The built-in predicates in Strawberry Prolog provide a one-step way to implement things that are either impossible or just too inefficient to program in the language itself.

Side effects

The built-in predicates in Strawberry Prolog also provide the facility to program actions other than just the working of the interpreter as a database engine. These actions have effect on the outer world, relative to Strawberry Prolog, and are not subject to 'undoing' in the way, say, variable instantiation is. That is why built-in predicates are often said causing side effects .