Using TChan with Timeout

Question

I have a TChan as input for a thread which should behave like this:

If sombody writes to the TChan within a specific time, the content should be retrieved. If there is n

Answer 1

Use registerDelay, an STM function, to signal a TVar when the timeout is reached. You can then use the orElse function or the Alternative operator <|> to select between the next TChan value or the timeout.

import Control.Applicative
import Control.Monad
import Control.Concurrent
import Control.Concurrent.STM
import System.Random

-- write random values after a random delay
packetWriter :: Int -> TChan Int -> IO ()
packetWriter maxDelay chan = do
  let xs = randomRs (10000 :: Int, maxDelay + 50000) (mkStdGen 24036583)
  forM_ xs $ \ x -> do
    threadDelay x
    atomically $ writeTChan chan x

-- block (retry) until the delay TVar is set to True
fini :: TVar Bool -> STM ()
fini = check <=< readTVar

-- Read the next value from a TChan or timeout
readTChanTimeout :: Int -> TChan a -> IO (Maybe a)
readTChanTimeout timeoutAfter pktChannel = do
  delay <- registerDelay timeoutAfter
  atomically $
        Just <$> readTChan pktChannel
    <|> Nothing <$ fini delay

-- | Print packets until a timeout is reached
readLoop :: Show a => Int -> TChan a -> IO ()
readLoop timeoutAfter pktChannel = do
  res <- readTChanTimeout timeoutAfter pktChannel
  case res of
    Nothing -> putStrLn "timeout"
    Just val -> do
      putStrLn $ "packet: " ++ show val
      readLoop timeoutAfter pktChannel

main :: IO ()
main = do
  let timeoutAfter = 1000000

  -- spin up a packet writer simulation
  pktChannel <- newTChanIO
  tid <- forkIO $ packetWriter timeoutAfter pktChannel

  readLoop timeoutAfter pktChannel

  killThread tid