[Haskell-cafe] Re: `Expect'-like lazy reading/Parsec matching on TCP sockets

Scott Bell sebell at gmail.com
Mon Apr 9 12:03:55 EDT 2007

> Have you got a complete (but preferably small) program showing the
> problem?


Here is the source and behavior that I'm seeing (Linux x86, under both
6.6 and 6.7-20070404:

module Main where

import System.IO
import System.IO.Unsafe
import System.Process
import Text.ParserCombinators.Parsec

main :: IO ()
main = do (_, h, _, p) <- runInteractiveCommand "telnet nyx.nyx.net"
          t <- hGetContentsTimeout h 15000
          print t >> terminateProcess p

hGetContentsTimeout :: Handle -> Int -> IO String
hGetContentsTimeout h t = do
  hSetBuffering stdin NoBuffering
  ready <- hWaitForInput h t; eof <- hIsEOF h	
  if ((not ready) || eof) then return []
    else do c <- hGetChar h
            s <- unsafeInterleaveIO (hGetContentsTimeout h t)
            return (c:s)

-- Behavior with threaded RTS, string is returned early because of EOF:
sebell at drei:~/src/remote$ ghc --make Remote.hs -o remote -threaded
[1 of 1] Compiling Main             ( Remote.hs, Remote.o )
Linking remote ...
sebell at drei:~/src/remote$ ./remote
"Trying\nConnected to nyx.nyx.net.\nEscape character is '^]'.\n"

-- Behavior with non-threaded RTS, proper timeout is observed:
sebell at drei:~/src/remote$ ./remote
"Trying\nConnected to nyx.nyx.net.\nEscape character
is '^]'.\n\n\n                   Welcome to Nyx, The Spirit of the
Night\n                                (303) 409-1401\n
        nyx.nyx.net --\n
nyx10.nyx.net --\n\n                         Free Public
Internet Access\n\n
===========================\n                          New user?
Login as new\n                         ===========================\n\n
      (If you get timed out, try later.  Nyx would be too slow to
use.)\n\n\n\n\r\n\r\nSunOS UNIX (nyx)\r\n\r\r\n\rlogin: "

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