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

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- http://www.yi-programmer.com/blog/

Michael Snoyman wrote:

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:

[...]

Thanks a lot. This conduit world is really new to me and feels a bit more complicated than enumerators, but at least I seem to be getting the right intuition. Greets, Ertugrul -- Key-ID: E5DD8D11 "Ertugrul Soeylemez " FPrint: BD28 3E3F BE63 BADD 4157 9134 D56A 37FA E5DD 8D11 Keysrv: hkp://subkeys.pgp.net/

Michael Snoyman wrote:

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:

I think I'm getting the conduit stuff, at least on a high level. As a little exercise I have ported a simplified variant of the 'netlines' enumerator to the conduit library. This is the code: import qualified Data.ByteString as B netLine :: (Resource m) => Int -> Sink B.ByteString m B.ByteString netLine n0 = sinkState (n0, B.empty) push (return . snd) where push (n, str') dstr' = return $ case B.elemIndex 10 dstr' of Nothing -> let dstr = B.take n dstr' str = B.append str' dstr in str `seq` StateProcessing (n - B.length dstr, str) Just i -> let (pfx, sfx) = B.splitAt i dstr' str = B.append str' (B.take n pfx) in str `seq` StateDone (Just . B.copy $ B.tail sfx) str netLines :: (Resource m) => Int -> Conduit B.ByteString m B.ByteString netLines n = sequenceSink () (\s -> fmap (\ln -> Emit s [ln]) (netLine n)) It reads a 256 MiB file with random data in 1.3 seconds and runs in constant memory for infinite lines. This is reassuring. But anyway, is this the proper/idiomatic way to do it, or would you go for a different direction? Greets, Ertugrul -- Key-ID: E5DD8D11 "Ertugrul Soeylemez " FPrint: BD28 3E3F BE63 BADD 4157 9134 D56A 37FA E5DD 8D11 Keysrv: hkp://subkeys.pgp.net/