Re: SWI Prolog forgets identity of query variables in CHR ???

Hello Uwe,

What I want is this:
> ?- c(X,Y).
> c(X,Y)

which is the corrct solution.

This one is wrong:

> ?- c(X,Y).
> c(_G30729,_G30730)


because it answers with a statement about two "local" variables fifferent from my "local", i.e. X and Y.

This is a minimalist example, of course, but when you want to derive more elaborate relationships among you query variables, it is a big problem.

I found a version of SICStus Prolog that uses the same CHR compiler as SWI, and it works the way I want, namely that my query variables X and Y are recognizable.


med venlig hilsen / best wishes
    Henning



On 26 Feb 2014, at 14:56, Uwe Lesta <[email protected]> wrote:

> Hello Henning,
> 
> What did you expect when queering  ?- c(X,Y). ?
> 
> Remember CHR select a constraint by matching the not by unification.
> If unification is needed the CHR constraint has to be
> 	c(X,Animal) <=> Animal = cow | d(X,pig).
> 
> Hope this helps
> 
> Uwe
> 
> 
>> -----Original Message-----
>> From: [email protected]
>> bonn.de [mailto:swi-prolog-bounces+lesta=sbs-
>> [email protected]] On Behalf Of Henning
>> Christiansen
>> Sent: Tuesday, February 25, 2014 12:43 PM
>> To: [email protected]
>> Subject: [SWIPL] SWI Prolog forgets identity of query variables in CHR
>> ???
>> 
>> Hi SWIPL team,
>> 
>> I observed strange problem that the identity of variables in CHR
>> queries are sometimes forgotten when the resylt is printed out.
>> I could isolate the problem into a one rule program.
>> For the two test queries, no rule is applied;
>> the first one behaves as you would expect, but in the second one, my
>> query variables gets renamed and thus the semantics of the answer is
>> wrong.
>> 
>> If I have a program with no rules, no misbehaviour is observed
>> 
>> (running on Mac OS 10.9.1)
>> 
>> best wishes
>> 
>>     Henning
>> 
>> 
>> The program:
>> 
>> :- use_module(library(chr)).
>> :- chr_constraint c/2, d/2.
>> c(X,cow) <=> d(X,pig).
>> 
>> Queries:
>> $ swipl
>> Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 6.6.1-DIRTY)
>> ....
>> 
>> ?- [program].
>> %  library(chr) compiled into chr 0.51 sec, 6,504 clauses
>> % program compiled 0.51 sec, 6,551 clauses
>> true.
>> 
>> ?- c(X,duck).
>> c(X,duck)
>> true .
>> c(X,duck)
>> 
>> ?- c(X,Y).
>> c(_G30729,_G30730)
>> true .
>> c(_G30729,_G30730)
>> 
>> 
>> 
>> 
>> 
>> Henning Christiansen
>> Professor of  Computer Science, PhD
>> Leader of Research group PLIS: Programming, Logic and Intelligent
>> Systems
>> Department of Communication, Business and Information Technologies
>> Roskilde University
>> P.O.Box 260, DK-4000 Roskilde, DENMARK
>> http://www.ruc.dk/~henning
>> 
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