Concurrency demos/Two reader threads

This is an example of using a main thread to send messages to 2 reader threads, over a single chan. It uses lazy IO to convert the Chan into a lazy list of filtered elements, and passes the filtered lists to the reader threads. The advantage of this design is a greatly simplified reader implementation (as it need do no IO).

--
-- ghc chantest.hs -o chantest
-- Usage: Enter "1", "2" or "0" to exit.
--

import System.IO
import Control.Concurrent
import Control.Concurrent.Chan
import Text.Printf

--
-- Rather tha wait on multiple channels, it strikes me that you could
-- have a single channel, and tag each thread's input. then lazily
-- stream the chans contents through a filter, passing the *pure* list
-- of filtered elements to a consuming thread
--
type Pipe = Chan (Either String String)

main :: IO ()
main = do
    chan      <- newChan :: IO Pipe
    s         <- getChanContents chan   -- lazy list of chan elements
    c1Thread  <- forkIO $ reader "c1" (catLeft  s) -- read only Lefts
    c2Thread  <- forkIO $ reader "c2" (catRight s) -- read only Rights
    writer chan
  where
    catLeft  ls = [x | Left  x <- ls]
    catRight ls = [x | Right x <- ls]

writer :: Pipe -> IO ()
writer chan = loop
  where
    loop = getChar >>= command
    command '0'  = print "done"
    command '1'  = writeChan chan (Left  "main: 1") >> loop
    command '2'  = writeChan chan (Right "main: 2") >> loop
    command '\n' = loop -- ignore
    command c    = printf "Illegal: %c\n" c         >> loop

reader :: String -> [String] -> IO ()
reader name xs = mapM_ (printf "%s %s\n" name) xs

Running this:

   $ ghc x.hs -threaded

   $ ./a.out           
   1
   c1 main: 1
   2
   c2 main: 2
   2
   c2 main: 2
   1
   c1 main: 1
   2
   c2 main: 2
   3
   Illegal: 3
   1
   c1 main: 1
   2
   c2 main: 2
   0
   "done"