A strange behavior of CHR package

Help! Pack
1

This is a further discussion about the issue on Github.

For the following program

%% test.pl 
:- use_module(library(chr)).
:- chr_constraint c/2.

c(K,_I), c(K,_J) <=> writeln('rule 1 fired').
c(_I,K), c(_J,K) <=> writeln('rule 2 fired').

?- consult(["test.pl"]).
true.

?- c(X,Y).
rule 2 fired
c($VAR(X),$VAR(Y)).

Although the 2nd rule has two heads and I only added one constraint c(X,Y), the rule still fired (?). This should be considered an unexpected behavior.

The expected behavior is

?- c(X,Y).
c($VAR(X),$VAR(Y)).

The “rule 2 fired” should not be printed, because there is only one constraint. It could matches the 1st constraint at head, but no partner constraint in the store.

Thanks.

2

Note that apparently the behaviour changed from v7.2 to v7.3 of SWI-Prolog. v7.2:

> swivm use 7.2
Now using SWI-Prolog v7.2.3
> swipl -s test.pl
Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 7.2.3)
Copyright (c) 1990-2015 University of Amsterdam, VU Amsterdam
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to redistribute it under certain conditions.
Please visit http://www.swi-prolog.org for details.

For help, use ?- help(Topic). or ?- apropos(Word).

?- c(X,Y).
c(_G39140,_G39150)
true .
c(_G39140,_G39150)

v7.3:

> swivm use 7.3
Now using SWI-Prolog v7.3.35
> swipl -s test.pl
Welcome to SWI-Prolog (threaded, 64 bits, version 7.3.35)
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.
Please run ?- license. for legal details.

For online help and background, visit http://www.swi-prolog.org
For built-in help, use ?- help(Topic). or ?- apropos(Word).

1 ?- c(X,Y).
rule 2 fired
c(X, Y).
3 Likes
3

There are quite a few changes in CHR between these two versions as well. Unfortunately the build system is different which makes bisecting non-trivial :frowning:

1 Like
4

Did Leuven make major changes? I guess the 2 directions to find the cause are
a) put their changes into the previous SWI build
b) check the newest SWI with the previous CHR code.