[Haskell-cafe] Conduit: Where to run monad stacks?

Niklas Hambüchen mail at nh2.me
Wed Aug 22 22:19:54 CEST 2012

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 $$
  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.


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