Toy actor that counts

counter.hs

#!/usr/bin/env stack
-- stack --resolver lts-10.8 runghc --package nqe-0.1.0.0

import Control.Monad
import Control.Concurrent.NQE

data CounterMessage = GetCounter (Reply Integer)

counter :: Inbox CounterMessage -> IO ()
counter inbox = do
    box <- newTVarIO 0
    forever $ do
        GetCounter reply <- receive inbox
        atomically $ do
            n <- readTVar box
            reply n
            modifyTVar box (+1)

printer :: Inbox CounterMessage -> IO ()
printer inbox = do
    replicateM_ 5 $ do
        i <- GetCounter `query` inbox
        print i

main :: IO ()
main = do
    inbox <- Inbox <$> newTQueueIO
    super <- Inbox <$> newTQueueIO
    supervisor KillAll super (actors inbox)
  where
    actors inbox = [counter inbox, printer inbox]