Pattern Matcher: NotLink AbsentLink not working

[linas]

Since AbsentLink requests that a certain pattern be absent from the atomspace, NotLink AbsentLink should have the semantics that the pattern is present. However, this currently fails. This is marked as an enhancement/feature-request, since it dates back over five years, to the original introduction of optional links in the pattern matcher.

Some examples:

;; print something and return an atom.
(define (sayit) (display "ola\n") (ConceptNode "trigger"))

;; threee slightly different bindlinks
(define fnd (BindLink
     (EvaluationLink
         (PredicateNode "room status")
         (VariableNode "$stat"))
   (ExecutionOutputLink
      (GroundedSchemaNode "scm: sayit") (ListLink))))

(define abs (BindLink
   (AbsentLink
     (EvaluationLink
         (PredicateNode "room status")
         (VariableNode "$stat")))
   (ExecutionOutputLink
      (GroundedSchemaNode "scm: sayit") (ListLink))))

(define nabs (BindLink
   (NotLink (AbsentLink
     (EvaluationLink
         (PredicateNode "room status")
         (VariableNode "$stat"))))
   (ExecutionOutputLink
      (GroundedSchemaNode "scm: sayit") (ListLink))))

If one runs this, one gets

(cog-bind fnd)  ;;; prints nothing, returns empty set.
(cog-bind abs) ;;; prints 'ola', returns 'trigger'
(cog-bind nabs)  ;; throws C++ exception

Now, define the following, and run again:

(EvaluationLink (PredicateNode "room status") (ConceptNode "occupied"))
(cog-bind fnd) ;;; prints 'ola', returns 'trigger'
(cog-bind abs) ;;; prints nothing, returns empty set
(cog-bind nabs) ;; throws c++ exception.

Hint: use cog-delete to delete the EvaluationLink, if you want to run this again.

This issue is closely related to issue #217 which says that AbsentLink is the same as NotLink-ChocieLink. It will probably be hard to implement, because the concept of absence can only be proved in the affirmative, not the negative: AbsentLinks are usual virtual links, that are evaluated only after all of thier arguments are already grounded.

This is marked "enhancement" because it is low-priority, it is not needed for any current projects; it is not blocking any work. It also has a reasonable work-around: just use an arity-1 ChoiceLink!

[linas]

Fixed in #2174

[linas]

Fixed in #2174