[Haskell-cafe] Conduit experiment: Is this correct?

Michael Snoyman michael at snoyman.com
Fri Feb 3 15:29:32 CET 2012

2012/2/3 Ertugrul Söylemez <es at ertes.de>:
> Hello there,
> I'm trying to build a server for testing the conduit and network-conduit
> packages.  As a contrived example the goal is to pick the first three
> lines from the client and send them back without the line feeds.  After
> that, I'd like to switch to a simple echo server.  This is the code:
>    module Main where
>    import Data.Conduit
>    import Data.Conduit.Binary as Cb
>    import Data.Conduit.List as Cl
>    import Data.Conduit.Network
>    handleClient :: Application
>    handleClient src snk =
>        src $$ do
>            (Cb.lines =$= Cl.isolate 3) =$ snk
>            snk
>    main :: IO ()
>    main = runTCPServer (ServerSettings 4000 Nothing) handleClient
> I'm not sure whether it is correct to use the 'snk' sink multiple times,
> and intuitively I'd say that this is wrong.  What would be the proper
> way to do this?
> Greets,
> Ertugrul

In this particular case, it will work due to the implementation of
snk. In general, however, you're correct: you should not use the same
sink twice.

I haven't thought about it much yet, but my initial recommendation
would be to create a new Conduit using SequencedSink, which takes the
three lines and then switches over to a passthrough conduit. The
result looks like this:

    module Main where

    import Data.Conduit
    import Data.Conduit.Binary as Cb
    import Data.Conduit.List as Cl
    import Data.Conduit.Network

    handleClient :: Application
    handleClient src snk = src $$ myConduit =$ snk

    main :: IO ()
    main = runTCPServer (ServerSettings 4000 Nothing) handleClient

    myConduit =
        sequenceSink 3 go
        go 0 = return $ StartConduit $ Cl.map id
        go count = do
            mx <- Cb.lines =$ Cl.head
            case mx of
                Nothing -> return Stop
                Just x -> return $ Emit (count - 1) [x]


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