G'day all.
On Thu, Jan 30, 2003 at 01:55:50PM -0000, Guest, Simon wrote:
> I'm trying to make a backtracking state monad using Ralf Hinze's
> backtracking monad transformer. My problem is that it won't backtrack
> very far.
>
> Suppose I try ( a >> b ) `mplus` c.
>
> If b fails, it should try c, but it doesn't rewind past a.
I added this to your source file:
testBACKTR :: (Monad m) => BACKTR m Int
testBACKTR
= ( return 1 >> M.mzero ) `M.mplus` (return 2)
main :: IO ()
main = putStrLn (show (observe testBACKTR :: Maybe Int))
The result is "Just 2", so I don't think there's anything wrong with
your implementation of BACKTR. I've compared it with my own
well-tested implementation and it seems identical modulo renamings.
In case you want to compare:
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/hfl/hfl/mtl/
I didn't follow the rest of the code, so I suspect the problem is
elsewhere. One place to look is here:
> -- backtracking state monad
> --
> type NDSM st a = BACKTR (SM st) a
You may have meant to stack the monad transformers in a different
order.
Cheers,
Andrew Bromage
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