You will have to use regular insert() instead of insertLogical(). This is because:

 

rule "reachDirect"
   salience 10
   when
       e : Edge(s1 : source, t1 : target)
       not( Reach(source == s1, target == t1) )
   then
       insertLogical( new Reach(e.getSource(),e.getTarget()) );
       System.out.println( "Reach " + e.getSource() + "," + e.getTarget() );
end
 

   Is initially true, but once the consequence is executed, the insert(new Reach()) will cause the pattern "not(Reach(...))" to become false and so the object is retracted, causing the recursion.

 

   []s

   Edson

 

2008/7/3, Paul Fodor <paul.i.fodor@gmail.com>:

On Thu, Jul 3, 2008 at 3:56 PM, Paul Fodor <paul.i.fodor@gmail.com> wrote:
> Dear Sir,
>
> I am new to Drools and I want to ask how can I implement the classical
> transitive closure in Drools. For instance we have a bunch of facts
> edge/2 and the transitive closure:
>
> reach(X,Y):- edge(X,Y).
> reach(X,Y):- edge(X,Z),reach(Z,Y).

My current version looks like the following, but it never terminates.
It re-derives reach(c,c) forever.

TransitiveClosure.drl :
package org.drools.examples

import org.drools.examples.TransitiveClosureExample.Edge;
import org.drools.examples.TransitiveClosureExample.Reach;

rule "reachDirect"
   salience 10
   when
       e : Edge(s1 : source, t1 : target)
       not( Reach(source == s1, target == t1) )
   then
       insertLogical( new Reach(e.getSource(),e.getTarget()) );
       System.out.println( "Reach " + e.getSource() + "," + e.getTarget() );
end

rule "reachIndirect"
   salience 10
   when
       e : Edge(s1 : source, t1 : target)
       r : Reach(t2 : target, source == t1 )
       not( Reach(source == s1, target == t2) )
   then
       insertLogical( new Reach(e.getSource(),r.getTarget()) );
       System.out.println( "Reach " + e.getSource() + "," + r.getTarget() );
end

TransitiveClosureExample.java :

package org.drools.examples;
import java.io.InputStreamReader;
import org.drools.RuleBase;
import org.drools.RuleBaseFactory;
import org.drools.StatefulSession;
import org.drools.audit.WorkingMemoryFileLogger;
import org.drools.compiler.PackageBuilder;
import org.drools.compiler.PackageBuilderConfiguration;

public class TransitiveClosureExample {
   public static void main(final String[] args) throws Exception {
       PackageBuilderConfiguration conf = new PackageBuilderConfiguration();
       final PackageBuilder builder = new PackageBuilder( conf );
       builder.addPackageFromDrl( new InputStreamReader(
TransitiveClosureExample.class.getResourceAsStream(
"TransitiveClosure.drl" ) ) );
       final RuleBase ruleBase = RuleBaseFactory.newRuleBase();
       ruleBase.addPackage( builder.getPackage() );
       final StatefulSession session = ruleBase.newStatefulSession();
       final WorkingMemoryFileLogger logger = new
WorkingMemoryFileLogger( session );
       logger.setFileName( "log/transitiveClosure" );

       final Edge edgeAB = new Edge( "a","b" );
       final Edge edgeBC = new Edge( "b","c" );
       final Edge edgeCD = new Edge( "c","d" );

       session.insert( edgeAB );
       session.insert( edgeBC );
       session.insert( edgeCD );

       session.fireAllRules();
       logger.writeToDisk();
       session.dispose();
   }

   public static class Edge {
       private String source;
       private String target;
       public Edge() {}
       public Edge(String source, String target) {
           super();
           this.source = source;
           this.target = target;
       }
       public String getSource() {
           return source;
       }
       public String getTarget() {
           return source;
       }
   }

   public static class Reach {
       private String source;
       private String target;
       public Reach() {}
       public Reach(String source, String target) {
           super();
           this.source = source;
           this.target = target;
       }
       public void setSource(String source) {
           this.source = source;
       }
       public String getSource() {
           return source;
       }
       public void setTarget(String target) {
           this.target = target;
       }
       public String getTarget() {
           return target;
       }
   }
}

Any sugestion?

Regards,
Paul Fodor
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