[Haskell] timing/timeout (how to express that in Haskell)

[Donald Bruce Stewart]

waldmann:
> What is the idiomatic way to say in (ghc) Haskell:
> "run this computation for at most  x  seconds"
> (e. g. it returns Boolean; imagine a primality test)
> so I want something :: Int -> a -> Maybe a
> with the guarantee that the result is
> Just x  with  x  in whnf, or  Nothing.
> I guess one answer is "that's not Haskell because
> that's not a function". Sure, but I think I need it
> anyways, so I would accept some  IO .. in the types.

This comes up occasionally, at least one solution is:

    watchdogIO :: Int  -- milliseconds
             -> IO a   -- expensive computation
             -> IO a   -- cheap computation
             -> IO a
    watchdogIO millis expensive cheap = 
        do mvar <- newEmptyMVar
           tid1 <- forkIO $ do x <- expensive
                               x `seq` putMVar mvar (Just x)
           tid2 <- forkIO $ do threadDelay (millis * 1000)
                               putMVar mvar Nothing
           res <- takeMVar mvar
           case res of
             Just x -> 
                 do info ("EXPENSIVE was used")
                    killThread tid2 `catch` (\e -> warn (show e))
                    return x
             Nothing ->
                 do info ("WATCHDOG after " ++ show millis ++ " milliseconds")
                    killThread tid1 `catch` (\e -> warn (show e))
                    cheap


Note that this does more than you want, but you get the idea. 
forkIO + killThread && threadDelay

If you code up a nice example, perhaps you coudl put it on the wiki,
under Idioms?

-- Don