[Haskell] Waiting on multiple Chan-nels
Setzer, Sebastian (ext)
sebastian.setzer.ext at siemens.com
Tue Dec 12 13:47:51 EST 2006
How do you wait on multiple channels, but read only from one of them
(the first that gets an entry)? Is there a library-function I missed
which already does this?
What do you think of this solution?
-- ghc chantest.hs -o chantest
-- Usage: Enter "1", "2" or "0" to exit.
main :: IO()
main = do
chan1 <- newChan
chan2 <- newChan
c1Thread <- forkIO (printChan "c1" chan1)
c2Thread <- forkIO (printChan "c2" chan2)
c12Thread <- forkIO (printChannels "c12" [chan1, chan2])
mainloop chan1 chan2
mainloop :: Chan String -> Chan String -> IO ()
mainloop chan1 chan2 = do
c <- getChar
where -- chan1 and chan2 are in scope, here.
go_on :: IO()
go_on = mainloop chan1 chan2
command :: Char -> IO()
command '0' = do
command '1' = do
writeChan chan1 "main: 1"
command '2' = do
writeChan chan2 "main: 2"
command '\n' = do -- ignore
command c = do
print ("illegal: " ++ [c])
-- return microseconds
seconds :: Int -> Int
seconds = (* 1000000)
printChan :: String -> (Chan String) -> IO()
printChan name chan = do
-- threadDelay (seconds 3)
-- print (name ++ " ready")
s <- readChan chan
printString name s
printChan name chan
printChannels :: String -> [Chan String] -> IO()
printChannels name channels = do
-- select (zip channels (repeat (printString name)))
select $ zip channels $ repeat $ printString name
printChannels name channels
printString :: String -> String -> IO ()
printString name s = print (name ++ ": " ++ s)
select :: [(Chan a, a -> IO())] -> IO ()
select channel_handler_pairs = do
threadDelay (seconds 3)
print "TODO: implement select"
TODO: implement select.
- create MVar
- For each element of the channel_handler_pairs - list, fork a
- wait on an MVar for the signal "I'm done"
- kill all helper threads (just to free the resources. They
In the helper-threads:
- try to signal "I'm ready" on the MVar.
If another thread was faster, unGetChan and exit
- call the handler
- signal "I'm done" and exit
- If the Thread gets killed between readChan and unGetChan, the
message is lost.
- if another thread reads between readChan and unGetChan,
its no fifo anymore (unlikely, because why would you want to
multiple threads on the same Chan? - You'd use dupChan for
purpose. Unless you want every "event" to be handled only
But then you would'nt use multiple threads, would you?)
- "a" must be the same for every channel-handler-pair
Thats easy: instead of channel-handler-tuples, put functions
MVar -> IO() into the list and provide a nice-looking operator
to create them
- Probably inefficient (lots of helper threads)
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