If this hook predicate succeeds it must instantiate the Action argument to the atom fail to make the operation fail silently, retry to tell Prolog to retry the operation or error to make the system generate an exception. The action retry only makes sense if this hook modified the environment such that the operation can now succeed without error.

undefined_predicate

Context is instantiated to a predicate indicator ([module]:<name>/<arity>). If the predicate fails, Prolog will generate an existence_error exception. The hook is intended to implement alternatives to the built-in autoloader, such as autoloading code from a database. Do not use this hook to suppress existence errors on predicates. See also unknown and section 2.14.

undefined_global_variable

Context is instantiated to the name of the missing global variable. The hook must call nb_setval/2 or b_setval/2 before returning with the action retry.