[rules-users] Locating facts via slot values

[David Sinclair]

Here you go WolfGang

rule shoot-3
    when
            $h : Hunter( $target : target)
                  Animal(this == $target)
    then
       $h.shoot( $a );
       retract( $a );
end

2009/4/3 Wolfgang Laun <wolfgang.laun at gmail.com>

> Given a working memory containing a large number of rather static facts
> which are from several subclasses (Ape, Bear, Crocodile,...) of some base
> class (Animal) and a single fact of class Hunter with a field target of type
> Animal, the shooting of an animal might be written with a rule like this:
>
> rule shoot-1
>     when
>         ?h : Hunter( ?target : target )
>         ?a : Animal()
>         eval(?target == ?a)
>     then
>        ?h.shoot( ?a );
>        retract( ?a );
> end
>
> Avoiding eval (which is said to be inefficient), it could also be written
> as
>
> rule shoot-2
>     when
>         ?a: Animal()
>         ?h : Hunter( target  == ?a )
>     then
>        ?h.shoot( ?a );
>        retract( ?a );
> end
>
> This, however, places the pattern with many instances up front, which is
> (AFAIK) not so good in a Rete implementation.
>
> Which one is to be preferred in Drools?
>
> Ideally, one might want to write
>
> rule shoot-3
>     when
>         ?h : Hunter( ?target : target )
>         ?a == ?target : Animal()                   ### not valid DRL
>     then
>        ?h.shoot( ?a );
>        retract( ?a );
> end
>
> This avoids eval, has the single instance fact up front, but it isn't
> available.
>
> -W
>
>
>
>
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