[rules-dev] feature request: undo-then

[Mark Proctor]

Geoffrey De Smet wrote:
> 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".
As I mentioned, i'd do it as a logical closure using an anonymous code 
block, in a similar manner that we do for logical insertions.

Mark
>
>
> 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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