CLIPS using (not (a-fact $?)) doesn't fire

Question

I'm having a slight difficulty in understanding how the not function interacts with pattern matching. I'd like to write a pattern match that goes "such a fact doesn't exist".

Namely, what I want is:

(defrule init-count
    (not (highest-debt ?))
    (catherines ?debt)
    =>
    (assert (highest-debt ?debt))
)

and then:

(defrule continue-count
    ?debt-fact <- (highest-debt ?h-debt)
    (? ?a-debt)
    (test (> ?a-debt ?h-debt))
    =>
    (retract ?debt-fact)
    (assert (highest-debt ?a-debt))
)

But for some reason not doesn't work for me here. Replacing not with (not (exists /*pattern*/)) doesn't seem to work either. Is there something I'm missing and/or a nice way to implement finding the highest number with rules like that?

Update

The answer that I have just discovered is this: the not pattern shouldn't be the first one ((declare (salience 0)) doesn't count either). So, the following code works correctly:

(defrule init-count
    (catherines ?debt)
    (not (highest-debt ?))
    =>
    (assert (highest-debt ?debt))
)

Sorry for bothering :(

Next time, try to format your code so others are able to read it. — pad
All the tutorial examples I've seen are formatted like that. Do you want me to add Lisp-style line breaks for every depth level? — Francis Drake
I meant formatting your code in the question. I didn't have comments on your coding styles. — pad
Sorry, I failed initially, but the current formatting looks kinda okay now, doesn't it? — Francis Drake
Ok, thank you. I was actually updating it when you posted your revision. The formatting help tip doesn't remind to put an empty string between plaintext and code blocks. — Francis Drake

Gary Riley Gary Riley · Accepted Answer · 2012-03-16T03:26:23

In versions 6.24 and earlier, when the not conditional element was the first pattern in a rule, the pattern (initial-fact) was added before it (this is described in section 5.4.9, Automatic Addition and Reordering of LHS CEs, in the Basic Programming Guide). The (initial-fact) fact is asserted when a (reset) command is performed. I think this is the cause of your problem. Your original rule would have worked if you'd performed a (reset) before asserting your other facts.

CLIPS using (not (a-fact $?)) doesn't fire

1 Answers