Hello, I have a CHR as follows: :- use_module(library(chr)). :- chr_constraint try/1. try(X), try(Y) <=> X == Y | writeln(succeed). then I declared a constraint, but got an unexpected result: ?- try(A). succeed try(A). The unexpected succeed seems to be because Prolog matches try(A) to both try…

See A strange behavior of CHR package However, if you only have grounded constraints, then you should be okay. try(X) by itself doesn’t make sense (at least to me) as a constraint.

CHR is not in active development. It is open source and could be fixed, but i dont know any experts on the internals are around to do so. Seems like something i would have to tackle if i got back into chr, but its not presently my focus.

I was scratching my head why this could be unexpected. I haven’t touched CLIPS for a while but I’m quite confident that it behaves the same. I’m 99% sure that this is the same behavior Soufflé uses too. It also aligns with my understading of RETE rule engines. For me the dissection is as follows: ?…

Thanks @xlii , that explains a lot! Now that I understand the mechanism and can adjust the expectation of how it behaves. However, the actual problem remains unsolved. Essentially the CHR guard needed here is to compare any two separate try/1 facts, iff there are at least two separate try/1 facts. I…

In addition to the last message, I’ve found something interesting. We can get a different result, if we disable the printout of CHR store: :- use_module(library(chr)). :- chr_constraint try/1. :- set_prolog_flag(chr_toplevel_show_store, false). try(X), try(Y) <=> X == Y | writeln(succeed). Then…

Keep in mind, you don’t have a duplicate facts, i.e. there’s no: cat(black). cat(black). only cat(black). So you need to be explicit that these facts are, in fact, separate, so: cat(A, Color), cat(B, Color) <=> A #= B | cat_pair(A,B,Color). Give facts identifier of some sort. In this case you…

[image] AZH: We can get a different result, if we disable the printout of CHR store: This might be actually confusing. I’m not sure what’s the phase of the printout (empty store wouldn’t make any printout, I suspect this is after-fact before-rule). Would have to check the docs. Other explan…

Thanks @xlii , assigning each fact a unique identifier is a solid solution!

CHR will have a rule like a(X) \ a(X) <=> true to eliminate duplicates. If that fired when you only have a single a(4), it would be bad. Efficiency note: it doesn’t matter for toy problems, but at scale, with optimizations on, the optimizations including hashing on integers. So using _vars is con…

Tricky CHR behaviour

Help!

AZH June 5, 2026, 11:17am 3

Thanks Ian!
From the post your shared, it is a pretty old issue.
Hope this will be fixed, sooner rather than later…

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Tricky CHR behaviour

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