swi-prolog/packages-chr

CHR: Leuven Constraint Handling Rules

packages-chr's Issues

Rule firing although guard should fail

I am using the 32-bit-Version of SWI-Prolog 8.2.0 on Windows 10 64-bit.

In the attached file, testchr.txt (rename to .pl before running), running testit should lead to none of the CHR rules to be executed.

testit adds the constraint c2(_Addr) to the store. For c2_rule to fire, c1(Dest) must fail. The expected behaviour is for c1(Dest) not to fail, as Dest is unified with _Addr from the testit-call, which is a variable and not an integer. Thus, the expected behaviour is for c2_rule not to fire.

However, when running testit, we get the following output:

>swipl
Welcome to SWI-Prolog (threaded, 32 bits, version 8.2.0)
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.
Please run ?- license. for legal details.

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

1 ?- use_module(testchr).
true.

2 ?- testit.
c2_rule(_97180)
true.

3 ?-

The output c2_rule indicates that the rule c2_rule was indeed executed.

The example comes from a much larger CHR code-base, but has been distilled down to the minimal example. As soon as the rule c1_rule is removed, the system behaves as expected.

I have also attached an execution trace in testchr_trace.txt.

Some warnings in the test suite

I don't know whether these are expected or not.

In SWI Prolog 8.3.5, running the test suite and grepping for WARN in the file build/Testing/Temporary/LastTest.log:

CHR test suite.  To run all tests run ?- test.

Running scripts from Tests  swipl-devel_original/packages/chr/Tests/dense_int.chr
. swipl-devel_original/packages/chr/Tests/fibonacci.chr
. swipl-devel_original/packages/chr/Tests/leq.chr
. swipl-devel_original/packages/chr/Tests/passive_check2.chr
. swipl-devel_original/packages/chr/Tests/passive_check.chr
. swipl-devel_original/packages/chr/Tests/primes.chr
================================================================================
CHR compiler WARNING:
    `--> Heads will never match or guard will always fail in rule number 4 at swipl-devel_original/packages/chr/Tests/primes.chr:24.
        This rule will never fire!
================================================================================
================================================================================
CHR compiler WARNING:
    `--> All heads passive in rule number 4 at swipl-devel_original/packages/chr/Tests/primes.chr:24.
        This rule never fires. Please check your program.
================================================================================
. swipl-devel_original/packages/chr/Tests/trigger_no_active_occurrence.chr
. swipl-devel_original/packages/chr/Tests/zebra.chr
. done

All tests passed

Unexpected rule fired.

Hi,

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 issue can be reproduced in Stable release - SWI-Prolog 8.4.1-1 for Microsoft Windows (64 bit), and Daily builds - swipl-w64-2022-01-31.exe (version 8.5.6-26-gee0b3fea1).

N.B. swish has no the same problem, but the constraint are not preserved, see https://swish.swi-prolog.org/p/unexpected_rule_fired.pl

Thanks.

The line number of "Undeclared constraint" is wrong when using `include`

The reproduce steps as follows.

1. Create two files.

main.pl

:- debug.
:- use_module(library(chr)).
:- chr_constraint c/1.
:- include('test.pl').

test.pl

test0 :- true.
test :- do_test.







c(X,Y,Z) <=> true.  %% line number 10

2. In REPL

?- consult(["main.pl"]).
================================================================================
CHR compiler ERROR: invalid syntax "c(_78356,_78358,_78360)".
    `--> Undeclared constraint _78370/_78372 in head of rule number 1 at e:/work-pl/prolog/code/include/main.pl:10.
	Constraint should be one of [c/1].
================================================================================
true.

Notice that the compiling error shows the wrong location main.pl:10. It is should be in test.pl:10.

Feature Request: Persistent constraint store in top-level

As already discussed in pull request #2 it is a common request to have a persistent constraint store in SWI's top-level. My suggestion was to have a new chr_option(persistent_store, OffOn). Unfortunately it is not enough to simply prevent the top-level's backtracking by changing the used b_setval into nb_setval for saving constraints.

@JanWielemaker already proposed some thoughts on this in #2:

If we want an incremental toplevel for CHR it might be better to see whether the normal Prolog toplevel could be changed from backtracking to a recursive loop. That is surely possible. Also there I'm not sure it is the right direction. I tend to prefer the SWISH toplevel which gives you a completely fresh environment for each query.

I am willing in implementing an alternative approach again, but I am not sure if the requested behaviour is ok for Jan and which would be the best starting point for implementation. Let's use this issue to track this down :)