Re: [Haskell-cafe] Spurious program crashes

22 Nov 2005

      On Mon, Nov 21, 2005 at 10:41:38PM +0000, Joel Reymont wrote:
...
STM would complicate things too much for me. At least I think so. I  
would love to use STM but I would need to fit it into "type  
ScriptState = ErrorT String (StateT World IO)" just to use the  
logger. I'm not THAT comfortable with monads.
I am talking about Software Transactional Memory, which is in
Control.Concurrent.STM. I think you confused it with State
Transformer Monad.

In your case STM would allow you to wait simultaneously on (T)MVar and
(T)Chan. It would look like this:

    logger :: TMVar () -> IO ()
    logger die =
        join $ atomically $
            (do x <- readTChan parent
                return $ do
                    putStrLn x
                    logger die)
            `orElse`
            (do takeTMVar die
                return (return ()))

but you have to modify the rest of code to use STM. I modified your
Conc.hs to use STM, but using the greater guarantees of STM you
could surely simplify it further (see the attached patch).
...
Let me see if I understand you correctly... Are you saying that I  
should be using getChanContents in the code below?
I am not proposing to use getChanContents. You are busy-waiting
on MVar and Chan. I just proposed a solution to stuff messages
and die-request into the same concurrency primitive, so you
can wait for both events using a single operation.

But you are right (below) that this bug doesn't explain the behaviour of
your program. It is "only" a performance bug.
...
logger :: Handle -> MVar () -> IO ()
logger h die =
    do empty <- isEmptyChan parent
       unless empty $ do x <- readChan parent
                         putStrLn x
                         hPutStrLn h x
       alive <- isEmptyMVar die
       when (alive || not empty) $ logger h die
...
I think using Maybe is a great trick but I'm curious why so few  
messages actually get taken out of the channel in the code above?
Actually, I am not sure. I just noticed that your code uses a bad
coding practice and could be improved. If I find some time I'll try to 
examine it more closely.
...
Are  you saing that with all the checking it does not get to pull
messages  out?
As it is, you code can impose a big performance penalty, but indeed
it shouldn't change the semantics. Perhaps I miss something.
...
I see clearly how using Maybe with getChanContents will work out  
perfectly. I don't understand why the above code is inefficient to  
the point of printing just a few messages (out of hundreds) out on  
Windows. I would like to understand it to avoid such mistakes in the  
future.
Yes, this is strange. Perhaps we're both missing something obvious.

Best regards
Tomasz

Re: [Haskell-cafe] Spurious program crashes

Tomasz Zielonka