We seem to have found a problem with logical dependencies in Jess. We
have a fact (b) that logically depends on a fact (a). If (b) is
retracted, it is automatically reasserted as long as (a) is still true.
Initially, we assert (a) as fact 0. This causes (b) to be asserted as
fact 1. We retract fact 1, causing (b) to be reasserted as fact 2.
Finally, we retract fact 0. At this point, (b) is not in working
memory, but a rule or query with (b) on the LHS still fires.
The code and output appear below. Looking at the trace, you'll note
that when (a) is retracted, fact 1 is consequently retracted (even
though it was already retracted) and fact 2 is not retracted (even
though it depends on (a)).
Code:
(defrule r
(logical (a))
(not (b))
=>
(assert (b)))
(defmodule mod)
(defrule bad-rule
(b)
=>
(printout t "BAD!!!" crlf))
(watch all)
(assert (a))
(run)
(retract 1) ;; (retract (b))
(run)
(retract 0) ;; (retract (a))
(focus mod)
(run)
(facts *)
Output:
Jess, the Rule Engine for the Java Platform
Copyright (C) 2006 Sandia Corporation
Jess Version 7.0p1 12/21/2006
==> f-0 (MAIN::a)
==> Activation: MAIN::r : f-0,
FIRE 1 MAIN::r f-0,
==> f-1 (MAIN::b)
==> Activation: mod::bad-rule : f-1
<== f-1 (MAIN::b)
==> Activation: MAIN::r : f-0,
<== Activation: mod::bad-rule : f-1
FIRE 1 MAIN::r f-0,
==> f-2 (MAIN::b)
==> Activation: mod::bad-rule : f-2
<== f-0 (MAIN::a)
<== f-1 (MAIN::b)
<== Focus MAIN
==> Focus mod
FIRE 1 mod::bad-rule f-2
BAD!!!
<== Focus mod
==> Focus MAIN
For a total of 0 facts in all modules.
begin:vcard
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email;internet:[EMAIL PROTECTED]
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