9.5. Select: Synchronous I/O multiplexing
The Select interface provides a Haskell wrapper for the select() OS call supplied by many modern UNIX variants. Select exports the following:
type TimeOut = Maybe Int
-- Nothing => wait indefinitely.
-- Just x | x >= 0 => block waiting for 'x' micro seconds.
-- | otherwise => block waiting for '-x' micro seconds.
hSelect :: [Handle]
-> [Handle]
-> [Handle]
-> TimeOut
-> IO SelectResult
type SelectResult
= ( [Handle] -- input handles ready
, [Handle] -- output handles ready
, [Handle] -- exc. handles ready
) |
Here's an example of how it could be used:
module Main(main) where import Select import IO main :: IO () main = do hSetBuffering stdin NoBuffering putStrLn "waiting for input to appear" hSelect [stdin] [] [] Nothing putStrLn "input ready, let's try reading" x <- getChar print x |
where the call to hSelect makes the process go to sleep until there's input available on stdin.
9.5.1. Using hSelect with Concurrent Haskell
In brief: don't. For two reasons:
hSelect will cause all your Haskell threads to block until the hSelect returns, much like any call to a foreign function.
You don't need to. Concurrent Haskell will let you do I/O on multiple file handles concurrently by forking threads, and if you need to assign a timeout, then this can be done using a combination of threadDelay (see ) and asynchronous exceptions (see ).