While using a Network.Socket converted to a Handle with socketToHandle, calling hReady or hWaitForInput on that handle will block.

This happens within ghci or when compiled with -threaded and executed with "+RTS -N2 -RTS" options.

I've attached a simple server.hs file that listens on a port. Once a client connects it loops until hReady returns true - then it will read from the port. Load it in gchi and execute something like:

servNumber "11333"

I've also attached a quick client.hs module to load in ghci to send data to the port. Load it in ghci and execute something like:

s1 <- openServer "localhost" "11333"
write2Server "blash" s1

Here's the code.

server.hs

import Network.Socket
import Control.Concurrent
import System.IO

-- main = servNumber "11333"

servNumber :: String -> IO ()
servNumber port = withSocketsDo $ do
  addrInfos <- getAddrInfo 
                    (Just (defaultHints {addrFlags = [AI_PASSIVE]}))
                    Nothing (Just port)
  let serveraddr = head addrInfos
  sock <- socket (addrFamily serveraddr) Stream defaultProtocol
  bindSocket sock (addrAddress serveraddr)
  listen sock 5
  procIncoming sock
 where
  procIncoming :: Socket -> IO ()
  procIncoming masterSock = do
    (conSock, _) <- accept masterSock
    forkIO $ doWork conSock
    procIncoming masterSock 

  doWork :: Socket -> IO ()
  doWork conSock = do
    h <- socketToHandle conSock ReadWriteMode
    hSetBuffering h LineBuffering
    loop h 

  loop :: Handle -> IO ()
  loop h = do 
    putStrLn "Calling hReady."
    ready <- hReady h
    --ready <- hWaitForInput h 1
    if ready 
        then hGetLine h >>= putStrLn >> loop h
        else loop h 

client.hs

import Network.Socket
import Control.Concurrent
import System.IO


openServer :: String -> String -> IO (Handle)
openServer hostname port = withSocketsDo $ do
  addrinfos <- getAddrInfo Nothing (Just hostname) (Just port)
  let serveraddr = head addrinfos
  sock <- socket (addrFamily serveraddr) Stream defaultProtocol
  connect sock (addrAddress serveraddr)
  h <- socketToHandle sock ReadWriteMode
  hSetBuffering h LineBuffering
  return h

write2Server :: String -> Handle -> IO ()
write2Server msg h = do
  hPutStrLn h msg
  hFlush h

closeServer :: Handle -> IO ()
closeServer h = hClose h