[Haskell-cafe] Conduit Best Practices for leftover data

Myles C. Maxfield myles.maxfield at gmail.com
Mon Apr 16 01:53:13 CEST 2012

>> 2. If you use connect-and-resume ($$+), the leftovers are returned as
>> part of the `Source`, and provided downstream.

I'm trying to figure out how to use this, but I'm getting a little bit
confused. In particular, here is a conduit that produces an output for
every 'i' inputs. I'm returning partial data when the input stream
hits an EOF (And I verified that the partial data is correct with
Debug.Trace), yet the output of 'partial' is ([[1,2,3,4,5]],[])
instead of ([[1,2,3,4,5]],[6,7,8]). Can you help me understand what's
going on?


import qualified Data.Conduit as C
import qualified Data.Conduit.List as CL

-- functionally the same as concatenating all the inputs, then
repeatedly running splitAt on the concatenation.
takeConduit :: (Num a, Monad m) => a -> C.Pipe [a1] [a1] m ()
takeConduit i = takeConduitHelper i [] []
  where takeConduitHelper x lout lin
          | x == 0 = C.HaveOutput (takeConduitHelper i [] lin) (return
()) $ reverse lout
          | null lin = C.NeedInput (takeConduitHelper x lout) (C.Done
(Just $ reverse lout) ())
          | otherwise = takeConduitHelper (x - 1) (head lin : lout) $ tail lin

partial :: (Num t, Monad m, Enum t) => m ([[t]], [[t]])
partial = do
  (source, output) <- CL.sourceList [[1..8]] C.$$+ (takeConduit 5 C.=$
  output' <- source C.$$ CL.consume
  return (output, output')

On Sun, Apr 15, 2012 at 2:12 PM, Myles C. Maxfield
<myles.maxfield at gmail.com> wrote:
> Thanks for responding to this. Some responses are inline.
> On Sat, Apr 14, 2012 at 8:30 PM, Michael Snoyman <michael at snoyman.com> wrote:
>> On Thu, Apr 12, 2012 at 9:25 AM, Myles C. Maxfield
>> <myles.maxfield at gmail.com> wrote:
>>> Hello,
>>> I am interested in the argument to Done, namely, leftover data. More
>>> specifically, when implementing a conduit/sink, what should the
>>> conduit specify for the (Maybe i) argument to Done in the following
>>> scenarios (Please note that these scenarios only make sense if the
>>> type of 'i' is something in Monoid):
>>> 1) The conduit outputted the last thing that it felt like outputting,
>>> and exited willfully. There seem to be two options here - a) the
>>> conduit/sink should greedily gather up all the remaining input in the
>>> stream and mconcat them, or b) Return the part of the last thing that
>>> never got represented in any part of anything outputted. Option b
>>> seems to make the most sense here.
>> Yes, option (b) is definitely what's intended.
>>> 2) Something upstream produced Done, so the second argument to
>>> NeedInput gets run. This is guaranteed to be run at the boundary of an
>>> item, so should it always return Nothing? Instead, should it remember
>>> all the input it has consumed for the current (yet-to-be-outputted)
>>> element, so it can let Data.Conduit know that, even though the conduit
>>> appeared to consume the past few items, it actually didn't (because it
>>> needs more input items to make an output)? Remembering this sequence
>>> could potentially have disastrous memory usage. On the other hand, It
>>> could also greedily gather everything remaining in the stream.
>> No, nothing so complicated is intended. Most likely you'll never
>> return any leftovers from the second field of NeedInput. One other
>> minor point: it's also possible that the second field will be used if
>> the *downstream* pipe returns Done.
> Just to help me understand, what is a case when you want to specify
> something in this field? I can't think of a case when a Conduit would
> specify anything in this case.
>>> 3) The conduit/sink encountered an error mid-item. In general, is
>>> there a commonly-accepted way to deal with this? If a conduit fails in
>>> the middle of an item, it might not be clear where it should pick up
>>> processing, so the conduit probably shouldn't even attempt to
>>> continue. It would probably be good to return some notion of where it
>>> was in the input when it failed. It could return (Done (???) (Left
>>> errcode)) but this requires that everything downstream in the pipeline
>>> be aware of Errcode, which is not ideal.I could use MonadError along
>>> with PipeM, but this approach completely abandons the part of the
>>> stream that has been processed successfully. I'd like to avoid using
>>> Exceptions if at all possible.
>> Why avoid Exceptions? It's the right fit for the job. You can still
>> keep your conduit pure by setting up an `ExceptionT Identity` stack,
>> which is exactly how you can use the Data.Conduit.Text functions from
>> pure code. Really, what you need to be asking is "is there any logical
>> way to recover from an exception here?"
> I suppose this is a little off-topic, but do you prefer ExceptionT or
> ErrorT? Any exception/error that I'd be throwing is just  a container
> around a String, so both of them will work fine for my purposes.
>>> It doesn't seem that a user application even has any way to access
>>> leftover data anyway, so perhaps this discussion will only be relevant
>>> in a future version of Conduit. At any rate, any feedback you could
>>> give me on this issue would be greatly appreciated.
>> Leftover data is definitely used:
>> 1. If you compose together two `Sink` with monadic bind, the leftovers
>> from the first will be passed to the second.
> You can do that???? That's so cool!I never realized that Pipes are
> members of Monad.
>> 2. If you use connect-and-resume ($$+), the leftovers are returned as
>> part of the `Source`, and provided downstream.
> This too is really neat :] I didn't realize how this worked.
>> Michael

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