RDF Resource Objects

In RDF graph model, a node is represented by either resource nodes or literals. A resource node is either a named node, which has a globally unique URI, or a blank node, which has no URI. A literal is a kind of string or number, or an instance of XMLLiteral. Figure 4.1, which is taken from RDF/XML Syntax Specification, shows an example of RDF graph. In the figure, a rectangle represents a literal and an ellipse represents an anonymous blank node or a resource node that contains a URI inside. In SWCLOS, Figure 4.1 is expressed as follows in S-expression.

Whereas rdf:about in SWCLOS looks like a property in RDF, it is not a property in RDF but an XML attribute that takes a place in RDF/XML format resource definition. The ex:editor, ex:homePage, ex:fullName, and dc:title are RDF properties in QName representation for the corresponding URIs on the edges in the figure. The terminology ‘rdf:Description’ that appears in S-expression for RDF graph representation just denotes a sequence for RDF description. With addForm function, this statement turns out a resource object. See the syntax in BNF described below. Precisely, the assertion above produces three CLOS objects, that is, two named resource objects and one anonymous resource object, in addition to two lisp strings.

4.2 Adding and Getting a Resource Form

SWCLOS allows users to add such a form as mentioned in the previous section, that is, nested object-centered representation of RDF graph into the lisp environment. However, if a resource name is not supplied, the created object is anonymous. See the following example. If you type the lines as the demonstration, a dialogue window will pop up twice at the line 5 for package name of uri “http://purl.org/net/” and “http://www.w3.org/TR/”. Please note to push ‘cancel’ button in this case. If you gave some package name for the query, SWCLOS would create the package and make a symbol, e.g., package-name-you-input:dajobe. Then, you would obtain a different look at line 6, where the symbol name package-name-you-input:dajobe would appear instead of ‘(rdf:Description (rdf:about "http://purl.org/net/dajobe/"))’ for the anonymous object.

Do not care about those warnings at line 5 at this time. It will be explained at Section 5.5. In this example, an RDF graph shown in Figure 4.1 is inputted through a form in S-expression in SWCLOS, and three objects are created in the environment. You can backwardly generate the form in RDF graph from the subjective resource object using get-form in the lisp top level.

The value of property ‘ex:editor’ is anonymous. The value of ‘ex:homePage’ is also anonymous, because SWCLOS did not obtain the name by your cancellation for its package name. Note that SWCLOS cannot identify anonymous objects by name, even if they have the URI in rdf:about attribute. (However, you can get anonymous objects through corresponding URIs with function iri-value.)

Function addForm interprets a form according to the following BNF syntax, where {A--Z} means any character from A to Z, and {SomeNode - element} means the possibility of SomeNode minus element. Also note ‘xyz’ means a sequence of character x, y, and z. #\x means a character x.

typically returns a resource object for form. In the top level, role is not supplied. Otherwise, role must be supplied in recursive call with form as role's filler.

When form is a lisp number, a lisp string, an instance of rdf:inLang, and an instance of rdfs:Literal, it is returned.
When form is a uri, it is returned if role is null (at top level), rdf:about, rdfs:isDefinedBy, or the role's range is xsd:anyURI or its subtype, else the uri is converted to the QName symbol and addForm is recursively called with the converted QName symbol.
When form is a symbol, its resource object (symbol value) is returned if it exists, else the resource is created using the range entailment rule.
When form is a resource object, it is returned.
If form is a lang form, the language environment is changed to the designated language. The scoping of language environment is inside this function calling.
If form is a lang-tag form, an rdf:inLang structure instance is created and returned.
If form is a type-tag form, then datum is interpreted as type-tag if possible and returns the result, otherwise signals an error.
When this function interprets a slot form or ‘(role filler)’, it recursively calls itself with parameter filler for form and role for role, and returns the result.
Otherwise, this function recursively calls for each of slot forms and the list of evaluated slots is obtained, then function addObject is invoked for the class(es) that is/are computed from type-tag and entailment rules with the evaluated slots. The result of addObject is returned.

A user usually do not use this function directly, instead a user should use defIndividual or defConcept (defResource) or defProperty macro for resource definition.
See also addForm in documentation in doc folder.

4. RDF Resource Objects

4.1 Resource Objects and the Syntax in S-expression

4.2 Adding and Getting a Resource Form