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 <[EMAIL PROTECTED]> 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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