Re: [rules-dev] feature request: undo-then
Geoffrey De Smet wrote:
How hard would it be to allow something like this in the DRL?
rule "myRule"
when
t: Total()
... // a bunch of complicated fact constraints
then
t.increment();
undo-then
t.decrement();
end
The current workarounds don't are clunky:
- Writing an negative (opposite) rule isn't efficient: it means
declaring the rule twice effectively. Also the negative rule is
usually using lots of or's and not's which isn't fast.
- Using logical inserts is a more heavy weight (= slower).
This sounds like my logical closure/stateful callback idea that I'd like
to do, where a closure is called when a rule is no longer true. So in
the "then" part you would do something using MVEL like lamba expressions:
logicalClosure( [a, b, c] { my closure code here } );
Where a, b, c are the variables to capture from the surrounding scope
and the my closure here is the code to be executed. In java it would
look something like
final Map vars = new HashMap();
vars.put( "a", a );
vars.put( "b", b);
vars.put( "c", c);
Closure closure = new Closure() {
private Map vars;
public void call() {
a = vars.get(a");
b = vars.get("b");
c = vars.get("c");
.... my closure code here;
}
public void setVars( Map vars ) {
this.vars = vars;
}
}
closure.setVars( vars );
so it forms a sort of encapsulated command that is called when the rule
is no longer true.
Mark
Here's another description of this feature request from the user
mailing list:
> Ah, well that's my point of ignorance, then. I was thinking in
> stateless mode. :) And to do what I suggest in a stateful manner
> would require a "modifyLogical" type functionality in place of the
> insertLogical. i.e.
>
> modifyLogical (object) {
> //do this when the condition obtains
> } else {
> //do this when the condition no longer obtains
> }
>
> Could be an interesting functionality to have, though.