## B.6 Hooks using the exception predicate

This section describes the predicate exception/3, which can be defined by the user in the module `user` as a multifile predicate. Unlike the name suggests, this is actually a hook predicate that has no relation to Prolog exceptions as defined by the ISO predicates catch/3 and throw/1.

The predicate exception/3 is called by the kernel on a couple of events, allowing the user to `fix' errors just-in-time. The mechanism allows for lazy creation of objects such as predicates.

exception(+Exception, +Context, -Action)
  Dynamic predicate, normally not defined. Called by the Prolog system on run-time exceptions that can be repaired `just-in-time'. The values for Exception are described below. See also catch/3 and throw/1.

  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 2.13.
  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`.