9:59 p.m.
Hi Wolfgang,
First, your rules work...
But for the second rule, I replaced the first Reachable() in the when
clause to Link(), and the result is still correct. Only if I remove
"no-loop true", the issue I had appeared.
So, I understand how "no-loop true" in this case helps to make the
result correct. But, do you see any scenarios where "no-loop" can
cause incorrect results? For example, not enough number of recursions?
Thanks.
-Simon
2011/3/8 Wolfgang Laun <wolfgang.laun(a)gmail.com>:
I think there is some fundamental error in deriving truths from
givens and
other derived facts that are then interpreted as given truths, and,
moreover, with subtly varying semantics. In terms of graph theory:
reachability is based on (directed) edges, but it does not establish
additional edges.
Deriving Reachability should be done by:
rule deriveLink
when
Link( $a: a, $b: b )
then
insertLogical(new Reachable($a,$b));
System.out.println( "ins reach " + $a + " " + $b );
end
rule deriveReachReach
no-loop true
when
Reachable( $a: a, $b: b )
Reachable( a == $b, $c: b != $a )
then
insertLogical(new Reachable($a,$c));
end
Ideally, I would like to use
not Reachable(a == $a, b == $c)
instead of the (last resort) no-loop in the second rule, but Drools' truth
maintenance is incomplete: it does not let your define the logical
dependency on part of the condition (i.e., excluding the CE "not" in this
case).
-W
On 8 March 2011 05:49, Simon Chen <simonchennj(a)gmail.com> wrote:
>
> What I had is a very simplified version of how calculating transitive
> closure could go wrong...
>
> Let's say we have two rules:
> rule 1
> when
> link(a,b)
> then
> insertLogical(new reachable(a,b))
>
> rule 2
> when
> link(a,b) reachable(b,c)
> then
> insertLogical(new reachable(a,c))
>
> Let's say, I have link(a,b), link(b,c), link(b,a), link(c,b). So,
> we'll have reachable(a,b), reachable(b,c), reachable(a,c), etc. But,
> after I retract link(a,b) and link(b,a), guess what, reachable(c,a)
> still exists! This doesn't sound right to me.
>
> But in Drools, this is possible, because we have:
> reachable(c,a) <- link(c,b), reachable(b,a)
> reachable(b,a) <- link(b,c), reachable(c,a)
>
> The problem here is that we actually inserted reachable(b,a) multiple
> times: first supported by link(b,a) and rule 1, and secondly by
> link(b,c) and reachable(c,a) and rule 2. When reachable(b,a) was
> inserted the second time, link(b,c) and reachable(c,a) become the
> additional supporting condition - maintained by the truth maintenance
> system. So, even if link(b,a) is retracted, reachable(b,a) still
> exists further supporting reachable(c,a).
>
> Is it clearer?
>
> Thanks.
> -Simon
>
> 2011/3/7 Edson Tirelli <ed.tirelli(a)gmail.com>:
> >
> > Simon,
> > The behavior seems correct to me as B is justified by either A or C
> > (or
> > both). Of course, from the initial state, A is required for C to first
> > exist, but once it starts to exist, your rules say that B and C justify
> > each
> > other and so both remain in memory.
> > This is design as intended, but do you think that is wrong?
> > Edson
> >
> > 2011/3/7 Simon Chen <simonchennj(a)gmail.com>
> >>
> >> Hi all,
> >>
> >> An interesting finding:
> >>
> >> I have three simple rules:
> >> rule "A2B"
> >> when
> >> A()
> >> then
> >> insertLogical(new B());
> >> end
> >> rule "B2C"
> >> when
> >> B()
> >> then
> >> insertLogical(new C());
> >> end
> >> rule "C2B"
> >> when
> >> C()
> >> then
> >> insertLogical(new B());
> >> end
> >>
> >> Basically, once we have an A(), we'll logically insert a B(). Once we
> >> have a B(), we'll logically insert a C(). Once we have a C(), we'll
> >> logically insert a B().
> >>
> >> So, I first insert an A(), print all the objects. Retract A(), and
> >> print all the objects. Here's what I got:
> >> com.sample.B@42
> >> com.sample.C@43
> >> com.sample.A@548997d1
> >> after retract!
> >> com.sample.B@42
> >> com.sample.C@43
> >>
> >> So, B() and C(), which should be logically depend on A(), somehow are
> >> not retracted. The problem I see is the truth maintenance system allow
> >> B() and C() to depend on each other, thus not affected by losing A().
> >>
> >> Is this a bug or my bad usage?
> >>
> >> Thanks.
> >> -Simon
> >> _______________________________________________
> >> rules-users mailing list
> >> rules-users(a)lists.jboss.org
> >> https://lists.jboss.org/mailman/listinfo/rules-users
> >
> >
> >
> > --
> > Edson Tirelli
> > JBoss Drools Core Development
> > JBoss by Red Hat @ www.jboss.com
> >
> > _______________________________________________
> > rules-users mailing list
> > rules-users(a)lists.jboss.org
> > https://lists.jboss.org/mailman/listinfo/rules-users
> >
> >
>
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