[Haskell-cafe] Timeouts that don't cause data growth.

Bas van Dijk v.dijk.bas at gmail.com
Tue Mar 23 16:02:24 EDT 2010

On Tue, Mar 23, 2010 at 8:23 PM, David Leimbach <leimy2k at gmail.com> wrote:
> Is this just a problem of spawning too many forkIO resources that never
> produce a result?

It looks like it. Lets look at the implementation of timeout:

timeout :: Int -> IO a -> IO (Maybe a)
timeout n f
    | n <  0    = fmap Just f
    | n == 0    = return Nothing
    | otherwise = do
        pid <- myThreadId
        ex  <- fmap Timeout newUnique
        handleJust (\e -> if e == ex then Just () else Nothing)
                   (\_ -> return Nothing)
                   (bracket (forkIO (threadDelay n >> throwTo pid ex))
                            (\_ -> fmap Just f))

We see a thread is forked that throws the Timeout exception to the
current thread after n microseconds. However when the current thread
finishes early this timeout thread will be killed. I assume that when
a thread is killed it can be garbage collected. (However we have to
watch out for [1]) So it's a big surprise to me that we're seeing this

Maybe you can file a bug report?

> I was thinking of trying something like the following in System.Timeout's
> place:
>> module Main where
>> import Control.Concurrent.MVar
>> import Control.Concurrent
>> import Data.Maybe
>> timeout :: Int -> IO a -> IO (Maybe a)
>> timeout time action = do
>>   someMVar <- newEmptyMVar   -- MVar is a Maybe
>>   timeoutThread <- forkIO $ nothingIzer time someMVar
>>   forkIO $ actionRunner action someMVar timeoutThread
>>   takeMVar someMVar >>= return
>>     where
>>       nothingIzer time mvar = threadDelay time >> putMVar mvar Nothing
>>       actionRunner action mvar timeoutThread = do
>>                         res <- action
>>                         killThread timeoutThread
>>                        putMVar mvar $ Just res
>> main :: IO ()
>> main = do
>>  res <- timeout (5 * 10 ^ 6) (getLine >>= putStrLn)
>>  case res of
>>     Nothing -> putStrLn "Timeout"
>>     Just x -> putStrLn "Success"

The original timeout obeys the following specification:

"The design of this combinator was guided by the objective that
timeout n f  should behave exactly the same as f as long as f doesn't
time out. This means that f has the same myThreadId  it would have
without the timeout wrapper. Any exceptions f might throw cancel the
timeout and propagate further up. It also possible for f to receive
exceptions thrown to it by another thread."

They implement this by executing the action in the current thread.
Yours executes the action in another thread.



[1] http://haskell.org/ghc/docs/latest/html/libraries/base-

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