[Info-gnuprologjava] Issue in dynamic predicates

[Aseem Rastogi]

Hi All:

I am using GNU Prolog for Java to solve a system of constraints. The algorithm 
starts with a bunch of constraints and then adds more constraints until it 
reaches a fixed point and then computes some solutions.

In my program, I represent the constraints using a dynamic predicate coerce/2. 
To add new constraints, I use asserta/1. I am facing a problem where, an assert 
doesn't seem to be taking effect. I am using 0.2.6 release of gnuprolog for 
java. My (relevant) code and debug messages are as follows:

Code:
-------

:- dynamic(coerce/2).

add_coercion(X, Y, Flag) :-
        write('Adding coercion from '), write(X), write(' to '), write(Y), nl,
        (
            coerce(X, Y)
            -> write('Exists, failing'), nl, fail
            ;  write('Did not exist, adding ...'), nl, asserta(coerce(X, Y)), 
write('Done with assert'), nl,
               Flag = n,
               true
        ).

reduce_fun3(X, Y, Flag) :-
        ....
        write(T), write(' to '), write(Y), nl,
        add_coercion(T, Y, Flag).

The code of add_coercion checks if coerce(X, Y) exists already. If it does, the 
predicate fails otherwise it asserts the predicate. reduce_fun3 calls 
add_coercion.

The debug messages are as follows:

int to x16
Adding coercion from int to x16
Did not exist, adding ...
Done with assert


int to x16
Adding coercion from int to x16
Did not exist, adding ...
Done with assert


int to x16
Adding coercion from int to x16
Did not exist, adding ...
Done with assert

As you can see, I keep getting the same messages again and again which means 
the assert is not taking effect.

Am I doing something wrong or is this a bug in GNU Prolog code ? Please let me 
know.

Thanks in advance,

-Aseem.