[Haskell-beginners] Learning about channels

[Benjamin Edwards]

On 25 May 2010 11:00, Daniel Fischer <daniel.is.fischer at web.de> wrote:

> Sorry for the ugly layout:
>
>
>                    else do
>                     if e then yield
>                         else
>                        do xs <- readChan ch
>                            atomically $ do x <- readTVar n
>                                            writeTVar n (x + sum xs)
>                     loop ch p n
>
> The point is, if the channel is empty, but the producer has not yet
> finished, don't try to read from the channel (that wouldn't work then), but
> give the producer the chance to produce the next chunk.
> Since thread-switching happens on allocation, don't just jump to the next
> iteration of the loop, but tell the thread manager "I have nothing to do at
> the moment, you can let somebody else run for a while".
>
> I have encountered cases where yield didn't work reliably (no idea whether
> that was my fault or the compiler's, but "threadDelay 0" instead of yield
> worked reliably).
>
> This is where I was getting it all  horribly wrong. I assumed that while
the loop thread blocked, the other thread would happily carry on producing
work. Is there anything I can read to get a better understanding of how the
haskell runtime manages these switches? Or is it the OS that takes care of
this..?

Here is my new function, with the a call to yield inserted :)

loop :: Chan [Integer] -> MVar () -> TVar Integer -> IO Integer
loop ch p n = do f <- isEmptyMVar p
                 e <- isEmptyChan ch
                 if not f
                   then atomically (readTVar n)
                   else
                     if e then yield >> loop ch p n

                          else do xs <- readChan ch
                                  atomically $ do x <- readTVar n
                                                  writeTVar n (x + sum xs)
                                  loop ch p n

Thanks for your help!

Ben
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