# Undecidable order (some cyclic terms) **URL:** https://swi-prolog.discourse.group/t/undecidable-order-some-cyclic-terms/9833 **Category:** Request For Comments **Created:** [September 23, 2026, 12:56pm UTC](https://swi-prolog.discourse.group/t/undecidable-order-some-cyclic-terms/9833 "2026-09-23T12:56:20Z") **Posts on this page:** 1 **Page:** 1
### Author: ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan) #### Post date: [September 23, 2026, 12:56pm UTC](https://swi-prolog.discourse.group/t/undecidable-order-some-cyclic-terms/9833/1 "2026-09-23T12:56:20Z")
A reported bug on standard order for cyclic terms (#1529) made it free to detect cyclic terms that have no defined order. As a result, SWI-Prolog now has a flag `incomparable` that can be set to `arbitrary` (default, old behaviour, i.e., different representation leads to different results and transitivity is not guaranteed) and `error`, raising `representation_error(standard_order(Sub1, Sub2))`, where the culprits are the cycles that cause the problem. So, now we get ```prolog 102 ?- set_prolog_flag(incomparable, error). true. 103 ?- X=f(X,0),Y=f(Y,1), sort([X,Y], L). ERROR: Cyclic terms have no standard order: @(S_1,[S_1=f(S_1,0)]) and @(S_1,[S_1=f(S_1,1)]) ERROR: In: ERROR: [16] sort([f(...,0),...],_280) ERROR: [14] toplevel_call(user:user: ...) at /home/jan/src/swipl-devel/build/home/boot/toplevel.pl:1678 ERROR: ERROR: Note: some frames are missing due to last-call optimization. ERROR: Re-run your program in debug mode (:- debug.) to get more detail. ``` In addition, the new partial\_compare/3 gives (regardless of the flag): ```prolog 109 ?- X=f(X,0),Y=f(Y,1), partial_compare(Diff, X, Y). X = f(X, 0), Y = f(Y, 1), Diff = incomparable(f(X, 0), f(Y, 1)). ```