Re: [eclipse-clp-users] On implementation of post_goal and EC_resume

Dear Joachim,

Thank you for your quick response! 

I'll appreciate it if you or any one here can provide me some pointers and references
on the Eclipse algorithm and implementaion of incremental answering of queries. 
(We may need to modify or re-design the algorithms for our purpose which
 needs a stronger requirement on incremental query answering).

best,
Yuanlin

> -----Original Message-----
> From: Joachim Schimpf [mailto:joachim.schimpf_at_infotech.monash.edu.au]
> Sent: Monday, June 22, 2009 9:25 AM
> To: 'eclipse-clp-users_at_lists.sourceforge.net'
> Subject: Re: [eclipse-clp-users] On implementation of post_goal and
> EC_resume
> 
> Zhang, Y wrote:
> >
> > We are working on answering a sequence of goals incrementally.
> > We noticed eclipse provides "post_goal" in its interface for C++.
> > I'd like to know how eclipse process the continuously added goals
> > (incrementally)?
> 
> Yes, it's incremental.  As long as your posted goals succeed,
> additional posted goals get appended to the resolvent (so they
> form a logical conjunction), you can imagine something like:
> 
> ?- get_posted_goals(G1), call(G1), get_posted_goals(G2), call(G2), ...
> 
> You can find some further discussion here
> http://eclipse-clp.org/archive/eclipse-users/0704.html
> http://eclipse-clp.org/doc/bips/kernel/externals/yield-2.html
> 
> 
> >
> > I also have a programming problem. I write this piece of code:
> >
> > ---
> > 	...
> >     ec_exec_string("compile('test.pl')",0);
> >
> >     post_goal(term(EC_functor("q",1),X));
> >     if (EC_resume(Start)==EC_succeed){
> >         ((EC_word)X).is_atom(&r1);
> >         cout << "succeed" << " X=" << r1.Name() << endl;
> >     }
> >     else cout << "failed q(X)" << endl;
> >
> >     post_goal(term(EC_functor("q", 2), X, Y));
> >     if (EC_resume(Start)==EC_succeed) {
> >       cout << "succeed";
> >       if (EC_succeed == ((EC_word)X).is_atom(&r1))
> >         cout << " X=" << r1.Name() << " ";
> >
> >       if (EC_succeed == ((EC_word)Y).is_atom(&r2)) {
> >         cout << " Y=" << r2.Name() << endl;
> >       }
> >       else cout << "bad Y " << endl;
> >     }
> >     else
> >         cout << "failed q(X,Y)" << endl;
> > 	...
> > ---
> > --- test.pl ---
> > q(a).
> > q(b).
> > q(b,c).
> > --- end of test.pl ---
> >
> > I got this output
> >
> > --
> > succeed X=a
> > succeed X=b bad Y
> > succeed X=b
> > succeed X=basf Y=c
> > ---
> >
> > In fact, I expect line 2 to be "succeed X=b Y=c". Any comments on
> > what's wrong about the program? Thank you.
> 
> In the following I have marked the "current execution point" with a ^.
> You first post p(X):
> 
> ?- p(X).
>     ^
> after resuming, you get the first solution X=a, so effectively the
> resolvent now looks like:
> 
> ?- p(a).
>         ^
> now you post q(a,Y):
> 
> ?- p(a), q(a,Y).
>           ^
> but after resuming, p(a,Y) fails, which means we backtrack to p(X)
> which succeeds with the second solution X=b:
> 
> ?- p(b).
>         ^
> But you see that the posted q(a,Y) goal was lost during backtracking
> because it was posted only conditionally for the first solution of
> p(X).
> 
> To achieve the result you expected, you have to post both goals
> together
> at the beginning, and only resume once.
> 
> 
> -- Joachim
> 
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