XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (460 page)

• When you want to indicate that an empty sequence is
  not
  allowed.
  
• When the type name is defined in a schema with no target namespace (unless the default function namespace is the null namespace, which would be unusual, and cannot arise in XSLT). When your type names are unprefixed, you have to use the more verbose
  cast
  as
  syntax.
  

The sections that follow describe all the rules for converting a supplied value to a target type. We'll start by considering the rules for converting from a primitive type to another primitive type and then go on to consider how derived types are handled.

Converting between Primitive Types

The type conversions described in this section start with an atomic value that is labeled with a primitive type. For these purposes we consider a primitive type to be either one of the primitive types defined in Part 2 of the XML Schema specification, or the XPath-defined atomic type
xs:untypedAtomic
.. Although in XPath the types
xs:integer, xs:dayTimeDuration
and
xs:yearMonthDuration
are sometimes treated as primitive, in this chapter we will treat them as derived types, which is what they actually are.

For reasons of space, few examples of conversions are given in this chapter. However, in the download files for this book there is a stylesheet,
cast.xsl
, which exercises all the conversions between primitive types. It requires a schema-aware XSLT processor.

The following table lists for each source type, the permitted destination types. The detailed rules for these conversions are then given in the subsequent sections, which for ease of reference are arranged alphabetically according to the type of the source value for the conversion.

Converting from anyURI

Converting from base64Binary

Converting from boolean

Converting from date

Destination Type	Rules
date	The value is returned unchanged.
dateTime	Returns the dateTime representing the instant in time at which the relevant date starts. The timezone (or the absence of a timezone) is retained unchanged. For example, the date 2008-04-04 becomes the dateTime 2008-04-04 T00:00:00 .
gDay	Returns a gDay value containing the same day component and timezone (or absence of a timezone) as the original date.
gMonth	Returns a gMonth value containing the same month component and timezone (or absence of a timezone) as the original date.
gMonthDay	Returns a gMonthDay value containing the same month and day components and timezone (or absence of a timezone) as the original date.
gYear	Returns a gYear value containing the same year component and timezone (or absence of a timezone) as the original date.
gYearMonth	Returns a gYearMonth value containing the same year and month components and timezone (or absence of a timezone) as the original date.
string	Returns the canonical lexical representation of the date, retaining the original timezone. For example, a date with no timezone might be converted to the string 2008-06-19 , while a date in the Pacific timezone might become 2008-06-19-08:00 . The timezone is represented as ±hh:mm , except for UTC which is represented by the single letter Z .
untypedAtomic	Returns the same result as converting to a string, but the result is labeled as untypedAtomic .