XForms Essentials

Computed Expressions

The preceding section discussed how addressing works in XPath. The language is also capable of performing simple computations that return strings, booleans, or numbers. XForms defines the term Computed Expression to represent this special usage of XPath.

Datatypes in XPath and XForms

The term datatype can get a little confusing during discussions on XForms, since both XPath and XML Schema contain the notion of a datatype—and the two don’t always agree on the details.

XPath expressions have four major datatypes: node-sets, strings, booleans, and numbers. (Note that only the expressions contain the notion of datatypes, and the Data Model itself is datatype free, although early drafts of XPath 2.0 appear to change this) Of these, string and boolean are similar to the XML Schema datatypes of the same name, although XML Schema booleans differ in that they allow both “1” and “true” for true, and “0” and “false” for false. Because of this, hacky workarounds are sometimes needed, as described later. The number datatype in XPath represents a double-precision floating point number under the rules defined by the IEEE (which includes positive and negative infinity, positive and negative zero, and a special ‘not a number’ value), and is thus comparable with the XML Schema datatype double. Node-sets do not have a convenient analog in XML Schema.

One other convention you might run into in the XPath specification is a datatype called “object”, which simply means ‘any datatype’. ...