Problem with backtracking monad transformer

[Guest, Simon]

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Hi,

> 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.

Andrew Bromage wrote:
> 
> I've compared it with my own
> well-tested implementation and it seems identical modulo renamings.

The similarity is explained by the fact that I looked at your example on the
Haskell Wiki as well as Ralf's paper, so thanks for that useful page.

> 
> In case you want to compare:
> 
> 	http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/hfl/hfl/mtl/

I will have a look at try to see what I am doing wrong.

> 
> 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.

This bit I don't understand.  I only have one monad transformer, which I use to 
transform my SM monad.

I have an uneasy feeling that my observe is in the wrong place (passed to runSM), 
but again, I can't see what else I should have done.

Thanks for the assurance, but I'm still head scratching for now.

cheers,
Simon

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