ipc2.hs

import           Control.Exception
import           Control.Monad        (forever, void)
import           Data.Char            (toUpper)
import           GHC.IO.Handle.FD     (fdToHandle)
import           Prelude
import           System.Exit
import           System.IO            (BufferMode (..), hGetLine, hPrint,
                                       hPutStrLn, hSetBuffering, hClose)
import           System.Posix.Process (exitImmediately, forkProcess)
import           System.Process       (createPipe, createPipeFd)
import           Text.Read            (readEither)

-- | Example using two processes
--
-- We want server/client to read only the messages that each should care about
-- In order to realize this, we need two proccesses with each of them providing
-- read/write handle.
--
-- These processes will then pass each others handle respectively and use it to
-- communicate with each other.
--
-- Server will take client's write handle and server's read handle.
--
-- Client will take server's write handle and client's read handle.
--
-- This allows the two proccesses to send the message to the other while
-- reading the response that other had sent.
exampleWithProcess :: IO ()
exampleWithProcess = do
    (clientReadHndl, clientWriteHndl) <- createPipe
    hSetBuffering clientReadHndl LineBuffering
    hSetBuffering clientWriteHndl LineBuffering

    processId <- forkProcess $ do
        (serverReadHndl, serverWriteHndl) <- createPipe
        hSetBuffering serverReadHndl LineBuffering
        hSetBuffering serverWriteHndl LineBuffering

        hPutStrLn serverWriteHndl "Ping"
        hPutStrLn serverWriteHndl "Pong"

        finally
            (forever $ do
                msg <- hGetLine serverReadHndl
                hPutStrLn clientWriteHndl (map toUpper msg)
            )
            (exitImmediately ExitSuccess)

    msg     <- hGetLine clientReadHndl -- PING
    msg2    <- hGetLine clientReadHndl -- PONG
    print msg
    print msg2