Logging in Threads

import Control.Concurrent (MVar, newEmptyMVar, putMVar, takeMVar, forkIO)

data Logger = Logger (MVar LogCommand)
data LogCommand = Message String | Stop (MVar ())

-- Create a Logger
mkLogger :: IO Logger
mkLogger = do
  m <- newEmptyMVar
  let l = Logger m
  _ <- forkIO (runLogger l)
  return l

-- Given a logger, read the logcommand and execute it.
-- Message => print the message and wait for the next command
-- Stop    => print "logger: stop" 
runLogger :: Logger -> IO ()
runLogger (Logger m) = undefined

-- Send a `Message` command to the logger.
logMessage :: Logger -> String -> IO ()
logMessage (Logger m) s = putMVar m (Message s)

-- Send a `Stop` command to the logger.
logStop :: Logger -> IO ()
logStop (Logger m) = undefined

-- Test, should print:
--  hello
--  bye
--  logger: stop
main :: IO ()
main = do
  l <- mkLogger
  logMessage l "hello"
  logMessage l "bye"
  logStop l

Je kunt zo vaak indienen als je wenst. Er wordt enkel rekening gehouden met je laatst ingediende oplossing.