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

[yi huang]

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.
>

Since Sink works in a CPS fashion, by which i mean every step it return a
new push close pair, i think it can be used multiple time.


>
> 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
>
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