How to replace a contradicting fact

[lazd]

Hey there,
The situation is I've pulled a set of facts from a database and asserted each one. Then, a rule runs that asserts a fact that contradicts a fact pulled from the database, so I want to retract the contradicting fact and assert the new fact.

i.e. the fact (status sX)  was pulled from the database and asserted, and a rule fires whose RHS contains (assert (status s1)), but I would like to retract (status sX) before asserting (status s1).

The following loops infinitely, as one would expect:

(assert(status s0))
(assert(someTrigger saysGo))

(defrule modifyStatus
    (someTrigger saysGo)
    ?x <- (status ?)
    =>
    (retract ?x)
    (assert (status s1))
)

Furthermore, (status sX) may or may not exist, so the rule doesn't even fire unless (status sX) is present.

I thought maybe using templates and (modify) was the answer, but it requires a fact specifier, which requires me to match the fact on the LHS, and again (status sX) may or may not exist, and this loops infinitely anyway:

(deftemplate person (slot status))

(assert(person (status s0)))
(assert(someTrigger saysGo))

(defrule modifyStatus
    (someTrigger saysGo)
    ?x <- (person (status ?))
    =>
    (modify ?x (status s1))
)

So basically, I need a simple way to replace (status sX) with a given (status sY), regardless of what X may be. Also, this rule might assert multiple facts, all of which may need to replace contradicting facts. Does the answer lie in templates?

Any ideas?

Thanks in advance,
Larry

[Gary Riley]

There are a couple of different ways you can prevent the rule from retriggering:

(defrule modifyStatus
(someTrigger saysGo ?v)
?x <- (person (status ?v))
=>
(modify ?x (status s1))
)

(defrule modifyStatus
?f <- (someTrigger saysGo)
?x <- (person (status ?))
=>
(retract ?f)
(modify ?x (status s1))
)

(defrule modifyStatus
(someTrigger saysGo)
?x <- (person (status ~?s1))
=>
(modify ?x (status s1))
)