Actually, there is no select statement in Haskell. I implement a makeshift "select" in this example. If you have a better idea, please send a Pull Request to my repository!
{-# LANGUAGE GADTs #-}
import Control.Monad
import Control.Concurrent
import Control.Concurrent.STM
main = do
c1 <- atomically newTQueue
c2 <- atomically newTQueue
forkIO $ do
threadDelay (1 * 1000000)
atomically $ writeTQueue c1 "one"
forkIO $ do
threadDelay (2 * 1000000)
atomically $ writeTQueue c2 "two"
forM_ [0..1] $ \i ->
select [ Case c1 $ \msg1 -> putStrLn $ "received " ++ msg1
, Case c2 $ \msg2 -> putStrLn $ "received " ++ msg2
]
class Selectable f where
tryRead :: f a -> STM (Maybe a)
instance Selectable TQueue where
tryRead = tryReadTQueue
data Select a where
Default :: IO a -> Select a
Case :: Selectable s => s b -> (b -> IO a) -> Select a
select :: [Select a] -> IO a
select [] = error "select: empty list"
select ((Default x):_) = x
select (x@(Case v f):xs) = do
var <- atomically $ tryRead v
case var of
Just b -> f b
Nothing -> select (xs ++ [x])$ time runhaskell select.hs
received one
received two
real 0m2.267s