The idea of backtracking is, that you try all options because you can't tell/calulate in advance which parameters lead to a solution; if you have some idea about where and how to limit your search, you may save a lot of time. In your example, you can limit your search by replacing the lines:

a <- lift $ mlist aList b <- lift $ mlist bList

with:

a <- lift $ mlist $ take 4 aList b <- lift $ mlist $ take 4 bList

In some cases you could use the "filter" function:

a <- lift $ mlist $ filter (< 5) aList b <- lift $ mlist $ filter (< 5) bList

Or "takeWhile":

a <- lift $ mlist $ takeWhile (< 5) aList b <- lift $ mlist $ takeWhile (< 5) bList

Etc. Met vriendelijke groet, Henk-Jan van Tuyl -- On Thu, 23 Nov 2006 22:25:23 +0100, isto wrote:

Hi all,

Weekly news had a link to article "Local and global side effects with monad transformers" and the following is from there (minor modification done):

import Control.Monad.List import Control.Monad.State import Control.Monad.Writer

test5 :: StateT Integer (ListT (Writer [Char])) Integer

test5 = do a <- lift $ mlist aList b <- lift $ mlist bList lift $ lift $ tell ("trying "++show a++" "++show b++"\n") modify (+1) guard $ a+b<5 return $ a+b

go5 = runWriter $ runListT $ runStateT test5 0

If the aList = [1..5] as well as bList, then there will be 25 tryings. If aList and bList are [1..10000000], there will be a lot of tryings...

However, guard is saying that we are interested in only values whose sum is less than 5.

Is it possible to control easily in the above example that when we have tried out pairs (1,1), (1,2), (1,3), (1,4), that now we are ready to stop trying from the bList? And then similarly when we finally arrive to a pair (4,1), that now we are ready to finish also with aList?

This would give a nice way to build branch & bounding algorithms, even though it does not take much more lines to do it in some other way.

br, Isto

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