Assertions on Simple Types
XSD 1.1 allows assertions on simple types to be defined. The mechanism is to define an
xs:assertion element as a child of the xs:restriction child
of the xs:simpleType element (that is, it acts as an additional facet). The
type must be an atomic type. The value of the test attribute of
xs:assert is an XPath expression.
The expression is evaluated with the value being validated supplied as the value of the
variable $value. This will be an instance of the base type: for example, if
you are restricting from xs:string, it will be a string; if you are
restricting from xs:date, it will be an xs:date; if you are
validating a list of integers, then $value will be a sequence of
integers.
If the effective boolean value of the expression is true, the value is valid. If the
effective boolean value is false, or if a dynamic error occurs while evaluating the
expression, the value is invalid. Currently no diagnostics are produced to indicate why
the value is deemed invalid, other than a statement that the xs:assertion
facet is violated. You can supply a message in a saxon:message attribute:
see saxon:message.
The XPath expression has no access to any part of the document being validated, other than the atomic value of the actual element or attribute node. So the validation cannot be context-sensitive.
The XPath expression may make calls on Java extension functions in the normal way: see Writing extension functions (Java). Allowing call-out to procedural programming languages means that you can perform arbitrary procedural validation of element and attribute values. Take care to disable use of extension functions if validating against a schema that is untrusted.
The following example validates that a date is in the past:
<xs:element name="date"> <xs:simpleType> <xs:restriction base="xs:date"> <xs:assertion test="$value lt current-date()"/> </xs:restriction> </xs:simpleType> </xs:element>The following example validates that a string is a legal XPath expression. This relies on the fact that a failure evaluating the assertion is treated as "false":
<xs:element name="xpath"> <xs:simpleType> <xs:restriction base="xs:string"> <xs:assertion test="exists(saxon:expression($value))" xmlns:saxon="http://saxon.sf.net/"/> </xs:restriction> </xs:simpleType> </xs:element>Note how the in-scope namespaces for the XPath expression are taken from the in-scope
namespaces of the containing xs:assert element.