<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><blockquote type="cite"><div><br>From: Quentin Moser <<a href="mailto:quentin.moser@unifr.ch">quentin.moser@unifr.ch</a>><br>Subject: Re: [Haskell-beginners] STM and IO<br><br>Your problem has nothing to do with lazyness; Haskell simply kills all<br>other threads when the main thread returns from main. You have to<br>somehow wait for them to complete in main or they won't have time to<br>run.</div></blockquote><div><br></div><div>Doh! </div><div>I've been bitten before...</div><br><blockquote type="cite"><div>Now onto the second problem: ignore me if I'm wrong, but it seems your<br>intent is to spawn 10 threads that will each try to run (writeTo store)<br>once. What your current code does is spawn one thread that sequentially<br>runs writeTo 10 times.</div></blockquote><div><br></div><div>yes you are right of course</div><div><br></div><blockquote type="cite"><div>Note: I haven't tried running any of this code, but it seems simple<br>enough to be confident in.<br></div></blockquote><br></div><div>I had to make a change to myFork to get it to compile, here's what I have now:</div><div><br></div><div><div>module Main where</div><div><br></div><div>import Control.Monad</div><div>import Control.Concurrent</div><div>import Control.Concurrent.STM</div><div>import System.IO</div><div>import Control.Exception (finally)</div><div><br></div><div>myFork :: IO () -> IO (MVar ())</div><div>myFork a = do </div><div> v <- newEmptyMVar</div><div> a `finally` (putMVar v ())</div><div> return v -- to honor the return type</div><div><br></div><div>myWait :: MVar () -> IO ()</div><div>myWait = readMVar </div><div><br></div><div>main = do</div><div> let fname = "store.txt"</div><div> fh <- openFile fname ReadWriteMode</div><div> store <- atomically $ newTMVar fh</div><div> waitMes <- 10 `replicateM` (myFork $ writeTo store)</div><div> mapM_ myWait waitMes</div><div><br></div><div>writeTo :: TMVar (Handle) -> IO ()</div><div>writeTo store = do</div><div> fh <- atomically $ takeTMVar store</div><div> text <- hGetContents fh</div><div> hPutStr fh (text ++ " some text ")</div><div> atomically $ putTMVar store fh</div></div><div><br></div><div>Now I get the following error : test3: store.txt: hPutStr: illegal operation (handle is closed)</div><div><br></div><div><div>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"</div><div><br></div><div><br></div><div>Intuitively I'd want to write something like : writeTo filename = atomically (do { s <- readFile filename ; writeFile filename (s ++ "blah") })</div><div>but the type system prevents me from doing IO within STM</div><div>I do not know how to go about sharing access to a file between multiple threads using STM... any pointers ?</div><div><br></div><div>Thanks</div><div><br></div><div>E.</div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div></div><div><br></div><div><br></div><br><div> <span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>--</div><div>Emmanuel Delaborde</div><div>Web Technologist</div><div>Cimex<br>53-55 Scrutton Street, London UK, EC2A 4PJ<br>T: +44 (0)20 7324 7780<br>F: +44 (0)20 7324 7781<br><a href="http://www.cimex.com/">http://www.cimex.com</a></div></div></span></div></span> </div><br></body></html>
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