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