[rules-users] "not exists MyObject" - is it a valid LHS condition?

[Edson Tirelli]

Vlad,

The conditional element "not" is the oposite of "exists". It means 
exactly what you want:

not MyObject( <constraints> )

Will evaluate to true if and only if there is no instance of MyObject in 
the working memory that match the given <constraints>.
Exists will evaluate to true if there is at least one object that 
matches the constraints:

exists MyObject( <constraints> )

So, you don't need to write "not exists"... write simply "not".

Having said that, 3.0.x will trigger an error if you write "not exists". 
Trunk will "optimize" and remove your "exists" effectivelly using only 
the "not".

[]s
Edson

Olenin, Vladimir (MOH) wrote:

> Hi,
>
> I wonder if the combination of ‘not’ and ‘exists’ keywords is a valid 
> one and therefore the construct like
>
> rule “myrule”
>
> when
>
> not exist MyObject(param matches “somepattern”)
>
> then
>
> System.out.println(“Object for pattern ‘somepattern’ doesn’t exisit”);
>
> End
>
> Thanks.
>
> Vlad
>
> PS: basically I need to trigger a rule if some set of facts is NOT 
> found and I need to trigger it only once.
>
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 Edson Tirelli
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