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 <m...@nh2.me> wrote: > 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 > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
