[Haskell-beginners] Concurrent vs GHC

[Bob Hutchison]

On 2012-06-30, at 1:51 PM, Mauricio Hernandes wrote:

> Eternal Gratitude for the help, it's working perfectly, I will consider the exceptions and other stuff now.
> 
> the code looks like this now
> 
> 
> import System.IO
> import Control.Concurrent
> import Data.List 
> import Control.Monad
> 
> main = do
>           finished <- newEmptyMVar
>           input <- newMVar  [1..30000]
>           ia <- newEmptyMVar
>           ib <- newEmptyMVar
>           ic <- newEmptyMVar
> 
>           forkIO $ do x <- readMVar input
>                       putMVar ia x
>                       putMVar finished ()
> 
>           forkIO $ do a <- readMVar ia
>                       putMVar ib ( sum a )
>                       putMVar finished ()
> 
>           forkIO $ do a <- readMVar ia
>                       putMVar ic ( reverse a )
>                       putMVar finished ()
> 
>           b <- readMVar ib
>           c <- readMVar ic
>           writeFile "somaEprod.txt" (show b ++ "\n") 
>           appendFile "somaEprod.txt" (show c)             
>           replicateM_ 3 (takeMVar finished)

Just another Haskell beginner here, so beware...

You've moved the readMVar out of a thread into the application. This means (I think) that you are waiting for values in both ib and ic in the application (rather than a fourth thread). In your specific program, for these to have values require that all three threads have completed so you don't need the finished MVar anymore. However, this is pretty fragile being completely dependent on the MVars being set exactly once as the threads complete (so if you modify the code you have to be careful). The found solution is also fragile as Felipe says in his post. I don't know what those libraries Felipe mentioned are but I think I'd be looking for them right about now if I were you :-)

Cheers,
Bob

> 
> 
> 
> 
> Valeu
> Mauricio
> 
> On Sun, Jul 1, 2012 at 12:24 AM, Felipe Almeida Lessa <felipe.lessa at gmail.com> wrote:
> Your application is exiting before your forkIOs get a chance to
> execute.  Instead of
> 
>  forkIO $ do
>    ...
>  forkIO $ do
>    ...
>  forkIO $ do
>    ...
> 
> use something like
> 
>  finished <- newEmptyMVar
> 
>  forkIO $ do
>    ...
>    putMVar finished ()
> 
>  forkIO $ do
>    ...
>    putMVar finished ()
> 
>  forkIO $ do
>    ...
>    putMVar finished ()
> 
>  replicateM_ 3 (takeMVar finished)
> 
> Doing so will avoid your program to exit until all threads have finished.
> 
> Note that the code above is extremely fragile: doesn't handle
> exceptions, you have to manually specify the number of threads that
> you opened, etc.  These are abstracted by some libraries on Hackage
> that you may use later for Real World Code (TM).
> 
> Cheers, =)
> 
> --
> Felipe.
> 
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----
Bob Hutchison
Recursive Design Inc.
http://www.recursive.ca/
weblog: http://xampl.com/so




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