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

[Michael Snoyman]

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 <michael at snoyman.com>:
> 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
>      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