[Haskell-cafe] What's the best way to give up?

[David F. Place]

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.
> 
> http://www.haskell.org/haskellwiki/Timing_out_computations
> 

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