Question

AFAIK TChan acts as a hub, every message sent is seen by others right ?!
i want a TChan that acts as a switch to send a message to specific thread, and also support broadcasting.
is there such thing ?

Answer 1

Edit: I re-read your question. This answer doesn’t quite address “selective send”, though it clarifies what a TChan can do.

The “broadcast” approach described below will wake up all listeners (though on the bright side, it won’t make 1000 copies of each item). To avoid this, use the Map approach as @Mikhail suggested. I did this in my chat server example.

---

A TChan is a FIFO queue:

• writeTChan adds an item to the end.

• readTChan reads an item from the beginning.

For example, the following example forks 10 threads which fight over a single channel:

import Control.Concurrent
import Control.Concurrent.STM
import Control.Monad

main = do
    chan <- newTChanIO

    forM_ [1..10] $ \i ->
        forkIO $
            forever $ do
                x <- atomically $ readTChan chan
                putStrLn $ "Thread " ++ show i ++ ": " ++ show x

    mapM_ (atomically . writeTChan chan) [1..1000]

    -- Wait for channel to empty out
    atomically $ do
        empty <- isEmptyTChan chan
        when (not empty) retry

Here, each item is read by exactly one thread.

In contrast, the following example “broadcasts” a stream of items to 10 threads, by making ten copies of the channel using dupTChan:

import Control.Concurrent
import Control.Concurrent.STM
import Control.Monad

main = do
    master <- newTChanIO

    forM_ [1..10] $ \i -> do
        chan <- atomically $ dupTChan master
        forkIO $
            forever $ do
                x <- atomically $ readTChan chan
                putStrLn $ "Thread " ++ show i ++ ": " ++ show x

    mapM_ (atomically . writeTChan master) [1..100]

    -- Give threads time to complete
    threadDelay 1000000

Now each thread gets all of the items written to the channel.

A couple subtleties to note:

• Items written to a channel prior to dupTChan will not appear in the new channel. If we called dupTChan from the child threads rather than the main thread, some writeTChans could happen first, meaning the children might not see all the items.

• Since nobody is reading the master channel, items written to it will pile up and will likely not be garbage collected. To avoid this caveat, use newBroadcastTChan to create the master channel.