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?
If I understood you correctly you seem to want a monad that offers something akin to Prolog's cut. You might want to take a look at http://okmij.org/ftp/Computation/monads.html#LogicT Cheers Ben