Hi ..Hmm .. maybe I explained it badly.For example I want my two processor to
do two tasks while they are sharing a variable . Is it not parallelism ? We
don't need MVar, as well ? I completely misunderstood !MozhganDate: Tue, 16 Dec
2008 04:03:59 -0700From: lrpal...@gmail.comto: mozhgan_...@hotmail.comsubject:
Re: [Haskell-cafe] MVar and Par ..CC: haskell-c...@haskell.org2008/12/16
Mozhgan Kabiri <mozhgan_...@hotmail.com>
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 .
This is fortunately impossible. par can never change the semantics of a
program; it just says "compute this in parallel now because we might need it
later", as opposed to just computing it later when it is demanded. Because
Haskell is referentially transparent, the answer it will get now vs. later will
always be the same.
Race conditions cannot happen with par. Perhaps you want to experiment with
concurrency rather than parallelism (this is the realm in which MVars lie). In
that case, look at the function forkIO, which spawns a new thread, and the MVar
operations.
Luke
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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