once/1 cuts caller choice points inconsistently across Prolog systems

Direct answer

Once/1 is intended to suppress choice points, but its interaction with the caller's context varies across Prolog implementations. In SWI-Prolog 9.x, once/1 acts as a hard cut that discards all choice points created inside the wrapped goal, but it does not remove choice points that existed before the call. In YAP 6.2.2, once/1 uses a softer cut that only eliminates choice points generated inside the goal, leaving pre-existing caller choice points intact. This can cause duplicate writes when the predicate backtracks. The ISO core specification treats once/1 as a goal-local cut; conforming implementations may differ on caller-level impact, so portable code should not rely on once/1 alone for choice-point suppression. A portable pattern to guarantee a single write without sacrificing needed backtracking uses a flag variable with a single-assignment guard:

( write_once(Flag) :- ( nonvar(Flag) -> true ; assert(Flag=1), write(...) ) ).

This pattern checks whether Flag has already been assigned; if not, it asserts the flag and performs the write, otherwise it skips. It guarantees a single write without eliminating the caller's legitimate backtracking points. Verification: compile a minimal test asserting p inside a predicate with once/1 and observe choice-point retention via (current_choice_points/2) or similar debugging hooks. Run the same test under SWI-Prolog 9.x and YAP 6.2.2 and compare choice-point counts. Note: version-specific behavior means the exact choice-point outcome may vary; testing on the target Prolog implementation is recommended before deployment. If you need a recommendation tailored to your system, please specify the Prolog implementation and whether the write appears inside a loop or recursive clause, as that context determines whether once/1 or a flag guard is the safer choice.