Editing Reification (computer science) (section)

== On Semantic Web ==

=== RDF and OWL ===
In [[Semantic Web]] languages, such as [[Resource Description Framework]] (RDF) and [[Web Ontology Language]] (OWL), a statement is a binary relation. It is used to link two individuals or an individual and a value. Applications sometimes need to describe other RDF statements, for instance, to record information like when statements were made, or who made them, which is sometimes called "[[provenance]]" information. As an example, we may want to represent properties of a relation, such as our certainty about it, severity or strength of a relation, relevance of a relation, and so on.

The example from the conceptual modeling section describes a particular person with <code>URIref person:p1</code>, who is a member of the <code>committee:c1</code>. The RDF triple from that description is
<syntaxhighlight lang="sparql">
  person:p1   committee:isMemberOf   committee:c1 .
</syntaxhighlight>
Consider to store two further facts: (i) to record who nominated this particular person to this committee (a statement about the membership itself), and (ii) to record who added the fact to the database (a statement about the statement).

The first case is a case of classical reification like above in UML: reify the membership and store its attributes and roles etc.:

<syntaxhighlight lang="sparql">
 committee:Membership        rdf:type              owl:Class .
 committee:membership12345   rdf:type              committee:Membership .
 committee:membership12345   committee:ofPerson    person:p1 .
 committee:membership12345   committee:inCommittee committee:c1 .
 person:p2                   committee:nominated   committee:membership12345 .  
</syntaxhighlight>

Additionally, RDF provides a built-in vocabulary intended for describing RDF statements. A description of a statement using this vocabulary is called a reification of the statement. The RDF reification vocabulary consists of the type <code>rdf:Statement</code>, and the properties <code>rdf:subject</code>, <code>rdf:predicate</code>, and <code>rdf:object</code>.<ref name="rdf">{{cite web|url=http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#reification |title=RDF Primer |publisher=W3.org |date= |accessdate=2010-10-09}}</ref>

Using the reification vocabulary, a reification of the statement about the person's membership would be given by assigning the statement a URIref such as <code>committee:membership12345</code> so that describing statements can be written as follows:
<syntaxhighlight lang="sparql">
 committee:membership12345Stat   rdf:type        rdf:Statement .
 committee:membership12345Stat   rdf:subject     person:p1 .
 committee:membership12345Stat   rdf:predicate   committee:isMemberOf . 
 committee:membership12345Stat   rdf:object      committee:c1 .
</syntaxhighlight>
These statements say that the resource identified by the <code>URIref committee:membership12345Stat</code> is an RDF statement, that the subject of the statement refers to the resource identified by <code>person:p1</code>, the predicate of the statement refers to the resource identified by <code>committee:isMemberOf</code>, and the object of the statement refers to the resource <code>committee:c1</code>. Assuming that the original statement is actually identified by <code>committee:membership12345</code>, it should be clear by comparing the original statement with the reification that the reification actually does describe it. The conventional use of the RDF reification vocabulary always involves describing a statement using four statements in this pattern. Therefore, they are sometimes referred to as the "reification quad".<ref name="rdf"/>

Using reification according to this convention, we could record the fact that <code>person:p3</code> added the statement to the
database by
<syntaxhighlight lang="sparql">
  person:p3    committee:addedToDatabase    committee:membership12345Stat .
</syntaxhighlight>
It is important to note that in the conventional use of reification, the subject of the reification triples is assumed to identify a particular instance  of a triple in a particular RDF document, rather than some arbitrary triple having the same subject, predicate, and object. This particular convention is used because reification is intended for expressing properties such as dates of composition and source information, as in the examples given already, and these properties need to be applied to specific instances of triples. 
Note that the described triple <code>(subject predicate object)</code> itself is not implied by such a reification quad (and it is not necessary that it actually exists in the database). This allows also to use this mechanism to express which triples do ''not'' hold.

The power of the reification vocabulary in RDF is restricted by the lack of a built-in means for assigning URIrefs to statements, so in order to express "provenance" information of this kind in RDF, one has to use some mechanism (outside of RDF) to assign URIs to individual RDF statements, then make further statements about those individual statements, using their URIs to identify them.<ref name="rdf"/>

=== In Topic Maps ===
In an [[Topic Maps|XML Topic Map]] (XTM), only a topic can have a name or play a role in an association. One may use an association to make an assertion about a topic, but one cannot directly make assertions about that assertion. However, it is possible to create a topic that reifies a non-topic construct in a map, thus enabling the association to be named and treated as a topic itself.<ref>[http://www.techquila.com/practical_intro.html Practical Introduction into Topic Maps] {{webarchive|url=https://web.archive.org/web/20090203202441/http://techquila.com/practical_intro.html |date=2009-02-03 }}.</ref>

=== ''n''-ary relations ===
In Semantic Web languages, such as RDF and OWL, a property is a binary relation used to link two individuals or an individual and a value. However, in some cases, the natural and convenient way to represent certain concepts is to use relations to link an individual to more than just one individual or value. These relations are called [[n-ary relations]]. Examples are representing relations among multiple individuals, such as a committee, a person who is a committee member and another person who has nominated the first person to become the committee member, or a buyer, a seller, and an object that was bought when describing a purchase of a book.

A more general approach to reification is to create an explicit new class and n new properties to represent an ''n''-ary relation, making an instance of the relation linking the {{var|n}} individuals an instance of this class. This approach can also be used to represent provenance information and other properties for an individual relation instance.<ref>{{cite web|url=http://www.w3.org/TR/swbp-n-aryRelations/ |title=W3C Defining N-ary relations on Semantic Web |publisher=W3.org |date= |accessdate=2010-10-09}}</ref>
<syntaxhighlight lang="turtle">
 :p1
      a       :Person ;
      :has_membership _:membership_12345 .
 _:membership_12345
      a       :Membership ;
      :committee :c1;
      :nominated_by :p2 .
</syntaxhighlight>

=== Vs. quotation ===
It is also important to note that the reification described here is not the same as "quotation" found in other languages. Instead, the reification describes the relationship between a particular instance of a triple and the resources the triple refers to. The reification can be read intuitively as saying "this RDF triple talks about these things", rather than (as in quotation) "this RDF triple has this form." For instance, in the reification example used in this section, the triple:
<syntaxhighlight lang="sparql">
  committee:membership12345   rdf:subject   person:p1 .
</syntaxhighlight>
describing the <code>rdf:subject</code> of the original statement says that the subject of the statement is the resource (the person) identified by the URIref <code>person:p1</code>. It does not state that the subject of the statement is the URIref itself (i.e., a string beginning with certain characters), as quotation would.