XQuery

Constructors and Casting

There are two mechanisms in XQuery for changing values from one type to another: constructors and casting.

Constructors

Constructors are functions used to construct atomic values with given types. For example, the constructor xs:date("2006-05-03") constructs an atomic value whose type is xs:date. The signature of this xs:date constructor function is:

xs:date($arg as xs:anyAtomicType?) as xs:date?

There is a constructor function for each of the built-in atomic types (both primitive and derived). The qualified name of the constructor is the same as the qualified name of the type. For the built-in types, constructor names are prefixed with xs to indicate that they are in the XML Schema namespace.

All of the constructor functions have a similar signature, in that they accept an atomic value and return an atomic value of the appropriate type. Because function arguments are atomized, you can pass a node to a constructor function, and its typed value is extracted. If you pass an empty sequence to a constructor, the result will be the empty sequence.

Unlike most other functions, constructor functions will accept arguments of any type and attempt to cast them to the appropriate type.^[*] The argument value must have a type that can be cast to the new type; otherwise, a type error is raised. Values of almost all types can be cast to and from xs:string and xs:untypedAtomic. The specific rules for casting among types are described in "Casting Rules," later in this chapter. ...