On Sat, Jul 18, 2009 at 10:51:26PM +0000, Maciej Piechotka wrote: ] Felipe Lessa writes: ] ] On Sat, Jul 18, 2009 at 12:40:15AM +0200, Maciej Piechotka wrote: ] ] ] I have a data structure of ] ] ] data Monad m => NntpConnection m = NntpConnection { ] ] ] input :: ByteString, ] ] ] output :: ByteString -> m () ] ] ] } ] ] ] ] ] ] I'd like to create echo structure such that the goes to output is going ] ] ] to (lazy) input. For sure it is possible to use network and IO monad - ] ] ] is is possible to do it purely? ] ] ] ] In words, not code: you may create a Chan of strict ByteStrings. ] ] On the output side you just append all the chunks of the lazy ] ] ByteString to the Chan, or you may copy the lazy ByteString in ] ] one chunk of strict ByteString. On the input side you ] ] "getContents" and "fromChunks". Should work, I guess. ] ] ] ] -- ] ] Felipe. ] ] ] ] Sorry - I don't follow: ] 1. How the output is suppose to get into input? Via the Chan. ] 2. Why combine getContents and fromChunks if getContents is overloaded for ] ByteString? I said getContents but I meant getChanContents :). ] 3. How to use getContents without IO - either network or posix pipe? It's not without the IO monad, but it works without doing I/O (e.g. networking, files) and portably. Ok, now with (literate Haskell) code :)

import Control.Concurrent.Chan import qualified Data.ByteString.Lazy.Char8 as L

data NntpConnection m = NntpConnection { input :: L.ByteString, output :: L.ByteString -> m () }

echoConnIO :: IO (NntpConnection IO) echoConnIO = do chan <- newChan inputChunks <- getChanContents chan -- lazy! return $ NntpConnection { input = L.fromChunks inputChunks, output = mapM_ (writeChan chan) . L.toChunks }

*Main Control.Concurrent> c <- echoConnIO *Main Control.Concurrent> forkIO $ L.putStrLn (input c) ThreadId 65 *Main Control.Concurrent> output c $ L.pack "Hello, world!\n" Hello, world! Note that the input bytestring will never end because NntpConnection doesn't have something like 'close :: m ()'. If you want something robust you'll probably move away from representing you input as a lazy bytestring anyway. -- Felipe.