>> 2. If you use connect-and-resume ($$+), the leftovers are returned as >> part of the `Source`, and provided downstream.
I'm trying to figure out how to use this, but I'm getting a little bit confused. In particular, here is a conduit that produces an output for every 'i' inputs. I'm returning partial data when the input stream hits an EOF (And I verified that the partial data is correct with Debug.Trace), yet the output of 'partial' is ([[1,2,3,4,5]],[]) instead of ([[1,2,3,4,5]],[6,7,8]). Can you help me understand what's going on? Thanks, Myles import qualified Data.Conduit as C import qualified Data.Conduit.List as CL -- functionally the same as concatenating all the inputs, then repeatedly running splitAt on the concatenation. takeConduit :: (Num a, Monad m) => a -> C.Pipe [a1] [a1] m () takeConduit i = takeConduitHelper i [] [] where takeConduitHelper x lout lin | x == 0 = C.HaveOutput (takeConduitHelper i [] lin) (return ()) $ reverse lout | null lin = C.NeedInput (takeConduitHelper x lout) (C.Done (Just $ reverse lout) ()) | otherwise = takeConduitHelper (x - 1) (head lin : lout) $ tail lin partial :: (Num t, Monad m, Enum t) => m ([[t]], [[t]]) partial = do (source, output) <- CL.sourceList [[1..8]] C.$$+ (takeConduit 5 C.=$ CL.consume) output' <- source C.$$ CL.consume return (output, output') On Sun, Apr 15, 2012 at 2:12 PM, Myles C. Maxfield <myles.maxfi...@gmail.com> wrote: > Thanks for responding to this. Some responses are inline. > > On Sat, Apr 14, 2012 at 8:30 PM, Michael Snoyman <mich...@snoyman.com> wrote: >> On Thu, Apr 12, 2012 at 9:25 AM, Myles C. Maxfield >> <myles.maxfi...@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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe