[Haskell] Waiting on multiple Chan-nels

Brandon Moore brandonm at yahoo-inc.com
Tue Dec 12 15:04:20 EST 2006

Setzer, Sebastian (ext) wrote:
> Hi,
> 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?

This is pretty hard with Control.Concurrent.Chan. They are simply not 
very easy to compose. STM is designed to be very easy to compose.
(The first word of the title of the first paper - "Composable memory 

If you can switch to TChan, you can use this function:

waitAny :: [TChan a] -> STM a
waitAny chans = msum (map readTChan chans)

which needs
import Control.Concurrent.STM.TChan (TChan, readTChan)
import Control.Monad (msum)
import Control.Monad.STM (STM) --also supplies instance MonadPlus STM


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