[Haskell-cafe] How to write Source for TChan working with LC.take?

[Hiromi ISHII]

Hello, there.

I'm writing a Source to supply values from TChan.
I wrote three implementations for that goal as follows:

~~~~
import Data.Conduit
import qualified Data.Conduit.List as LC
import Control.Monad.Trans
import Control.Concurrent.STM
import Control.Monad

sourceTChanRaw :: MonadIO m => TChan a -> Source m a
sourceTChanRaw ch = pipe
  where
    pipe = PipeM next (return ())
    next = do
      o <- liftIO $ atomically $ readTChan ch
      return $ HaveOutput pipe (return ()) o

sourceTChanState :: MonadIO m => TChan a -> Source m a
sourceTChanState ch = sourceState ch puller
  where
    puller ch = StateOpen ch `liftM` (liftIO . atomically $ readTChan ch)

sourceTChanYield :: MonadIO m => TChan a -> Source m a
sourceTChanYield ch = forever $ do
  ans <- liftIO . atomically $ readTChan ch
  yield ans
~~~~

Namely, one using raw Pipe constructors directly, using `sourceState` and `yield`.
I tested these with GHCi.

~~~~
ghci> ch <- newTChanIO :: IO (TChan ())
ghci> atomically $ replicateM_ 1500 $ writeTChan ch ()
ghci> sourceTChanRaw ch $$ LC.take 10
[(),(),(),(),(),(),(),(),(),()]
ghci> sourceTChanState ch $$ LC.take 10
[(),(),(),(),(),(),(),(),(),()]
ghci> sourceTChanYield ch $$ LC.take 10
*thread blocks*
~~~~

First two versions' result is what I exactly expected but the last one not: the source written with `yield` never returns value even if there are much enough value.

I also realized that following code runs perfectly as I expected:

~~~~
ghci> ch <- newTChanIO :: IO (TChan ())
ghci> atomically $ replicateM_ 1500 $ writeTChan ch ()
ghci> sourceTChanRaw ch $= LC.isolate 10 $$ LC.mapM_ print
[(),(),(),(),(),(),(),(),(),()]
ghci> sourceTChanState ch $= LC.isolate 10 $$ LC.mapM_ print
[(),(),(),(),(),(),(),(),(),()]
ghci> sourceTChanYield ch $= LC.isolate 10 $$ LC.mapM_ print
[(),(),(),(),(),(),(),(),(),()]
~~~~

So, here is the question:

    Why the Source using `yield` doesn't work as expected with LC.take?

Or, might be

    Semantically, what behaviour should be expected for LC.take?


Thanks,

-- Hiromi ISHII
konn.jinro at gmail.com