[rules-users] Reasoning over hierarchies.
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?
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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).
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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.