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

On Fri, Aug 24, 2012 at 5:03 PM, Niklas Hambüchen wrote:

Hello Michael,

yes, that does certainly help, and it should definitely be linked to.

The remaining question is:

Is it possible to have something like transPipe that runs only once for the beginning of the pipe?

It seems desirable for me to have conduits which encapsulate monads. Imagine you have to conduits dealing with stateful encryption/decryption and one data-counting one in the middle, like:

decryptConduit $= countConduit $= encryptConduit

Would you really want to combine the three different internal monads into one single monad of the whole pipe, even though the internal monads are implementation details and not necessary for the operation of the whole pipe?

I don't disagree with your analysis, but I don't think it's generally possible to implement the desired transPipe. (If someone can prove otherwise, I'd be very happy.) It *might* be possible via some (ab)use of `monad-control` and mutable variables, however.

The idea with a Ref inside a Reader sounds like a workaround, but has the same problem of globalizing/combining effects, somewhat limiting composability of conduits.

I wouldn't say that we're globalizing effects at all. It should theoretically be possible to write some function like: stateToReader :: MonadIO m => StateT r m a -> ReaderT (IORef r) m a And then `transPipe` will function on the resulting Pipe without issue. Michael

Niklas

On 24/08/12 06:51, Michael Snoyman wrote:

...

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 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

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