XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (458 page)

The rule for
SingleType
is confusing at first sight. It means that the
AtomicType
may optionally be followed by a question mark.

In all these cases, the
AtomicType
must correspond to the name of an atomic type (that is, a simple type that is not a list type or a union type) that is present in the static context for the XPath expression. Most commonly, this will be one of the built-in types such as
xs:integer
or
xs:date
, but it can also be a user-defined type. The type name is written as a
QName
, and its namespace prefix must therefore have been declared to reference the
targetNamespace
of the schema in which the type is defined. If the name has no prefix, the default namespace for elements is used; in XSLT, this will be the null namespace, unless the
xpath-default-namespace
attribute has been set to identify a different namespace.

A question mark after the type name means that an empty sequence is allowed as the value. For example, the expression
@A
cast
as
xs:integer
will fail if the attribute
A
does not exist, but the expression
@A cast as xs:integer?
will succeed, returning an empty sequence.

The
castable
as
expression returns
true
if the corresponding
cast
as
expression would succeed, and
false
if the corresponding
cast
as
expression would fail. For example, the string
2009-02-29
is not a valid date, so the expression
“2009-02-29”
castable
as
xs:date
returns
false
.