Query the pirate data with Prolog

ClioPatria is primarily a Prolog-based development platform. In this lesson you learn to query the RDF data from the Prolog command line. This is a preparation for the next step, where you learn to create simple Prolog programs.

Asking queries from the terminal is a practical step in constructing and debugging more complex programs.

Basic queries

The loaded RDF can be queried using the predicate rdf/3. This predicate does not perform any form of reasoning: it only matches triples that literally appear in the database. First we need some notation. URIs (resources) are represented as Prolog atoms. E.g., the URI for the event suspicious vessel spotted is represented by the following syntax in Prolog (note the single quotes).

'http://semanticweb.cs.vu.nl/poseidon/ns/instances/etype_suspicious'

This is both unpleasant to type and read. Therefore, ClioPatria extends Prolog to use the prefix notation if a suitable prefix is known. The query below illustrates that Prolog writes answers that contain a URI using the prefix notation. The second query illustrates reusing previous bindings in the interactive toplevel and proves that X was indeed bound to an atom and not to a term <prefix>:<local>.

?- X = 'http://semanticweb.cs.vu.nl/poseidon/ns/instances/etype_suspicious'.
X = poseidon:etype_suspicious.

?- atom($X).
true.

Note that if the <local> part is not a valid Prolog atom, it must be quoted, as illustrated in the query below.

?- rdf(rdfs:'Class', rdfs:comment, Comment).
Comment = literal('The class of classes.').

Literals are represented by a term literal(Value), where Value is either a plain atom, a term type(Type,Value) or a term lang(Lang,Value). rdf/3 also supports a form literal(Query,Value) to match part of a literal.

Exercises

Use rdf/3 to obtain all the properties and their values for the resource poseidon:etype_suspicious.
What happens if we use the query below (note the missing quotes around Class):
```
?- rdf(rdfs:Class, rdfs:comment, Comment).
```
Tip: type 'n' (nodebug) (possibly repeated) to get back to the toplevel. Can you understand what happened?
Use an rdf-query to find how poseidon:etype_suspicious is related to events in the event model.
Combine the above with setof/3 and length/2 to count the number of such events in the database.

Queries that imply reasoning

The RDF libraries contain a number of building blocks that perform partial reasoning on the RDF data. It notably supports rdfs:subPropertyOf using rdf_has/3 and its transitive variant rdf_reachable/3

Exercises

Actor types have the type sem:'ActorType' and are organised in a hierarchy. What is/are the root(s) of this hiearchy? Tip: consider exercise 3.
Use rdf_reachable/3 to enumerate the members of the hiearchies found above. You can use two different RDF predicates for this query. Which ones?

You are now ready to Pack your queries as Prolog predicates.

See also: - Tutorial index