[rules-dev] feature request: undo-then

[Greg Barton]

OK, here's where combining the else and undo would get freaky. :)

rule
when
1: Foo()
2: Bar()
3: Bas()
then
  //Everything
undo-then
  //When Everything no longer matches
else 3:
  //Foo and Bar, No Bas
undo-else 3:
  //When (Foo and Bar, No Bas) no longer matches
else 2:
  //Foo, No Bar, Bas unknown
undo-else 2:
  //When (Foo, No Bar, Bas unknown) no longer matches
else 1:
  //No Foo, Bar and Bas unknown
undo-else 1:
  //When (No Foo, Bar and Bas unknown) no longer matches
end

The else clause basically is shorthand that generates another rule, one that's triggerd when the indexed indexed fails.  The undo would fit right in there, and should be triggered immediately when the indexed condition fails to fail. :) i.e. say "else 3" fired first, then condition 3 matches, (and 1,2 still match) then "undo-else 3" and "then" would fire in that order.

Maybe that's just too much. :)

--- On Thu, 2/19/09, Geoffrey De Smet <ge0ffrey.spam at gmail.com> wrote:

> From: Geoffrey De Smet <ge0ffrey.spam at gmail.com>
> Subject: Re: [rules-dev] feature request: undo-then
> To: rules-dev at lists.jboss.org
> Date: Thursday, February 19, 2009, 5:42 AM
> What would an "else clause" do?
> Imagine a rule with matches on 5 different fact sets.
> How many times would the else part match?
> - none (because it matches at least one)
> - a very lot (because it matches on any fact set that
> isn't that one of those 5)
> 
> The "undo-then" is another concept:
> it matches when a rule that matched before (in a previous
> fireAllRules), now no longer matches.
> "undo-then" probably isn't the best name, so
> better suggestions are welcome, but "else"
> isn't a good name for it as it's not about "not
> matching" but about "no longer matching".
> 
> 
> Anyway, I 've been thinking and it wouldn't work
> for all use cases in drools-solver:
> rule
>   when
>     q1 : Queen()
>     q2 : Queen()
>     eval(q1.getY() - q2.getY() < 10)
>   then
>     a.add(q1.getY() - q2.getY());
>   undo-then
>     a.subtract(q1.getY() - q2.getY())
> end
> The y of a queen changes, so the subtract wouldn't
> subtract the exact same number that was added.
> Any way we could work around that, or is there no avoiding
> insertLogical?
> 
> With kind regards,
> Geoffrey De Smet
> 
> Mark Proctor schreef:
> > Greg Barton wrote:
> >> --- On Wed, 2/18/09, Geoffrey De Smet
> <ge0ffrey.spam at gmail.com> wrote:
> >> 
> >>   
> >>> The current workarounds [to undo-then] 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.
> >>>     
> >> 
> >> This would be made easier by...drum roll
> please...the else clause! :)
> >>   
> > yes we want OPSJ style else statements, edson has an
> idea on how to do that, just a matter of time :(
> >> Now, if there was the else clause plus
> undo-then/closures you could probably write an entire
> complex ruleset in one rule.  Might as well just use perl,
> then. :P   
> > heh, that's true you would have potentially
> encapsulated 4 possible executions in a single rule.
> >> 
> >>      
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> >> 
> >> 
> >>   
> > 
> > 
> >
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