[rules-users] Reasoning over hierarchies.

[Anstis, Michael (M.)]

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.
		
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