RE: JESS: [EXTERNAL] Jess asserts fact that is not true

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 u=
sing 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: [email protected] [mailto:[email protected]] On=
 Behalf Of Wolfgang Laun
Sent: Tuesday, June 26, 2012 9:59 PM
To: [email protected]
Subject: Re: JESS: [EXTERNAL] Jess asserts fact that is not true

On 26/06/2012, Dwight Hare <[email protected]> wrote:
> I am trying to reason about the passage of time in my rules. I would=20
> like to write the following rule:
>
> (defrule Test
>                     ?r <- (logical (CurrentTime))
>                     (logical (Condition (time ?time&:(> (+ ?time 5)
> ?r.t))))
>                 =3D>
>                     (assert (Condition-met))
> )
>
> Where "CurrentTime" is a fact that holds the current time, and I=20
> change the slot value "t" as time progresses. The rule fires if the=20
> "time" slot in the Condition plus 5 is greater than the current time=20
> (that is, is true within the last 5 time increments). But this syntax=20
> 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))=
))
                =3D>
                    (assert (Condition-met))
)

which avoids all that global rigmarole?

More below.

>
> My first alternative was to declare a defglobal called "*time*" and=20
> have that change value as time progresses. But there is no way to make=20
> the global a "logical" dependency of the asserted condition-met so it=20
> doesn't get retracted when the global changes in value sufficient to=20
> make the rule no longer be true.
>
> I came up with a workaround where in addition to the defglobal=20
> "*time*" I assert a Fact called CurrentTime that holds the same value i=
n the "t" slot.
> Now my rule is:
>
> (defrule Test
>                     ?r <- (logical (CurrentTime (t ?t&:(=3D ?t ?*time*)=
)))
>                     (logical (Condition (time ?time&:(> (+ ?time 5) =20
> ?*time*))))  ; Note I use the global here instead of the dotted var but=
 the value is the same
>                 =3D>
>                     (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 patt=
ern because you change the CurrentTime fact, and so this pattern is re-ev=
aluated.

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=20
> same value as the global "*time*" and there is a Condition whose=20
> "time" slot plus
> 5 is greater than that time, then assert the Condition-met. My java=20
> code sets the global to "4", asserts the fact "(CurrentTime (t 4))",=20
> and then asserts the fact "(Condition (time 0))". Since all the=20
> conditions are met the rules engine asserts the fact=20
> "(MAIN::Condition-met (globtime 4) (facttime 4))". All is well though=20
> a bit clumsy. Now I want to move time forward where the=20
> "Condition-met" should no longer be true. I change the global to 10=20
> 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=20
> (globtime 4) (facttime 4))" but then mysteriously asserts=20
> "(MAIN::Condition-met (globtime 10) (facttime 10))". Since the=20
> Condition time of 0 plus 5 is not greater than 10, it should not have a=
sserted 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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