run.

Doh! I've been bitten before...

Now onto the second problem: ignore me if I'm wrong, but it seems your intent is to spawn 10 threads that will each try to run (writeTo store) once. What your current code does is spawn one thread that sequentially runs writeTo 10 times.

yes you are right of course

Note: I haven't tried running any of this code, but it seems simple enough to be confident in.

I had to make a change to myFork to get it to compile, here's what I have now:

module Main where

import Control.Monad import Control.Concurrent import Control.Concurrent.STM import System.IO import Control.Exception (finally)

myFork :: IO () -> IO (MVar ()) myFork a = do v <- newEmptyMVar a `finally` (putMVar v ()) return v -- to honor the return type

myWait :: MVar () -> IO () myWait = readMVar

main = do let fname = "store.txt" fh <- openFile fname ReadWriteMode store <- atomically $ newTMVar fh waitMes <- 10 `replicateM` (myFork $ writeTo store) mapM_ myWait waitMes

writeTo :: TMVar (Handle) -> IO () writeTo store = do fh <- atomically $ takeTMVar store text <- hGetContents fh hPutStr fh (text ++ " some text ") atomically $ putTMVar store fh

Now I get the following error : test3: store.txt: hPutStr: illegal operation (handle is closed)

reading the doc about hGetContents, I found that : "Computation hGetContents hdl returns the list of characters corresponding to the unread portion of the channel or file managed by hdl, which is put into an intermediate state, semi-closed. In this state, hdl is effectively closed"

Intuitively I'd want to write something like : writeTo filename = atomically (do { s <- readFile filename ; writeFile filename (s ++ "blah") }) but the type system prevents me from doing IO within STM I do not know how to go about sharing access to a file between multiple threads using STM... any pointers ?

Thanks

E.

-- Emmanuel Delaborde Web Technologist Cimex 53-55 Scrutton Street, London UK, EC2A 4PJ T: +44 (0)20 7324 7780 F: +44 (0)20 7324 7781 http://www.cimex.com

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