On Tue, Jun 15, 2004 at 06:29:59PM -0400, S. Alexander Jacobson wrote:
I'd like a function that blocks until data is available on one of two channels and then returns that data wrapped in an Either. Here is my naive implementation:
readEitherChan::Chan a -> Chan b -> Either a b readEitherChan a b = do var <- newEmptyMVar forkIO (readChan a >>= putMVar var . Left) forkIO (readChan b >>= putMVar var . Right) val <- readMVar return val
eitherChan a b left right = readEitherChan a b >>= either left right
But creating an MVar and starting two threads feels like a lot of overhead for a simple operation.
You can also loose some elements from both channels. I think it would be safer to create once new Chan (Either a b), and then read from it. createEitherChan :: Chan a -> Chan b -> IO (Chan (Either a b)) createEitherChan a b = do ab <- newChan forkIO (sequence_ (repeat (readChan a >>= writeChan ab . Left))) forkIO (sequence_ (repeat (readChan b >>= writeChan ab . Right))) return ab Best regards, Tom -- .signature: Too many levels of symbolic links