The Initialization Declaration

The initialization/1 predicate is an important complement to the embedded directive construct :- Goal appearing in a file being consulted or compiled, and can in many cases not only replace the directive, but also make the code work better when used in stand-alone programs and runtime systems.

The main reason for this is that :- Goal directive is executed at compile-time, not when the file in which the construct occurs is actually loaded into a running system. This causes no problems within development systems, but if we want to save states and compile programs into qof-files, link them together, and later start them up again, problems arise because:

The initialization/1 predicate, on the other hand, provides a way of letting initialization routines be called when a file is actually loaded or a system containing the file is started up. This allows for correct initialization in stand-alone programs and runtime systems; therefore a recommended programming style is to use initialization/1 instead of a bare :- Goal construct whenever appropriate.

In the following figure, Goal_1 might typically be an operator declaration and Goal_2, an initialization predicate that modifies the database.


Embedded directives (goal_1) vs. initialized goals (goal_2)

The initialization goal, Goal_2, is defined to be run when: