Folding lines in conduit

September 24, 2013

GravatarBy Michael Snoyman

This blog post is in response to a post by Gabriel Gonzalez on perfect streaming. The issue he raises is a good one: how to do efficient processing of chunks of data, where each chunk may contain a large amount of data that should be streamed.

While I appreciate the stated problem, I dislike the given solution. Gabriel's approach is to invert the entirety of the stream processing, forcing the user to have to explicitly pull data out of a producer instead of the more usual, declarative semantics most of the streaming data libraries (enumerator, conduit, and pipes) adhere to. (In fact, this critique is not limited to the specific technique employed here, but to the general approach to chunked data employed by pipes-bytestring and pipes-parse. I'm writing up a more detailed blog post on that subject separately.)

Nonetheless, Gabriel's criticism of the simplistic approach the lines function takes in conduit is valid. While the function is convenient for many common use cases, it is not robust enough for data with large lines. But instead of inverting the entirety of the streaming library to fix the problem, I'd rather just reuse a more common idiom in Haskell: the fold.

If you want to just jump in, the full code is available on School of Haskell. There are some auxilary functions in there that should really be in Data.Conduit.Text itself. But let's focus on the foldLines function:

foldLines :: Monad m
          => (a -> ConduitM Text o m a)
          -> a
          -> ConduitM Text o m a
foldLines f =
    start a = CL.peek >>= maybe (return a) (const $ loop $ f a)

    loop consumer = do
        a <- takeWhileText (/= '\n') =$= do
            a <- consumer
            return a
        dropText 1
        start a

The logic here is pretty straight-forward, let's just step through it. The user provides an initial value for the accumulator, which is initially passed to the start helper function. start peeks at the input stream; if there's no data in the stream, we immediately return the accumulator. If there is data in the stream, peek ensures that the data will be put back on the stream. We use const to ignore that initial chunk (we'll consume it in a moment), and pass the accumulator to the user supplied function to get the user's consuming function.

loop is able to use all the glory of standard conduit composition. We use the takeWhileText function to stream everything up to the first newline character to the user's consumer. sinkNull is employed to ensure the entire input is consumed, regardless of the behavior of the user function. Notice also that the user is unable to accidentally consume more content than the current line. After all of that, we return the new accumulator value, use dropText to drop the newline character on the stream, and then we start again with the new accumulator.

To demonstrate usage of this function, I've included a small function for getting the line number and character count for all the lines in a file.

type LineNumber = Int
type CharCount = Int
data LineStat = LineStat !LineNumber !CharCount

myFunc :: Monad m => LineNumber -> ConduitM Text LineStat m LineNumber
myFunc number' = do
    count <- CL.fold (\count t -> count + T.length t) 0
    yield $ LineStat number count
    return number
    number = number' + 1

showLineStat :: LineStat -> Text
showLineStat (LineStat number count) = T.concat
    [ "Line number "
    , T.pack $ show number
    , " has character count of: "
    , T.pack $ show count

main :: IO ()
main = runResourceT
     $ CB.sourceFile "input.txt"
    $$ CT.decode CT.utf8
    =$ void (foldLines myFunc 0)
    =$ showLineStat
    =$ unlinesText
    =$ CT.encode CT.utf8
    =$ CB.sinkHandle stdout

myFunc is able to use all the standard conduit machinery to do its work. It uses a standard fold to sum up the length of each line, and then yields a LineStat for each line. For this kind of usage, it would probably make sense to include some kind of a scan-like function, but folding works fine.

main itself is a bit verbose, since it has to deal with character encoding/decoding issues. (Tangentially, I've considered adding file read/write support to Data.Conduit.Text, but don't like adding implicit character encoding/decoding. I'd be interested on hearing people's thoughts on this.)

Bonus points: if you feel like optimizing this, try using blaze-builder-conduit and turn showLineStat into a function of type LineStat -> Builder.

I think this style of solution is far preferable to having to reinvent the wheel to deal with chunked data. Unfortunately, this style of coding seems to be out of reach for pipes. To understand why, let's look at the implementation of takeWhileText, the powerhouse underneath foldLines:

takeWhileText :: Monad m
              => (Char -> Bool)
              -> Conduit Text m Text
takeWhileText p =
    loop = await >>= maybe (return ()) go
    go t =
        case T.span p t of
            (x, y)
                | T.null y -> yield x >> loop
                | otherwise -> yield x >> leftover y

The logic is pretty simple. Loop over the input. If there is no input, we're done. If there is an input chunk available, split it based on the provided predicate. If the second part (y) is null, it means that the entire chunk matched the predicate, so we should yield the chunk and continue looping.

If y is not null, then we simply yield the piece that did match (x) and then return the rest as leftovers, to be consumed later. To me, this is the most natural way to express the algorithm. However, pipes does not provide leftover support, and therefore has to invert streaming data to instead be explicit pulling from a producer. As I mentioned before, I'll go into more details in a later blog post on the problems with this approach.

Gabriel's FreeT trick is interesting, and certainly clever. Given the constraints of pipes, it seems like a good solution to the problem. However, let's not confuse a constrained good solution with the ideal solution. In my opinion, being able to reuse commonly used abstractions is far preferable to needing to invent lots of clever hacks.


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