On Sat, 2008-09-06 at 11:10 -0400, Brandon S. Allbery KF8NH wrote:
On 2008 Sep 6, at 7:30, David F. Place wrote:
Say I have a function solve which is a constraint solver. It reconfigures its input to be a solution. If there is no solution, it returns the input.
solve :: a -> Either a a solve input output = maybe (Left input) Right $ solve' input
If there is a solution, it finds it in a few seconds. If there is no solution, it goes away for days proving that. So, I'd like to give up on it if it doesn't return in a few seconds.
Thanks, Arnar, Christopher and Brandon. I looked into all your suggestions. I came up with this: import Control.Exception import System.Timeout apt :: Int -> (a -> a) -> a -> IO (Either a a) apt time f x = do { ans <- timeout time $ evaluate (f x) ; return $ maybe (Left x) Right ans } test x = sum [1..x] test2 x = length $ repeat x Which seems to do what I want for test: *Main> apt 100 test 100 Right 5050 *Main> apt 100 test 1000 Right 500500 *Main> apt 100 test 10000 Right 50005000 *Main> apt 100 test 100000 Left 100000 *Main> apt 100 test 1000000 Left 1000000 *Main> apt 100 test 10000000 Left 10000000 But, the following sets the cpu spinning and can't be interrupted. *Main> apt 100 test2 100