[rules-users] Re: not exists / forall(not) logically equivalent, behave differently
edson,
thanks a lot for your answer.
now i came to a new problem.
i stated the rule as you said:
rule "no dog without a tail"
when
$dog :Dog()
not( Tail(dog == $dog) )
then
// fire rule
end
but now, if i do
workingMemory.insert(dog);
workingMemory.insert(dog.getTail());
workingMemory.fireAllRules();
modifyRetract(dogHandle);
dog.changeHashCode(); // do something that changes the hashCode of the dog
modifyInsert(dogHandle);
workingMemory.fireAllRules();
in the second fireAllRules() the rule fires, even though there is a
tail for the dog.
am i not allowed to do an opperation that changes the hashCode of the
modified object between the
modifyRetract() and modifyInsert() ?
should i totally retract the object, and insert it again instead?
thank you, tim
I believe there is an open ticket for a bug when using
"not(exists())" in 5.0. I did not investigated yet, but I believe this is a
regression in the new asymmetric matching algorithm in 5.0. Anyway, we will fix it before
releasing the final version.
Meantime, just use:
not( Tail(dog == $dog) )
Since the "exists" is redundant there because "not" is the
existential operator, not the "boolean" operator.
[]s
Edson
2008/10/6 tim tim <timbaermannextra@...>
hello there,
i have a rule, that should fire, when a required part of a whole is missing.
lets say the whole is a dog, and the required part the tail.
then the rule should fire, when there is a dog without a tail.
i stated the rule in two logically equivalent ways.
1) when we have a dog, and all existing tails belong to other
peoples dogs, the rule should fire:
rule "no dog without a tail; forall"
when
$dog :Dog()
forall($t : Tail()
Tail(dog != $dog))
then
// fire rule
end
2) when we have a dog and no tail exists that belongs to this dog
the rule should fire:
rule "no dog without a tail; not exists"
when
$dog :Dog()
not( exists( Tail(dog == $dog)))
then
// fire rule
end
and they really do behave similar, but not the same..
workingMemory.insert(dog);
workingMemory.insert(dog.getTail());
workingMemory.fireAllRules();
// here both did not fire, but when i go on:
workingMemory.modifyRetract(dogHandle)
workingMemory.modifyInsert(dogHandle, dog)
workingMemory.fireAllRules();
// now the not(exists(..)) version did fire, whereas the
forall(..) version did not.
of course both rules should not have fired, nothing changed for
the dogs relationship
with its tail.
i am using the 5.0.0.snapshot version of drools but i can not
imagine that this is an
unknown bug, as this is rather basic predicate logic.
what am i missing?
best, tim