Hi Hakell Cafe,

I'm struggling to understand my unambitious IterIO code that somehow manages to work.

Basically I run an echo server that is supposed to read from a socket line by line and write back to the socket with all the characters in the line reversed:

import Control.Exception

import Control.Monad
import Control.Monad.Trans

import Data.IterIO
import Data.IterIO.Inum

import Network
import System.IO

import System.IO.Error (isEOFError)
import qualified Data.ByteString.Lazy as L

import qualified Data.ByteString.Lazy.Char8 as C

iterHandle' :: (MonadIO m) => Handle -> IO (Iter L.ByteString m (), Onum L.ByteString m a)

iterHandle' = iterHandle

main = withSocketsDo $ do
sListen <- listenOn (PortNumber 8000)

putStrLn "Listening on Port 8000"
forkIO $ forever $ do

(sSession, hostname, port) <- accept sListen
hSetBuffering sSession NoBuffering

putStrLn ("Connected to " ++ hostname ++ ":" ++ show port)
forkIO $ do

(iter, enum) <- iterHandle' sSession
enum |$ inumReverseLines .| iter

putStrLn "Press <CTRL-D> to quit."
exitOnCtrlD

inumReverseLines :: (Monad m) => Inum L.ByteString L.ByteString m a

inumReverseLines = mkInum $ do
line <- lineI

return (L.reverse (L.concat [line, C.pack "\n"]))

exitOnCtrlD = try getLine >>= either
(\e -> unless (isEOFError e) $ ioError e)

(const exitOnCtrlD)

When I run it, it does this:

asdfghc7@hoggy-nn:/home/hoggy$ nc localhost 8000
1234

4321
4321

1234
abcde

edcba

The red lines are the replies from my echo server.

But all I've done is:

enum |$ inumReverseLines .| iter

inumReverseLines = mkInum $ do

line <- lineI
return (L.reverse (L.concat [line, C.pack "\n"]))

No attempt was made to reverse more than one line - at least as far as I can see. What have I done wrong that it should work so well?

Also, is there a better way to do this?

Cheers,

-John