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Note: This issue has been resolved:

  • No more chunking.
  • isolate cannot force consumption.

Thinking out loud: my current problem with the Conduit type

The main issue with implementing a Conduit (i.e., the pipe between a source and sink, not the overall package) is that of data loss. For example, take the following code:

import qualified Data.Conduit as C
import qualified Data.Conduit.List as CL
import Control.Monad.IO.Class (liftIO)

main = C.runResourceT $ CL.sourceList [1..10] C.$$ do
    strings <- CL.map show C.=$ CL.take 5
    liftIO $ putStr $ unlines strings
    rest <- CL.fold (+) 0
    liftIO $ putStrLn $ "Sum is: " ++ show rest

You would expect it to print the numbers 1 to 5 on individual lines, and then print the sum of the numbers 6 to 10. Instead, it prints that the sum is "0". This is because the map conduit has already converted the integers to strings, and those values are now lost to the system.

However, the problem is worse than this, because the output will change based on the chunking behavior. For example, the following code (perhaps correctly) gives the sum as 40:

import qualified Data.Conduit as C
import qualified Data.Conduit.List as CL
import Control.Monad.IO.Class (liftIO)
import Data.Monoid (mappend)

main = C.runResourceT $ (CL.sourceList [1..5] `mappend` CL.sourceList [6..10]) C.$$ do
    strings <- CL.map show C.=$ CL.take 5
    liftIO $ putStr $ unlines strings
    rest <- CL.fold (+) 0
    liftIO $ putStrLn $ "Sum is: " ++ show rest

For the record, I believe that enumerator suffers from the same problem. There's another problem in conduit: isolate currently only does half of its job. We want isolate to both prevent a sink from consuming too much data, and force the sink to consume all the data directed at it. In other words, the output from the following program should be the numbers 11 to 15:

import qualified Data.Conduit as C
import qualified Data.Conduit.List as CL
import Control.Monad.IO.Class (liftIO)
import Data.Monoid (mappend)

main = C.runResourceT $ CL.sourceList [1..15] C.$$ do
    _ <- CL.isolate 5 C.=$ CL.consume
    _ <- CL.isolate 5 C.=$ return ()
    rest <- CL.consume
    liftIO $ print rest

Instead, however, the second isolate call consumes no data, and the result is the numbers 6 to 15.

Update: I've decided that the issue of isolate not consuming all data is not a real issue. Instead, I'm adding sinkNull to help out with forcing consumption.

Possible solutions

One simple solution (on branch isolate-fix-1) to both problems is to set up the fuse/connect operators (actually, I think just =$) to always pipe all data into a conduit until it returns leftovers. This solves the ambiguity issue: we will consistently return the same results despite chunking. And it also means that isolate works. There are two downsides:

  1. This can be very inefficient, forcing consumption of an entire source even if its not needed.

  2. We're defaulting to throwing away data.

You can argue that (2) is acceptable, and if someone wants to preserve data, they can either write a specialized conduit that can (somehow, I'm not quite sure how) return leftovers early, or use isolate to make sure only some data is consumed.

Another solution I'm thinking of, which I have no yet solidified, is changing the definition of conduit somehow. Here are a bunch of ideas I have:

  • We have the concept of closing from the left (i.e., the source is closed), where no leftover values need to be returned, versus closed from the right (i.e., the sink is closed), where no output values are produced.

  • Conduits are required to return data piecemeal, i.e., no chunking. As soon as the consuming sink finishes processing, the conduit is notified before it processes any more content. This is in effect the opposite of isolate-fix-1. (It also doesn't solve the isolate issue.)

  • When closing a conduit (from the right using the terminology above), the conduit can provide a new sink which will consume as much input as it wants and then provide leftovers.

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