The known Keyword

Last revised 6/4/2021. Contact us.

The SADL grammar include the keyword "known". It is useful in rules, queries, and tests. it is most often used as a negated concept, as in this query.

Ask: select p where p is a Person and age of p is not known.

This query asks for all instances of Person where age is not known, that is where there is no triple in the knowledge base with that instance of Person as subject and age as predicate.

However, it is possible to use it in a non-negated statement. For example,

Test: fprop of MyFoo3 is known.

This test will pass if there is a triple in the knowledge base with MyFoo3 as subject and fprop as predicate, regardless of the object of the triple.

The translation of known and not known in rules, queries, and tests depends upon usage and query engine or reasoner. Here are some examples.

  1. In a SPARQL query, "age of p is not known" translates into "OPTIONAL {?p <http://sadl.org/Issue442.sadl#age> ?v2} . FILTER (!bound(?v2)) ".
  2. In a rule, "age of p is not known" translates, when Jena is the reasoner, to "noValue(?p, http://sadl.org/Issue442.sadl#age)" where noValue is a Jena built-in function.