Re: Getting different results than Schrijvers

I took this to the CHR mailing list and found the answer.

First, the weird choice points thing is a result of, for some reason,
having chr_option(debug, on) be the default in SWI-Prolog.  This is not
true for YAP (the only other Prolog I have installed which had CHR baked
in).  There was also some surprise expressed in the CHR mailing list that
this was the default.  That being said, adding this line of code to the
neq.pl file solved that particular issue:

:- chr_option(debug,off).

The other one is a case of bad code.

neq(X,Y) <=> X \= Y | true.

This breaks one of the requirements of CHR: variables cannot be bound in
the guard.  Since X \= Y is \+ X = Y, the runtime is screwing up because
there is an attempt to unify.  The solution is to either use unifiable/3
like this:

neq(X, Y) <=> unifiable(X, Y, _) | true.

Or to explicitly check for instantiation:

neq(X, Y) <=> ground(X), ground(Y) | X \= Y.



On 31 March 2014 01:01, Anne Ogborn <[email protected]> wrote:

> This example by Schrijvers (slide 94 of his presentation online) :-
> chr_constraint neq/2. neq(X,X) <=> fail.
> neq(X,Y) <=> X \= Y | true. ?- neq(a,a).
> No
> ?- neq(a,b).
> Yes
> ?- neq(A,B).
> neq(A,B) However in  SWI-Prolog (Multi-threaded, 64 bits, Version
> 7.1.9-11-g3c17aed) 2 ?- neq(A,B).
> false.
>
> Anybody know whats going on?
> _______________________________________________
> SWI-Prolog mailing list
> [email protected]
> https://lists.iai.uni-bonn.de/mailman/listinfo.cgi/swi-prolog
>



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