[rules-users] Binding Not

[Mark Proctor]

Joshua Undesser wrote:
> I have a question on the return value of a not statement........
>   
'not' has no return value, and you can't bind to it - after all it is 
the absence of something, how can you bind to nothing.
> I have two fairly large rules in respect to the LHS when declarations.  
> They are identical except for the fact that in one of them, I am checking to
> see if one more object exits, while in the other I am checking to see if it
> doesn't exist.   It seems silly to try have duplicate rules expressively
> typed when this is the only difference.   So what i tried to do was bind the
> value of the not statement as shown below....
>
> rule "Rule Name"
>   no-loop = true
>   when
>        .........
>
>       $msg : (or Message (description == "OK")
>                      not Message (description == "OK"))
>        .........
>   then
>      if ($msg == null) {
>           createMessage("OK");  // 
>      }
>
>      .... rest of RHS.....
> end
>
> I figured if the Message object didn't exist, null would be returned and
> assigned to the $msg variable.   But it was not.    For now I have it
> separated into two rules, but logically I would like to combine them, as
> every other part of the LHS and RHS match.
>   
nope, doesn't work that way, you can't bind to a not.
> I also tried a few different variants as shown below thinking it would work,
> but didn't.  
>
> rule "Rule Name"
>   no-loop = true
>   when
>        .........
>
>       (or $msg : Message (description == "OK")
>            not Message (description == "OK"))
>        .........
>   then
>      if ($msg == null) {
>           createMessage("OK");  // 
>      }
>
>      .... rest of RHS.....
> end
>
>
> I even tried the above with the entire "then" side encapsulated in a
> try/catch block to catch the null pointer that gets thrown, but no luck.
>
> Anyone have any suggestions?   Just seems like there should be a way to do
> this.
>   
nope.
>
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