[Haskell-cafe] Re: MVar and Par ..

Mozhgan Kabiri mozhgan_kch at hotmail.com
Tue Dec 16 07:40:50 EST 2008

Hi , 

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.


> To: haskell-cafe at haskell.org
> From: haskell at list.mightyreason.com
> Date: Tue, 16 Dec 2008 12:34:23 +0000
> Subject: [Haskell-cafe] Re: MVar and Par ..
> Mozhgan Kabiri wrote:
> > Hi .. Hope you are doing well . I've just joined this group.
> Hi.
> > Recently, I am struggling to do some simple experiment with haskell 
> > language about parallelism and wrong answers that we can get while using 
> > a shared variable .
> Your goal is still unclear.
> Are you trying to create an example which shows unexpected answers?
> Are you trying to create an example which shows the expected answer?
> > I tried to write a simple program, for example calculationg 'n=n+1' few 
> > times.And then I tried to do it in parallel by using 'par' and 'pseq' . 
> > The aim was to get the wrong answer because we have to share a variable 
> > here,and without using 'MVar' function we will get the wrong answer for 
> > the calculation .
> MVar is a mutable storage cell.  One can use forkIO to create IO threads which 
> race to change MVar, and if this is done badly then you can get unexpected answers.
> One cannot use the pure "par" and "pseq" to launch IO threads, so you cannot use 
> "par" and "pseq" to create race conditions, so one can only get the expected answer.
> > I don't know how to write it in parallel in order to get a wrong answer 
> > when we don't use MVar,because we have a shared variable here. I read 
> > about MVars as well,but also I don't know how to combine MVar and Par 
> > together to get the program to work.
> I do not immediately see how MVar and "par" can be sensibly combined at all.
> > I wrote this :
> > 
> > module Main where f :: Int -> Int -> Int f i n = g 1 i n where g x i n | 
> > x <= i = g (x+1) i (n+1) | otherwise = n main :: IO () main = do 
> > putStrLn "starting..." let r = f 10 5 putStrLn (show r) putStrLn "finished"
> > I want to make to work in parallel by using 'Par'.And also use MVar for 
> > this simple example to work.
> > All of the example about MVar are a little bit complicated and I 
> > couldn't figure it that how can I write one,the same !
> > 
> > Can any one help me with this ? I want a simple example that I can feel 
> > the need of MVar when I run my program in parallel and while I am using 
> > a shared variable.
> > 
> > Regards; Mozhgan
> I have run out of time, so I leave this part of your question to others.
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