I agree that the behavior is a bit confusing (Dan Burton just filed an
issue about this[1], I'm guessing this email is related).
I put up a wiki page[2] to hopefully explain the issue. Can you review
it and let me know if it helps? If so, I'll link to it from the
Haddocks.
Michael
[1] https://github.com/snoyberg/conduit/issues/67
[2] https://github.com/snoyberg/conduit/wiki/Dealing-with-monad-transformers
On Wed, Aug 22, 2012 at 11:19 PM, Niklas Hambüchen
Today I was surprised that transPipe is called for every chunk of data going through my pipe, rendering the StateT I put in useless, because it was always restarted with the initial value.
It would be nice to have some explanation about this, as it makes it easy to write compiling code that has completely unexpected behaviour.
I wrote this function (also on http://hpaste.org/73538):
conduitWithState :: (MonadIO m) => Conduit Int (StateT Int m) String conduitWithState = do liftIO $ putStrLn $ "Counting Int->String converter ready!" awaitForever $ \x -> do i <- lift get lift $ modify (+1) liftIO $ putStrLn $ "Converting " ++ show x ++ " to a string! " ++ "Processed so far: " ++ show i yield (show x)
and ran it like this:
countingConverterConduit :: (MonadIO m) => Conduit Int m String countingConverterConduit = transPipe (\stateTint -> evalStateT stateTint 1) conduitWithState
main :: IO () main = do stringList <- CL.sourceList [4,1,9,7,3] $= countingConverterConduit $$ CL.consume print stringList
However, the output is not what I expected, but only:
Processed so far:1 Processed so far:1 ...
Dan Burton proposed a fix, making the whole sink-conduit-source construction run on the StateT:
main = do stringList <- flip evalStateT 1 $ ...
So the question is: What is the rationale for this?
I was expecting that if I have an IO pipe in my main conduit, I could easily run stuff on top of that in parts of the pipe.
Thanks Niklas
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