[rules-users] top n - accumulate function parameters

[Edson Tirelli]

   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 at 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
 JBoss, a division of Red Hat @ www.jboss.com
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