Re: Is there a way to determine whether a call has been tabled?

Hi Brian,

Currently, from the user point of view, there is no way to check if a
tabled call is already tabled or not. The tabling engine automatically
detects when a call is already completed and in such cases simply
consumes answers from the table space without executing the clauses.

For what I understand, the behaviour you want to implement, can be
solved, for instance, by using a global variable in the tabled code.
Something like that:

fib(_,_) :- set_value(fib_executing_clauses,1).
fib(1, 1) :- ...
fib(X, Y) :- ...

call_fib(X, Y) :- set_value(fib_executing_clauses,0),
                 fib(X, Y),
                 get_value(fib_executing_clauses,T),
                 (T = 0 -> write('Tabled success'), nl ; true).

Ricardo Rocha

> Howdy all,
>
> I'm using tabling in my Prolog program, and when calling a tabled
> predicate, I would like to determine whether the call I'm about to
> make has been tabled already or not. Is there a way to query the
> predicate's table before (or after) making the call so I can determine
> whether the call will succeed via a tabled answer or because the
> predicate itself was executed?
>
> My situation is more complex than this example, but it will suffice to
> describe the behavior I would like:
>
> :- table fib/2.
>
> fib(1, 1) :- write('Base case'), nl.
> fib(X, Y) :- X1 is X - 1, X2 is X - 2, fib(X1, Y1), fib(X2, Y2), Y is
> Y1 + Y2, write('Inductive case'), nl.
>
> call_fib(X, Y) :- fib(X, Y), (was_tabled(fib(X, Y)) -> write('Tabled
> success'), nl ; true /* do nothing */).
>
>
> Regardless of whether fib is actually executed, I still need to
> execute the call to write, so I would like a way to determine whether
> a call succeeds / succeeded because it was already in the table or
> because the predicate was actually executed.
>
> I do realize that the general principle is "actions taken inside
> tabled predicates can't be expected to always be executed, so actions
> that must always be taken should not be placed in tabled predicates."
> Unfortunately, in my case, the required actions are essentially
> (non-I/O) data processing instructions that cannot be removed from the
> tabled predicate but nevertheless must be executed every time the
> tabled predicate is called, regardless of whether it succeeds or not.
>
> Is there a way to do this?
>
> Thanks,
> ~Brian W. DeVries

> ------------------------------------------------------------------------------
> Open Source Business Conference (OSBC), March 24-25, 2009, San Francisco, CA
> -OSBC tackles the biggest issue in open source: Open Sourcing the Enterprise
> -Strategies to boost innovation and cut costs with open source participation
> -Receive a $600 discount off the registration fee with the source code: SFAD
> http://p.sf.net/sfu/XcvMzF8H
> _______________________________________________ Yap-users mailing list Yap-users@... https://lists.sourceforge.net/lists/listinfo/yap-users