On 06/08/2010 16:04, Anstis, Michael (M.) wrote:

IRC: interesting topic: use of lists using "from" in LHS and iterated over in RHS
HI,

There was an interesting discussion today on IRC; a user "jochen" reported NPEs when trying to iterate lists in the RHS constructed in the LHS using "from collect".

I was wondering how Drools handles lists constructed in the LHS; if the list contained 5 elements but another rule retracted one of those elements would the list available in the RHS contain 4 or 5 elements?

Here's a simplification of what was posted at http://pastebin.com/yKGSxFkA

Rule 1
when
        $cheese : ArrayList() from collect(CheeseFact());
then
        for(int i=0; i<$cheese.size(); i++) {
                //Something useful…
        }
end

What if there was another rule that did something like this:-

Rule 2
When
        $cf : CheeseFact(type == "chedder")
Then
        retract($cf);
end

Each time this "rule 2" fires and retracts a CheeseFact that was in $cheese from rule1, the $cheese list will be reduced by one. So there should be no null pointers here.

Mark

What would be the list Rule 1 iterates?

I'd assume since I believe activation execution to be serial it would depend on the agenda's conflict resolution strategy or rule salience?

What would be the learned opinion?

With kind regards,

Mike
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