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Hi ..<br><br>Thanks ..<br>But I keep get error when I run the program in order to see the result . It says 'var' is not in the scope.<br><br>Mozhgan<br><br><hr id="stopSpelling">Date: Tue, 16 Dec 2008 05:54:40 -0700<br>From: lrpalmer@gmail.com<br>To: mozhgan_kch@hotmail.com<br>Subject: Re: [Haskell-cafe] Re: MVar and Par ..<br>CC: haskell@list.mightyreason.com; haskell-cafe@haskell.org<br><br>Okay, that's a bit clearer. Control.Parallel is not what you want; that is for parallelizing pure code (and is very nice inside its little domain). But you want concurrent code: multiple threads that change state at the same time. That is in Control.Concurrent.<br>
<br>In particular, the "shared variable" is Data.IORef.<br><br>Simple example from which you should be able to extrapolate.<br><br><font face="courier new,monospace">import Control.Concurrent<br>import Control.Concurrent.MVar<br>
import Data.IORef<br><br>threadBody = do<br> x <- readIORef var<br> writeIORef var (x+1)<br><br>main = do<br> var <- newIORef 0<br> -- spawn two threads<br> forkIO threadBody<br> forkIO threadBody<br>
<br> -- look at the value of the variable<br> x <- readIORef var<br> print x<br></font><br>You can scatter threadDelay throughout this code to mess with the scheduler and show how a failure could happen.<br><br>
Conversion to MVars should be pretty straightforward.<br><br>Luke<br><br><br><div class="EC_gmail_quote">2008/12/16 Mozhgan Kabiri <span dir="ltr"><<a href="mailto:mozhgan_kch@hotmail.com">mozhgan_kch@hotmail.com</a>></span><br>
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<br>Hi , <br><br>Yeah , first I want to get unexpected answer ! And then I want to prove that when I use MVar I will get the write answer.I want to show that when we have a shared variable, we should use MVar.Besides we use for example two processors to do different tasks.But the point is these processors have to share a variable and I want to show it with MVar.But I don't know how to write a simple code to show it while undrestand it.<br>
<br>Mozhgan<br><br>> To: <a href="mailto:haskell-cafe@haskell.org">haskell-cafe@haskell.org</a><br>> From: <a href="mailto:haskell@list.mightyreason.com">haskell@list.mightyreason.com</a><br>
> Date: Tue, 16 Dec 2008 12:34:23 +0000<br>> Subject: [Haskell-cafe] Re: MVar and Par ..<div class="EC_Ih2E3d"><br>> <br>> Mozhgan Kabiri wrote:<br>> > Hi .. Hope you are doing well . I've just joined this group.<br>
> <br>> Hi.<br>> <br>> > Recently, I am struggling to do some simple experiment with haskell <br>> > language about parallelism and wrong answers that we can get while using <br>> > a shared variable .<br>
>
<br>> Your goal is still unclear.<br>> Are you trying to create an example which shows unexpected answers?<br>> Are you trying to create an example which shows the expected answer?<br>> <br>> > I tried to write a simple program, for example calculationg 'n=n+1' few <br>
> > times.And then I tried to do it in parallel by using 'par' and 'pseq' . <br>> > The aim was to get the wrong answer because we have to share a variable <br>> > here,and without using 'MVar' function we will get the wrong answer for <br>
> > the calculation .<br>> <br>> MVar is a mutable storage cell. One can use forkIO to create IO threads which <br>> race to change MVar, and if this is done badly then you can get unexpected answers.<br>> <br>
> One cannot use the pure "par" and "pseq" to launch IO threads, so you cannot use <br>> "par" and "pseq" to create race conditions, so one can only get the expected answer.<br>
> <br></div>> > I don't kno
w how to write it in parallel in order to get a wrong answer <br><div class="EC_Ih2E3d">> > when we don't use MVar,because we have a shared variable here. I read <br>> > about MVars as well,but also I don't know how to combine MVar and Par <br>
> > together to get the program to work.<br>> <br>> I do not immediately see how MVar and "par" can be sensibly combined at all.<br>> <br>> > I wrote this :<br>> > <br>> > module Main where f :: Int -> Int -> Int f i n = g 1 i n where g x i n | <br>
> > x <= i = g (x+1) i (n+1) | otherwise = n main :: IO () main = do <br>> > putStrLn "starting..." let r = f 10 5 putStrLn (show r) putStrLn "finished"<br>> > I want to make to work in parallel by using 'Par'.And also use MVar for <br>
> > this simple example to work.<br>> > All of the example about MVar are a little bit complicated and I <br>> > couldn't figure it that how can I write one,the same !<br>> > <br></div>> &gt
; Can any one help me with this ? I want a simple example that I can feel <br><div class="EC_Ih2E3d">> > the need of MVar when I run my program in parallel and while I am using <br>> > a shared variable.<br>> > <br>
> > Regards; Mozhgan<br>> <br>> I have run out of time, so I leave this part of your question to others.<br>> <br>> _______________________________________________<br>> Haskell-Cafe mailing list<br>> <a href="mailto:Haskell-Cafe@haskell.org">Haskell-Cafe@haskell.org</a><br>
> <a href="http://www.haskell.org/mailman/listinfo/haskell-cafe">http://www.haskell.org/mailman/listinfo/haskell-cafe</a><br><br></div><div class="EC_Ih2E3d"><hr>Explore the seven wonders of the world <a href="http://search.msn.com/results.aspx?q=7+wonders+world&mkt=en-US&form=QBRE">Learn more!</a></div>
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