Question

I intend to build on this code, found here
However, I notice I can telnet to this server on the local host. Can’t from another computer. I did a quick nmap scan, which reported that nothing was listening on the port I had selected.
For purposes of troubleshooting, I had shut down my firewall, so I’ve ruled that out as a possible problem.

Clues from haskell windows programmers would be appreciated.

Answer 1

This is for future new haskellers.
I based my code on this example.
I made improvements based on this reddit thread, and suggestions made above. The import statements are still sloppy, but fixing them is left as the proverbial “exercise for the reader”. I invite any additional suggestions leading to improvements.

import Network.Socket
import Control.Monad
import Network
import System.Environment (getArgs)
import System.IO
import Control.Concurrent (forkIO)

main :: IO ()
main = withSocketsDo $ do
   [portStr] <- getArgs
   sock <- socket AF_INET Stream defaultProtocol 
   let port = fromIntegral (read portStr :: Int)
       socketAddress = SockAddrInet port 0000 
   bindSocket sock socketAddress
   listen sock 1
   putStrLn $ "Listening on " ++ (show port)
   sockHandler sock

sockHandler :: Socket -> IO ()
sockHandler sock' = forever $ do
    (sock, _) <- Network.Socket.accept sock'
    handle <- socketToHandle sock ReadWriteMode
    hSetBuffering handle NoBuffering
    forkIO $ commandProcessor handle

commandProcessor :: Handle -> IO ()
commandProcessor  handle = forever $ do
    line <- hGetLine handle
    let (cmd:arg) = words line
    case cmd of
        "echo" -> echoCommand handle arg 
        "add" -> addCommand handle arg 
        _ -> do hPutStrLn handle "Unknown command"

echoCommand :: Handle -> [String] -> IO ()
echoCommand handle arg = do
    hPutStrLn handle (unwords arg)

addCommand :: Handle -> [String] -> IO ()  
addCommand handle [x,y] = do
    hPutStrLn handle $ show $ read x + read y
addCommand handle _ = do
    hPutStrLn handle "usage: add Int Int"