[rules-users] Reasoning over hierarchies.

[David Sinclair]

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.
>
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>
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