XForms Essentials

Going Deep: The XPath Data Model

In the XPath view of things, elements, attributes, text, comments, processing instructions, and even namespaces are represented internally as nodes connected in a tree shape. Some nodes, such as elements, may have child nodes, while others, such as attributes, have no children, as restricted by XML rules. A special node, called the root node, serves as the ultimate ancestor node.

XML Information Set Mapping

XPath 1.0 was completed in November 1999 with a section outlining the XPath Data Model and an appendix defining the mapping to the then-unfinished W3C specification called XML Information Set or “infoset” for short. This formalized description of the XML Data Model, available at http://www.w3.org/TR/xml-infoset/, was completed in October 2001. Subsequently, an errata to XPath, at http://www.w3.org/1999/11/REC-xpath-19991116-errata/, finalized the infoset mapping.

Example 3-2 shows a short XML document, and Figure 3-1 shows how that document would be represented by a tree of nodes.

In this example, note that neither the XML declaration nor the DOCTYPE declaration produce any nodes. Thus, these XML data structures are effectively invisible to XPath and, by extension, XForms. On the other hand, notice how each element node has two namespace nodes attached: one for the xmlns:html declaration on the root element, which applies throughout, and one for the built-in declaration of the xml prefix, as seen in the attribute xml:lang. Even a short document ...