Greg Barton wrote: 
OK, here's where combining the else and undo would get freaky. :)
Btw did you see my otherwise proposal?

So we would create otherwise-groups, these would ne named groups, like activation groups.
Person( location == "london" )
Person ( location == "berlin" )
Person( location == OTHERWISE )

Assuming the three patterns above where each in three different rules. What this means is for the given named otherwise-group if a field marked as OTHERWISE is not matched by any other rules in that group then it's true. Otherwise has long been a feature in decision tables, but it's not something I've seen supported in technical rules.

Something else I want to add, particularly focused on systems automations is a repeat attribute. We already have a duration attribute, that delays the firing of a rule, but what about the rule firing again say 30 seconds later if it's still true. So I'd like to add a repeat command that takes either a integer duration or a cron like scheduling statement, which may also be combined with an initial duration rule.

I've also got ideas around a "execution-group" these would be groups who's behaviour would be user defined, probably around some sort of event system, which would provide more flexability than the current hard coded group behaviours. These execution-groups in theory could contain custom conflict resolution strategies and also be able to emulate our existing execution control modes too.

So as you can see betwee logical closures, else, otherwise-groups, repeats and execution-groups there is still a lot more we can do to improve our expressiveness. It is, imho, a necessary step if we are to take rule engines further to get mainstream adoption as a generic pramming tool - because currently other than very narrow use cases rules engines suck for generic programming compared to other languages, where with enough flexability they should actually excel. Another key aspect for this will be the adoption of properly ontology support, so people start creating models that are appropriate for rule engines and not bastardising their existing deeply nested pojo models.


Mark


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@gmail.com> wrote:

  
From: Geoffrey De Smet <ge0ffrey.spam@gmail.com>
Subject: Re: [rules-dev] feature request: undo-then
To: rules-dev@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@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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