Timeouts in async haskell package

AsyncTimeouts.hs

{-# LANGUAGE DeriveDataTypeable #-}

import Control.Concurrent (threadDelay)
import Control.Exception (Exception,throw)
import Control.Monad (void)
import Control.Concurrent.Async (Async,race,wait,withAsync)
import Data.Typeable (Typeable)

data ThreadTimeoutException = ThreadTimeoutException
     deriving (Show, Typeable)

instance Exception ThreadTimeoutException

waitTimeout :: Async a -> Int -> IO (Either ThreadTimeoutException a)
waitTimeout t i =
  race (threadDelay i >> throw ThreadTimeoutException) (wait t)

asyncTimeout_ :: IO a -> Int -> IO ()
asyncTimeout_ f i =
  withAsync f $ \a1 ->
  withAsync (threadDelay i) $ \a2 ->
  void $ race (wait a1) (wait a2)

Should line 14 be return ThreadTimeoutException rather throw?

added a package for AsyncTimeouts.hs https://github.com/joe9/async-timeouts

This should be simplified by returning a Maybe.