XML SCHEMA2 defines facilities for defining simple types to be used in XML Schema as well as other XML specifications. It is influenced by earlier work on datatypes such as ISO 11404.
An XML Schema simple type d is characterised by a value space, V(d), which is a non-empty set, a lexical space, L(d), which is a non-empty set of Unicode strings, and a set of facets, F(d), each of which characterizes a value space along independent axes or dimensions.
XML Schema simple types are divided into disjoint built-in simple types and derived simple types. Derived datatypes can be defined by derivation from primitive or existing derived datatypes by the following three means:
- Derivation by restriction, i.e., by using facets on an existing type, so as to limit the number of possible values of the derived type.
- Derivation by union, i.e., to allow values from a list of simple types.
- Derivation by list, i.e., to define the list type of an existing simple type.
Relevance
In Phase 1, the format must support literals and common functions and operators for at least: text strings (xsd:string), 32-bit signed integers (xsd:int), unlimited-size decimal numbers (xsd:decimal), Boolean values xsd:boolean), and list structures.
In Phase 2, more expressive datatypes and operators are expected.
Impact on Design
[todo]
For More Information
JeffPan and Ian Horrocks. OWL-Eu: Adding Customised Datatypes into OWL. In Proc. of the Second European Semantic Web Conference (ESWC 2005), pages 153-166, 2005. In "Journal of Web Semantics". To appear. An online version is available here.