built-in predicate




Returns an answer independent of the solutions of the engine goal. Fails if not called from within an engine. This predicate is usually used when the engine goal is a call to a recursive predicate processing terms from the engine term queue.

This predicate blocks until the returned answer is consumed.

Note that this predicate should not be called as the last element of a conjunction resulting in an engine goal solution as, in this case, an answer will always be returned. For example, instead of (threaded_engine_yield(ready); member(X,[1,2,3])) use (X=ready; member(X,[1,2,3])).


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

Modes and number of proofs

threaded_engine_yield(@term) - zero_or_one




% returns the atom "ready" as an engine answer:
..., threaded_engine_yield(ready), ...