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

2012/2/3 Michael Snoyman

2012/2/3 Ertugrul Söylemez :

...

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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