---+ 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][</help/source/doc_for?object=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][</help/source/doc_for?object=rdf/3>]] also supports a form
literal(Query,Value) to match part of a literal.

---+++ Exercises

  1. Use [[rdf/3][</help/source/doc_for?object=rdf/3>]] to obtain all
  the properties and their values for the resource
  =|poseidon:etype_suspicious|=.

  2. 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?

  3. Use an rdf-query to find how =|poseidon:etype_suspicious|= is
  related to events in the event model.

  4. 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][</help/source/doc_for?object=rdf_has/3>]] and its
transitive variant
[[rdf_reachable/3][</help/source/doc_for?object=rdf_reachable/3>]]


---++ Exercises

  1. 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][LoadPirates.txt]].

  2. Use [[rdf_reachable/3][</help/source/doc_for?object=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][PiratePredicates.txt]].

@see [[Tutorial index][Piracy.txt]]