[rules-users] Reasoning over hierarchies.

[Mark Proctor]

you can do something like this to drive backward chaining queries. 
Notice it inserts a new fact to drive the query down the graph, but 
deletes that driver once all it's  children have been evaluated. It uses 
the -5 salience to ensure it's done after all the children have been 
evaluated. So just insert an initial FindAncestor fact call 
fireAllRules() and it'll println if it's an ancesor, you could ofcourse 
have it insert a result object, or set a value on the originally 
inserted FindAncestor.

rule "query parent"
  when
    FindAncestor( $name == name, $ancestor : ancestor )
    $child : Person(  name == $name, $parent : parent )
    $parent : Person( this == $parent, this != ancestor)
  then
    insert( new FindAncestor( $parent, $ancestor ) )
end
 
rule "retract query parent driver"
  salience - 5
  when
    $f : FindAncestor( $name == name, $ancestor : ancestor )
    $child : Person(  name == $name, $parent : parent )
    $parent : Person( this == $parent, this != ancestor)
  then
    retract( $f )
end
 
rule "ancestor found"
  when
    FindAncestor( $name == name, $ancestor : ancestor )
    $child : Person(  name == $name, $parent : parent )
    $parent : Person( this == $parent, this == ancestor)
  then
     println( "is ancestor" )
end
 
rule "retract query ancestor found driver"
  salience - 5
  when
    $f : FindAncestor( $name == name, $ancestor : ancestor )
    $child : Person(  name == $name, $parent : parent )
    $parent : Person( this == $parent, this == ancestor)
  then
    retract( $f )
end

Mark
Faron Dutton wrote:
> Thanks. I'll look into this.
>
> On Wed, Jan 21, 2009 at 9:46 AM, Anstis, Michael (M.) <manstis1 at ford.com> wrote:
>   
>> I wonder whether a custom accumulate function could be used to move your
>> logic out of the object model?
>>
>> Not that accumulate was (probably) meant for this sort of function but the
>> "init" method could create the empty set and "accumulate" perform your
>> iteration below?
>>
>> Just the musing of an idle mind.
>>
>> ________________________________
>> From: rules-users-bounces at lists.jboss.org
>> [mailto:rules-users-bounces at lists.jboss.org] On Behalf Of David Sinclair
>> Sent: 21 January 2009 14:28
>> To: Rules Users List
>> Subject: Re: [rules-users] Reasoning over hierarchies.
>>
>> You could have something along the lines
>>
>> class Person {
>>      Person parent;
>>
>>      Collection getAncestors() {
>>            Collection<Person> ancestors = new HashSet<Person>();
>>
>>             Person currentAncestor = parent;
>>
>>             while(currentAncestor != null) {
>>                    ancestors.add(currentAncestor);
>>                    currentAncestor = currentAncestor.getParent();
>>             }
>>      }
>> }
>>
>> query isAncestor(String a, String b)
>>    p: Person(name = a)
>>    c: Person(name = b, ancestors contains p)
>> end
>>
>> that should do it
>>
>> dave
>>
>> On Wed, Jan 21, 2009 at 8:54 AM, Faron Dutton <fgdutton at gmail.com> wrote:
>>     
>>> I know this has probably been asked before but I cannot find any mention
>>> of
>>> it. How does one reason over a transitive (recursive) relation in Drools?
>>>
>>> -----------------------------------------------------------
>>>
>>> The classic example from Prolog:
>>>
>>> -- The relation parent(P,C) says that P is a parent of C.
>>> parent(P,C).
>>>
>>> -- The predicate ancestor(A,B) implies that A is an ancestor
>>> -- of B if A is a parent of B or A is a parent of C and C
>>> -- is an ancestor of B.
>>> ancestor(A,B) :- parent(A,B).
>>> ancestor(A,B) :- parent(A,C), ancestor(C,B).
>>>
>>> -- The query ancestor(bob,frank) asks if bob is an ancestor
>>> -- of frank.
>>> ?- ancestor(bob,frank).
>>>
>>> -----------------------------------------------------------
>>>
>>> In Drools, I can find the parent using
>>>
>>> query isParent(String a, String b)
>>>    p: Person(name = a)
>>>    c: Person(name = b, parent = p)
>>> end
>>>
>>> likewise, I can find the grandparent using
>>>
>>> query isGrandparent(String a, String b)
>>>    g: Person(name = a)
>>>    p: Person(parent = g)
>>>    c: Person(name = b, parent = p)
>>> end
>>>
>>> I am unable to formulate the query isAncestor.
>>>
>>> _______________________________________________
>>> rules-users mailing list
>>> rules-users at lists.jboss.org
>>> https://lists.jboss.org/mailman/listinfo/rules-users
>>>       
>> _______________________________________________
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>>
>>
>>     
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>   

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