Blackboard and attributes on a naked variable

[infradig]

This works...

$ swipl
Welcome to SWI-Prolog (threaded, 64 bits, version 9.3.21-90-ge96c60c-DIRTY)
?- freeze(A,writeln(here)), nb_setval(abc, f(A)).
freeze(A, writeln(here)).
?- b_getval(abc,A).
A = f(_A),
freeze(_A, writeln(here)).
?- b_getval(abc,A), A=f(123).
here
A = f(123).

but this doesn't...

?- freeze(A,writeln(here)), nb_setval(abc, A).
freeze(A, writeln(here)).
?- b_getval(abc,A).
freeze(A, writeln(_)).
?- b_getval(abc,A), A=123.
_2576
A = 123.

[JanWielemaker]

The toplevel loop is a backtracking loop, so all backtrackable actions are lost between queries. You can set the Prolog flag toplevel_mode to recursive to turn the toplevel into a recursive loop.

[infradig]

Sorry, I meant to use non-backtrackable, pasted the wrong text...

?- freeze(A,writeln(here)), nb_setval(abc, f(A)).
freeze(A, writeln(here)).
?- nb_getval(abc,A), A=f(123).
here
A = f(123).

?- freeze(A,writeln(here)), nb_setval(abc, A).
freeze(A, writeln(here)).
?- nb_getval(abc,A), A=123.
_6760
A = 123.
?- 

[JanWielemaker]

There is indeed something wrong:

?- freeze(A,writeln(here)), nb_setval(abc, A).
freeze(A, writeln(here)).

?- nb_getval(abc, X), get_attrs(X, L).
L = att(freeze, user:writeln(_A), []),
freeze(X, writeln(_A)).