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

I thought about it a bit more. The problem would actually be *very* easy to solve if conduit exported one extra function: a connect function that returned a Sink instead of running it. Then you could do: bsrc <- bufferSource src sink2 <- (bsrc $= Cb.lines $= Cl.isolate 3) `connectReturnSink` snk bsrc $$ sink2 That might be generally useful in other places as well, I'm not sure. Michael 2012/2/3 Michael Snoyman :

2012/2/3 Ertugrul Söylemez :

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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      where        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]

Michael