JESS vs DROOLS : Backward chaining

Question

I'm trying to replace Jess with Drools as backward chaining rule engine in our project. I was looking for simple examples about how backward chaining is done with Drools. Interestingly, there's only 1 same example on every site (which I don't get how it's BC but let's forget it for now).

Very trivial example of BC in Jess:

//q is a fact template with a slot named 'n'
//when there's a q with n==8 print something
//I need a q with n==8 to fire a rule so I will insert it myself!

(deftemplate q (slot n))
(do-backward-chaining q)
(defrule printq (q (n 8))   =>  (printout t "n is eight! yeah!" crlf))
(defrule iNeedn8 (need-q (n 8)) => (assert (q (n 8))))
(reset)
(run 1)
//fires printq and prints to console...

Equivalent in Drools:

package com.example;

declare Q
    n : int
end

rule "print q"
when
    Q(n == 8)
then
    System.out.println("n is eight by drools!");
end

//I'M LOST HERE! HELP!

How can I achieve same behaviour with Drools?

Answer 1

In Drools, the general idea of BC is to use queries. In addition to your rule "print q" you need:

query noQ( int $num )
    Goal(num==$num) and not Q(num == $num)
end

rule goal when
    Goal( $n: num )
    noQ($n;)
then
    Q q = new Q($n);
    insert( q );
end

rule go when
then
    insert( new Goal( 8 ) );
end

There is no way to instruct Drools to detect the missing fact all by itself; you have to provide the goal and queries to "bridge the gap".

Answer 2

Inspired from the Jess feature, there is an experimental, in-development feature that gives you a similar behavior. Here is how a test would look like:

@Test
public void testFindQ8() {
    String droolsSource =
        " package org.drools.abductive.test;                 " +
        "                                                    " +
        " import " + Abducible.class.getName() + ";          " +
        " global java.util.List list;                     \n " +
        "                                                    " +
        " declare Q                                          " +
        "    @Abducible                                      " +
        "    id : int @key                                   " +
        " end                                             \n " +
        "                                                    " +
        " query foo( int $x )                                " +
        "    @Abductive( target=Q.class )                    " +
        "    not Q( $x; )                                    " +
        " end                                             \n " +
        "                                                    " +
        " rule R1                                            " +
        " when                                               " +
        "    $x := foo( 8 ; )                                " +
        " then                                               " +
        "    System.out.println( 'R1 returned ' + $x );      " +
        " end                                             \n " +
        "                                                    " +
        " rule R2                                            " +
        " when                                               " +
        "    $q : Q( 8; )                                    " +
        " then                                               " +
        "    System.out.println( 'We have 8!' );             " +
        " end                                                ";
    /////////////////////////////////////

    KieHelper kieHelper = new KieHelper();
    kieHelper.addContent( droolsSource, ResourceType.DRL ).build().newKieSession().fireAllRules();


}