13.3 State initialization

The initialization/1 and initialization/2 directive may be used to register goals to be executed at various points in the life cycle of an executable. Alternatively, one may consider lazy initialization which typically follows the pattern below. Single threaded code can avoid using with_mutex/2.

:- dynamic x_done/0.
:- volatile x_done/0.

x(X) :-
    x_done,
    !,
    use_x(X).
x(X) :-
    with_mutex(x, create_x),
    use_x(X).

create_x :-
    x_done,
    !.
create_x :-
    <create x>
    asserta(x_done).