The MVEL trick works.  That was exactly what I was hoping to find.
 
thanks,
 
-Jess

From: rules-users-bounces@lists.jboss.org [mailto:rules-users-bounces@lists.jboss.org] On Behalf Of Edson Tirelli
Sent: Tuesday, November 11, 2008 3:40 PM
To: Rules Users List
Subject: Re: [rules-users] top n - accumulate function parameters


   Accumulate functions support only a single parameter, as they are defined now, but would be great if someone can come up with a good way of adding support to variable number of parameters.

   Having said that, you can probably try a quick trick for your rules. The way accumulate function works now is that it takes an expression as the parameter, evaluate the expression and sends the result of the expression to the accumulate function implementation. Example:

... accumulate( Cheese( $price: price ),
                       sum( $price * 10 ) )

   So the engine will calculate $price * 10 and will send the result into the function implementation.
   So, if you are using MVEL dialect, you can use the list creation simplified syntax to pass multiple parameters to the function. I never tried, but should work:

rule xyz
    dialect "mvel"
when
    ...
... accumlate( Cheese( $price : price ),
                     averageTop( [10, $price ] ) )
then
  ...
end

    The syntax [ ] will create a list in MVEL and will pass it into the method call:

    public void accumulate(Serializable context, Object value) {
          List values = (List) value;
          ...
     }

     Problem is you only have the parameters available in the accumulate() and reverse() call... you don't have access to them in the init() and result() call.

     If you want to improve this, you are more than welcome!

    []s
    Edson

 

2008/11/11 Evans, Jess <JEvans@collegeboard.org>
I'm trying to average top N items.  I can of course collect the items and do the rest in the consequence, but thought it might be a scenario applicable to a custom accumulate function.  However, the accumulate function interface methods only take one parameter.  If I were to declare a function top(N, things) within an accumulate call would drools autobox the parameters into some sort of collection? 
 
Maybe this is a naive solution anyway.  I am still learning to think declaratively.
 
 
cheers,
 
-Jess

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 Edson Tirelli
 JBoss Drools Core Development
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