CODEWITHSUNDEEP
pythonclipsclipspy
greenlantern
greenlantern

Reputation: 414

why retracted fact template is still available?

I am trying to modify a fact of a fact-template in clips using ClipsPY. Following code retracts and reasserts the fact. Why is it that after the fact is retracted, the slot value s_2 is not nil on reassert?

run.py

import clips

clips_env = clips.Environment()

def py_pfact():
    for fact in clips_env.facts():
        print(fact)

def py_modify(p):
    print("\nfacts:")
    py_pfact()

    p.retract()
    print("\nfacts after retracted:")
    py_pfact()

    p["s_1"] = clips.Symbol("v_2") 
    p.assertit()
    print("\nfacts after re_assert:")
    py_pfact()

clips_env.define_function(py_pfact)
clips_env.define_function(py_modify)

clips_env.load("KB.clp")
clips_env.reset()
clips_env.run()

This is the clp file

;; KB.clp
(deftemplate t
            (slot s_1 (type SYMBOL))
            (slot s_2 (type SYMBOL))
    )

    (defrule main-intent
            (initial-fact)
            =>
            (assert (t (s_1 v_1) (s_2 v_2)))
    )

    (defrule rule_1
            ?p<-(t (s_1 ?v&~v_2))
            =>
            (py_modify ?p)
    )

The output is:

facts:
(initial-fact)
(t (s_1 v_1) (s_2 v_2))

facts after retracted:
(initial-fact)

facts after re_assert:
(initial-fact)
(t (s_1 v_2) (s_2 v_2))

I expected output to be (t (s_1 v_2) (s_2 nil)), however s_2 is not nil but previous set value v_2 which was set before the fact retracted.

Upvotes: 0

Views: 167

Answers (1)

noxdafox
noxdafox

Reputation: 15060

Asserting and retracting a fact does not modify it. It simply adds/removes it from the engine knowledge base.

You can see a deftemplate as a class and a fact created from it like an object. You can extend the engine knowledge base by adding and removing facts either from the API or through the rules.

# define a fact template within the engine
environment.build('(deftemplate foo (slot bar) (slot baz))')
template = environment.find_template('foo')

# create a new fact and set its values
fact = template.new_fact()
fact['bar'] = 1
fact['baz'] = 1

# assert the fact within the engine
fact.assertit()

assert fact in environment.facts()

# retract it
fact.retract()

assert fact not in environment.facts()
assert fact is fact  # fact is the same object as before

fact['baz'] = 2

assert fact is fact  # still the same object

fact.assertit()

assert str(fact) == '(foo (bar 1) (baz 2))'

Upvotes: 0

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