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* <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> 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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