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The Conduit package by Michael Snoyman is an approach to the streaming data problem. It is meant as an alternative to enumerators/iterators, hoping to address the same issues with different trade-offs based on real-world experience with enumerators.

Conduit statements always have a Source and a Sink for data. These can be anything read- or writeable, for example a file, a network connection or simply stdin/stdout. Between the Source and the Sink you can use conduits to modify stream elements.

A very simple example for an application using conduits would be the "uppercase copy" of a text file: The sourceFile streams its characters to the uppercase conduit before all elements are stored to a new file by the sinkFile command.

sourceFile "myfile.txt" $= uppercase $$ sinkFile "mycopy.txt"

For more detailed information about conduits, head over to Michaels github and take a look at the haddock documentation on hackage.

A Network Protocol with Conduits

The main goal of this library is a simple-to-use network library based on conduits which supports sending and receiving of single and multiple "messages" through one single server/client connection.

Part of the Conduit project is the network-conduit package, which provides basic functions for streaming binary data over a network connection:

type Application m = AppData m -> m ()

appSource :: AppData m -> Source m ByteString
appSink   :: AppData m -> Sink ByteString m ()

runTCPServer :: (MonadIO m, MonadBaseControl IO m)
             => ServerSettings m -> Application m -> m ()
runTCPClient :: (MonadIO m, MonadBaseControl IO m)
             => ClientSettings m -> Application m -> m ()

For example, you could copy a file over network by using sourceFile and appSink in the client...

runTCPClient (clientSettings ..) $ \appData ->
    sourceFile "myfile.txt" $$ appSink appData

... and appSource with sinkFile in the server:

runTCPServer (serverSettings ..) $ \appData ->
    appSource appData $$ sinkFile "myfile-networkcopy.txt"

With this very rudimentary API, it is difficult to distinguish between separate ByteString packages ("messages"). To solve this problem, the conduit-network-stream library wraps every ByteString into a tiny header which contains its exact length. This header is designed such that it's not only possible to send single ByteString packages but also lists of ByteStrings:

send1    :: (Monad m, Sendable a m)
         => AppData m -> Source (Stream m) a -> m ()
sendList :: (Monad m, Sendable a m)
         => AppData m -> Source (Stream m) a -> m ()

Using these functions, it is very simple to receive those ByteString packages. All you have to supply is a ByteString sink (Note that these are lazy ByteStrings):

receive :: AppData m
        -> Sink ByteString (Stream m) b
        -> m (ResumableSource (Stream m) ByteString, b)

To avoid two different functions for "fresh" (AppData m) and "old" (ResumableSource) sources I introduced a type class with instances for both:

class Streamable source m where
      receive :: source
              -> Sink ByteString (Stream m) b
              -> m (ResumableSource (Stream m) ByteString, b)

instance Streamable (AppData m)                             m where ...
instance Streamable (ResumableSource (Stream m) ByteString) m where ...

With this function you could then for example define a function which would receive protocol-buffer messages:

receiveProtoBuff :: (Streamable source m, ReflectDescriptor msg, Wire msg)
                 => source
                 -> Sink msg (Stream m) b
                 -> m (ResumableSource (Stream m) ByteString, b)
receiveProtoBuff src sink = receive src $ toMsg =$ sink
    toMsg = Data.Conduits.List.mapMaybe $ \bs ->
              case messageGet bs of
                   Right (msg,_) -> Just msg
                   Left  _       -> Nothing

A simple client/server application with this package (and -XOverloadedStrings) could look like this:

client = runTCPClient myClientSettings $ \appData -> do

    -- send one single `ByteString`
    send1 appData $
        Data.Conduit.yield "Hello world!"

    -- send a `ByteString` list
    sendList appData $
        mapM_ Data.Conduit.yield ["This", "is", "a", "list", "of", "words."]

server = runTCPServer myServerSettings $ \appData -> do

    (next, bs) <- receive appData $

    -- print: "Hello world!"
    liftIO $
        mapM_ print bs

    (next', bs') <- receive next $

    -- print: "This"
    --        "is"
    --        "a"
    --        "list"
    --        "of"
    --        "words."
    liftIO $
        mapM_ print bs'

    -- close the conduit stream
    close next'

Note that receive automatically terminates as soon as the end of a list/block is reached, without closing the connection. All conduits inside a receive will only manipulate the elements of this one block/list.

Also note that I deviated from the original Source/Sink model by accepting a Sink/Source argument to the send/recive functions instead of using the standard $$ operator to pipe one to the other. This has a couple of reasons:

  • Convenience: I found that in practice, there is very little difference between a Source and a Sink conduit. Usually it doesn't matter whether you write src $= conduit $$ sink or src $$ conduit =$ sink. Having just one argument to each function simplifies the API a bit.

  • Type safety: Using the send and receive functions I can wrap the "inner" conduit into a Stream newtype. This makes it impossible to use a stream with this encoding outside of its intended context. (Despite this point I still exported the ~~ operator, a lifted version of $$. There are a number of scenarios where this is useful, but try to avoid whenever possible)

  • Immutable source: Applying a conduit to a Source changes the source for all "resumed" instances aswell. This would make it impossible to decode the stream properly later on. By using receive this problem can be avoided.

The Protocol

A detailed explanation of the used protocol/header is at the top of Conduit.Network.Stream.Header.