[Haskell-cafe] MVar and Par ..

[Mozhgan Kabiri]

Hi ..

Hope you are doing well . I've just joined this group. 
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 .
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 .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 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

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