FilterLink

The FilterLink is a proposed (unimplemented) atom type that performs filtering of a set, to reject set elements that do not match a pattern. In many ways, it behaves like GetLink, except that it does not search the entire atomspace; instead, it only searches it's input set.

A minor variation of this idea is presented at the bottom of the PutLink article. In that case, filtering and substitution is performed in one step, with the result behaving like a BindLink (instead of a GetLink, as described here).

Another variation of the FilterLink can be found in the MapLink, which acts as a dual to PutLink: the MapLink un-beta-reduces expressions, extracting values from them. It performs type checking as it does so, and so also implements a kind-of filtering.

It seems like it might be possible to implement FilterLink entirely with MapLink ... in which case, FilterLink is not really needed?

Definition

Filters could be specified as shown below, or by using a SignatureLink.

The FilterLink would have the form

FilterLink
   <filter definition>
   SetLink
      <set of items to be filtered>

Execution of the FilterLink would return a SetLink of everything that passed through the filter.

Thus, for example:

FilterLink
  EvaluationLink           # This defines the filter
     PredicateNode "foo"
     ListLink
        Variable $x
        Variable $y
  SetLink                  # This is the set to be filtered
     EvaluationLink
        PredicateNode "foo"
        ListLink
           Atom A
           Atom B
     EvaluationLink
        PredicateNode "bar"
        ListLink
           Atom C
           Atom D

would extract the values of A and B from the "foo" clause, but ignore the "bar" clause, because it does not match the input filter.

Signatures as filters

Note that the variable declarations in the filter above play no significant role. Thus, it might be better to define filters with SignatureLinks. Thus, the above example can be written as:

FilterLink
   SignatureLink
      EvaluationLink           # This defines the filter
         PredicateNode "foo"
         ListLink
            TypeNode "ConceptNode"
            TypeNode "ConceptNode"
  SetLink                  # This is the set to be filtered
     EvaluationLink
        PredicateNode "foo"
        ListLink
           ConceptNode A
           ConceptNode B
     EvaluationLink
        PredicateNode "bar"
        ListLink
           ConceptNode C
           ConceptNode D
     EvaluationLink
        PredicateNode "foo"
        ListLink
           WordNode E
           WordNode F

Notice that the variable parts in the above expression are now type-constrained to be ConceptNodes. Thus, as a result, the third input, "foo E F" would be rejected because E and F are WordNodes, not ConceptNodes.