Rdf Primer / quote 26 of 55
Typed literals and external datatypes
The common practice in programming languages or database systems is to provide this additional information about how to interpret a literal by associating a datatype with the literal, in this case, a datatype like decimal or integer. [..] An RDF typed literal is formed by pairing a string with a URIref that identifies a particular datatype. [...] Unlike typical programming languages and database systems, RDF has no built-in set of datatypes of its own, such as datatypes for integers, reals, strings, or dates. Instead, RDF typed literals simply provide a way to explicitly indicate, for a given literal, what datatype should be used to interpret it. The datatypes used in typed literals are defined externally to RDF, and identified by their datatype URIs. (There is one exception: RDF defines a built-in datatype with the URIref rdf:XMLLiteral to represent XML content as a literal value. [...] Not all datatypes are suitable for use in RDF. For a datatype to be suitable for use in RDF, it must conform to the conceptual framework just described. This basically means that, given a character string, the datatype must unambiguously define whether or not the string is in its lexical space, and what value in its value space the string represents. For example, the basic XML Schema datatypes such as xsd:string, xsd:boolean, xsd:date, etc. are suitable for use in RDF. However, some of the built-in XML Schema datatypes are not suitable for use in RDF. For example, xsd:duration does not have a well-defined value space, and xsd:QName requires an enclosing XML document context.