strange PresentLink, AbsentLink, NotLink, AndLink interactions

[Alexander Gabriel]

Hi guys,

I'm somewhat confused regarding the results of this experiment. Am I doing something wrong or are there bugs responsible for the results?

It seems to me like (NotLink (PresentLink ..)) doesn't produce the correct results,

(NotLink (AndLink (PresentLink ...))) produces the same results. (AndLink (NotLink (PresentLink)) doesn't produce any results and neither does (AbsentLink ...).

I hope you can help me understand this.

Best regards and stay safe in these corona times,

Alex

The output:

hungry people

expected: (SetLink (ConceptNode "me"))

(SetLink (ConceptNode "me"))

not hungry people (NotLink (PresentLink  ...) )

expected: (SetLink (ConceptNode "you") (ConceptNode "hungry"))

(SetLink (ConceptNode "you") (ConceptNode "hungry") (ConceptNode "me"))

not hungry people (AndLink (NotLink (PresentLink ...)))

expected: (SetLink (ConceptNode "you") (ConceptNode "hungry"))

(SetLink)

not hungry people (NotLink (AndLink (PresentLink ...)))

expected: (SetLink (ConceptNode "you") (ConceptNode "hungry"))

(SetLink (ConceptNode "you") (ConceptNode "hungry") (ConceptNode "me"))

not hungry people (AbsentLink  ...)

expected: (SetLink (ConceptNode "you") (ConceptNode "hungry"))

(SetLink)

The code (also attached):

    atomspace = AtomSpace()

    initialize_opencog(atomspace)

    me = ConceptNode("me")

    you = ConceptNode("you")

    hungry = ConceptNode("hungry")

    StateLink(me, hungry)

    query = GetLink(

                TypedVariableLink(VariableNode("someone"), TypeNode("ConceptNode")),

                PresentLink(StateLink(VariableNode("someone"), hungry)))

    who_is_hungry = execute_atom(atomspace, query)

    print("\nhungry people")

    print('expected: (SetLink (ConceptNode "me"))')

    print(who_is_hungry)

    query1 = GetLink(

                TypedVariableLink(VariableNode("someone"), TypeNode("ConceptNode")),

                NotLink(PresentLink(StateLink(VariableNode("someone"), hungry))))

    who_isnt_hungry1 = execute_atom(atomspace, query1)

    print("\nnot hungry people (NotLink (PresentLink  ...) )")

    print('expected: (SetLink (ConceptNode "you") (ConceptNode "hungry"))')

    print(who_isnt_hungry1)

    query2 = GetLink(

                TypedVariableLink(VariableNode("someone"), TypeNode("ConceptNode")),

                AndLink(NotLink(PresentLink(StateLink(VariableNode("someone"), hungry)))))

    who_isnt_hungry2 = execute_atom(atomspace, query2)

    print("\nnot hungry people (AndLink (NotLink (PresentLink ...)))")

    print('expected: (SetLink (ConceptNode "you") (ConceptNode "hungry"))')

    print(who_isnt_hungry2)

    query3 = GetLink(

                TypedVariableLink(VariableNode("someone"), TypeNode("ConceptNode")),

                NotLink(AndLink(PresentLink(StateLink(VariableNode("someone"), hungry)))))

    who_isnt_hungry3 = execute_atom(atomspace, query3)

    print("\nnot hungry people (NotLink (AndLink (PresentLink ...)))")

    print('expected: (SetLink (ConceptNode "you") (ConceptNode "hungry"))')

    print(who_isnt_hungry3)

    query4 = GetLink(

                TypedVariableLink(VariableNode("someone"), TypeNode("ConceptNode")),

                AbsentLink(StateLink(VariableNode("someone"), hungry)))

    who_isnt_hungry4 = execute_atom(atomspace, query4)

    print("\nnot hungry people (AbsentLink  ...)")

    print('expected: (SetLink (ConceptNode "you") (ConceptNode "hungry"))')

    print(who_isnt_hungry4)

[Linas Vepstas]

Hmm. Indeed, two of the variations seem buggy. Part of the problem is that the patterns you wrote are ill-formed, and the pattern engine is trying to guess what you meant, and is guessing incorrectly. This "trying to guess what you meant" is a kind-of historical design accident.

What I think you meant was  `(And (Present (Variable "someone")) (...other constraints...))`   but since there is no explicit ` (Present (Variable "someone"))` while the other constraints are negative constraints, the engine (sometimes) implicitly assumes that `(Present (Variable "someone"))` was desired.

The pattern engine is a satisifiability solver, not a classical-logic engine. It has no built-in "law of the excluded middle"; it uses intuitionist logic only.

When you have only negative constraints, then the engine implicitly adds one positive constraint:  `(Present (Variable "someone"))` which is satsified by the entire universe --- it invokes a search over the entire universe, and applies the negative constraints to weed  down the results. Very CPU and RAM-intensive.

By contrast Absent is a positive constraint: there is no search done at all (no wastage of CPU/RAM); the constraint is applied to whatever is given. But since you had no other positive constraints, the range is over the empty set. So the result is the empty set.

The two variants: `(NotLink (PresentLink  ...) )` and (NotLink (AndLink (PresentLink ...))) seem broken, Can you open a bug report for that? 





-- Linas

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