[Haskell-beginners] Concurrent vs GHC

Mauricio Hernandes maukeshigomu at gmail.com
Thu Jul 12 07:42:50 CEST 2012


Hi Guys, thanks again for the time reading this thread.
I was busy so I couldn't modify the code, but now it looks like this:


main = do
    finished <- atomically $ newTChan
    chan <- atomically $ newTChan
    f <- openOffline "filename"
    forkIO $ writeFile chan
    forkIO $ reaFile f chan finished
    a <- readTChan finished
    show a


So chan is the variable for communication and finished the variable used to
readFile tell the main thread that the file is over,

It seems more neat than the previous code,
but I have a hard time understanding the sentence "...you have to manually
specify the number of threads that you opened..." in Felipe's email. What
does it mean? How do I specify automatically how many threads do I want?


Thanks a lot

Mau :)





On Mon, Jul 2, 2012 at 10:18 PM, Bob Hutchison <hutch-lists at recursive.ca>wrote:

>
> On 2012-07-02, at 9:10 AM, Bob Hutchison wrote:
>
>
> 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
>
>                                                      ^^^^^ finished (sorry)
>
> 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
>
>
>
>
> _______________________________________________
> Beginners mailing list
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>
>
>   ----
> Bob Hutchison
> Recursive Design Inc.
> http://www.recursive.ca/
> weblog: http://xampl.com/so
>
>
>
>
>
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