built-in predicate



threaded_exit(Goal, Tag)

Retrieves the result of proving Goal in a new thread. This predicate blocks execution until the reply is sent to the this message queue by the thread executing the goal. When there is no thread proving the goal, the predicate generates an exception. This predicate is non-deterministic, providing access to any alternative solutions of its argument.

The argument of this predicate should be a variant of the argument of the corresponding threaded_call/1 or threaded_once/1 call. When the predicate argument is subsumed by the threaded_call/1 or threaded_once/1 call argument, the threaded_exit/1 call will succeed iff its argument is a solution of the (more general) goal.

The threaded_exit/2 variant accepts a threaded call identifier tag generated by the calls to the threaded_call/2 and threaded_once/2 predicates. Tags shall be regarded as an opaque term; users shall not rely on its type.


This predicate requires a backend Prolog compiler providing compatible multi-threading primitives. The value of the read-only threads flag is set to supported when that is the case.

Modes and number of proofs

threaded_exit(+callable) - zero_or_more
threaded_exit(+callable, +nonvar) - zero_or_more


Goal is a variable:
Goal is neither a variable nor a callable term:
type_error(callable, Goal)
No thread is running for proving Goal in the Object calling context:
existence_error(thread, Object)
Tag is a variable:


To retrieve an asynchronous goal proof result:
To retrieve an asynchronous message to self result:
To retrieve an asynchronous message result: