[Haskell-beginners] STM and IO

emmanuel.delaborde emmanuel.delaborde at cimex.com
Thu Apr 9 12:20:19 EDT 2009

> From: Quentin Moser <quentin.moser at unifr.ch>
> 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.

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 ?



Emmanuel Delaborde
Web Technologist
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