How do I fire a rule if an object with certain characterstics does not
exists.

For example my class Foo, if I have a rule:

rule "Identify Foos with values"

when
Foo(stringProp=="blah", intProp==5)
then
System.err.println("A Foo was found!");
end

So how do I check for lack of existence of an object with certain
characteristics

I tried:
rule "Flag missing Foos with values"

when
not(Foo(stringProp=="blah", intProp==5))
then
System.err.println("A proper foo does not exist");
end

That's the way to do it.

I also tried:
rule "Flag missing Foos with values"

when
not(exists(Foo(stringProp=="blah", intProp==5)))
then
System.err.println("A proper foo does not exist");
end

Since not( Foo(...)) means "if no Foo(...) exists", the addition of "exists" is superfluous and generally considered bad style (even though some systems accept it to mean just "not".

-W

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