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 :- Goal construct calls Goal only once, when the file is compiled, not when a saved state containing the file is restored.
• Goal is called at compile-time, which means that if you use qpc to compile source code into a qof-file, your directives will be run during this compilation, not when you load the qof-file or start up a stand-alone system to which the qof-file has been linked.

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.

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

• a source file with a :- initialization Goal directive is loaded into a running system (using compile, consult etc.)
• a stand-alone program or runtime system is started up, and some file linked to the system had a :- initialization Goal directive. If several files had such directives, the order in which the goals are run is not defined.
• a saved state is restored, and some file loaded to the saved system had a :- initialization Goal directive, or initialization(Goal) was called before the state was saved. If several initialization goals were defined in the system, the order in which they are run upon a restore is not defined.
• a QOF file is loaded into a running system, and the source file that was compiled into the qof file had a (:- initialization Goal) directive.