[Xsb-development] storage module and backtracking

[john D. <j0h...@gm...>]

I'm trying to use global variables with backtracking to capture and print
the steps of the derivation.  The example program at the end of this message
demonstrates the issue I'm having.

If I load the program in SWI Prolog, then the output is as follows.

[SWI transcript]
?- init.
Yes
?- happy(bob).
Yes
?- output.
unhappy(dan) => happy(bob)
Yes

During execution, step(happy(alice, carol)) is also stored in the global
variable, but it is removed during backtracking, as per the semantics of the
SWI-Prolog b_setval/2 function.

Now, I try to use the storage module to accomplish the same effect in XSB
Prolog.  In the documentation, it appears to have the same
backtracking/nonbacktracking options, but the backtracking isn't working the
same way.

I define compatibility functions to map the SWI getval/setval functions to
XSB storage, but I get the following transcript, which indicates that the
irrelevant step(happy(alice,carol)) was not removed during backtracking.  Is
this a bug in the module implementation or a flaw in my expectation, and if
it's the latter, how I can I accomplish what I want?

[XSB transcript]
| ?- init.
yes
| ?- happy(bob).
yes
| ?- output.
happy(alice) => happy(carol)
unhappy(dan) => happy(bob)
yes

-------------------------
% for XSB-Prolog only
:- import storage_delete_keypair/3, storage_insert_keypair/4,
          storage_delete_keypair_bt/3, storage_insert_keypair_bt/4,
          storage_find_keypair/3 from storage.

nb_setval(K, V) :- storage_delete_keypair(global, K, _),
  storage_insert_keypair(global, K, V, _).
b_setval(K, V) :- storage_delete_keypair_bt(global, K, _),
  storage_insert_keypair_bt(global, K, V, _).
nb_getval(K, V) :- storage_find_keypair(global, K, V).
b_getval(K, V) :- storage_find_keypair(global, K, V).
% end of XSB-Prolog functions

happy(alice).
friend(carol, alice).
enemy(bob, carol).
enemy(bob, dan).

happy(X) :- friend(X,Y), happy(Y),
  b_getval(out, R), b_setval(out, [step(happy(Y),happy(X)) | R]),
    b_getval(out, Q), nb_setval(out2, Q).
happy(X) :- enemy(X,Y), \+(happy(Y)),
  b_getval(out, R), b_setval(out, [step(unhappy(Y),happy(X)) | R]),
    b_getval(out, Q), nb_setval(out2, Q).


init :- nb_setval(out, []).

output([]).
output([step(happy(Y),happy(X)) | R]) :- output(R),
  write(happy(Y)), write(' => '), writeln(happy(X)).
output([step(unhappy(Y),happy(X)) | R]) :- output(R),
  write(unhappy(Y)), write(' => '), writeln(happy(X)).

output :- nb_getval(out2, R), output(R).