[rules-users] Roll Back Rule / Rule Transactions?

[Mark Proctor]

You could use truth maintenance, this won't remove something from a 
list, but it will remove a fact from the working memory when a rule is 
no longer true.

Mark
Michael Suzio wrote:
> Anytime you modify a object, candidate rules are going to be 
> re-examined.  You may need some state variables in addition to this 
> (to control when rules fire, avoid loops, etc), but the basic rules 
> could be:
>
> rule AddVcr
> when
>   $c: Cart(contents contains "TV", contents excludes "free VCR")
> then
>  $c.add("free VCR");
>  modify($c);
> end
>
> rule RemoveVcr
> when
>  $c: Cart(contents excludes "TV")
> then
>  $c.remove("free VCR");
>  modify($c);
> end
>
> You can fill in the rest.  Like I said, you probably need some state 
> variables so things don't loop, need to exclude the case where a 
> non-free VCR is legitimately in the cart, etc.  But your basic 
> question should be answered with this approach.  Don't think 
> procedurally, think in terms of a living system when the changes in 
> objects trigger somewhat asynchronous cascades of matching rules.  
> Calling modify() on a fact is like restarting the whole matching 
> algorithm for any rules that had that fact included in their (possible 
> candidate) constraints.
>
> On 2/13/07, *Matt Johnston* <MJohnston at pubgroup.com 
> <mailto:MJohnston at pubgroup.com>> wrote:
>
>     I am working on rules for an online shopping cart. Depending on the
>     products in the cart, the customer may get certain discounts.
>
>     If cart contains a TV, add a free VCR.
>
>     I can apply these rules, but what about rolling them back?
>
>     So in the example above, the rule automatically added a free VCR
>     because
>     the cart contained a TV. If the customer removes the TV, I need to
>     roll
>     back the VCR rule and also remove the VCR from the cart.
>
>     Does anyone know how to do this? Is it possible?
>
>     Matt Johnston
>     Manager of Internet Content
>     Publishing Group of America
>     p: 615-468-6053
>     f: 615-468-6153
>
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