Duh. I had early on gotten into the habit of assigning the LHS clause to a
variable (as I did below in ?r - ...) and then accessing the slots using
dotted vars that I had completely forgotten this alternative way of assigning a
slot value to a var in a rule. It works correctly now without the global and it
retracts the fact when I change the value representing the current time.
Apologies for wasting everyone's time with this stupid question.
Dwight
-Original Message-
From: owner-jess-us...@sandia.gov [mailto:owner-jess-us...@sandia.gov] On
Behalf Of Wolfgang Laun
Sent: Tuesday, June 26, 2012 9:59 PM
To: jess-users@sandia.gov
Subject: Re: JESS: [EXTERNAL] Jess asserts fact that is not true
On 26/06/2012, Dwight Hare d.h...@paritycomputing.com wrote:
I am trying to reason about the passage of time in my rules. I would
like to write the following rule:
(defrule Test
?r - (logical (CurrentTime))
(logical (Condition (time ?time:( (+ ?time 5)
?r.t
=
(assert (Condition-met))
)
Where CurrentTime is a fact that holds the current time, and I
change the slot value t as time progresses. The rule fires if the
time slot in the Condition plus 5 is greater than the current time
(that is, is true within the last 5 time increments). But this syntax
is not allowed because you cannot access a dotted variable (the ?r.t) here.
Why not simply
(defrule Test
(logical (CurrentTime ?ct))
(logical (Condition (time ?time:( (+ ?time 5) ?ct
=
(assert (Condition-met))
)
which avoids all that global rigmarole?
More below.
My first alternative was to declare a defglobal called *time* and
have that change value as time progresses. But there is no way to make
the global a logical dependency of the asserted condition-met so it
doesn't get retracted when the global changes in value sufficient to
make the rule no longer be true.
I came up with a workaround where in addition to the defglobal
*time* I assert a Fact called CurrentTime that holds the same value in the
t slot.
Now my rule is:
(defrule Test
?r - (logical (CurrentTime (t ?t:(= ?t ?*time*
(logical (Condition (time ?time:( (+ ?time 5)
?*time* ; Note I use the global here instead of the dotted var but the
value is the same
=
(assert (Condition-met (globtime ?*time*)(facttime
?r.t)))
)
The fallacy is buried in the second pattern which compares the
(unchanged) Condition with the value in the global *time*. You know that this
has changed, but the Jess Engine doesn't. It works in the first pattern because
you change the CurrentTime fact, and so this pattern is re-evaluated.
Don't use globals in LHS patterns unless they are immutable.
-W
This says that if there is a CurrentTime fact whose slot t has the
same value as the global *time* and there is a Condition whose
time slot plus
5 is greater than that time, then assert the Condition-met. My java
code sets the global to 4, asserts the fact (CurrentTime (t 4)),
and then asserts the fact (Condition (time 0)). Since all the
conditions are met the rules engine asserts the fact
(MAIN::Condition-met (globtime 4) (facttime 4)). All is well though
a bit clumsy. Now I want to move time forward where the
Condition-met should no longer be true. I change the global to 10
and modify the CurrentTime fact and set the t slot to 10 too.
Then I run the rules engine and it retracts the (MAIN::Condition-met
(globtime 4) (facttime 4)) but then mysteriously asserts
(MAIN::Condition-met (globtime 10) (facttime 10)). Since the
Condition time of 0 plus 5 is not greater than 10, it should not have
asserted this.
Here is a dump of the facts in the rules engine at the end:
0: (MAIN::CurrentTime (t 10))
1: (MAIN::Condition (time 0))
3: (MAIN::Condition-met (globtime 10) (facttime 10))
I see no justification for the 3rd fact given the only rule I have.
Is there a better way of accomplishing this?
Dwight
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