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This specification defines elements and attributes that can be used as annotations in [RELAX NG] schemas. The purpose of these annotations is to support some of the features of XML 1.0 DTDs that are not supported by RELAX NG.
This is a working draft constructed by the editors. It is not an official committee work product and may not reflect the consensus opinion of the committee. Comments on this document may be sent to email@example.com.
RELAX NG [RELAX NG] provides an annotation capability. In a RELAX NG schema, RELAX NG-defined elements can be annotated with child elements and attributes from other namespaces. The goal of this specification is to facilitate transition from XML 1.0 DTDs to RELAX NG schemas by defining annotations that support some of the features of XML 1.0 DTDs that are not supported by RELAX NG.
RELAX NG itself performs only validation: it does not change the infoset [XML Infoset] of an XML document. Most of the features of XML 1.0 DTDs that are not supported by RELAX NG involve modification to the infoset. In XML 1.0, validation and infoset modification are combined in a monolithic XML processor. It is a goal of this specification to provide a clean separation between validation and infoset modification, so that a wide variety of implementation scenarios are possible. In particular, it should be possible to make the infoset modifications implied by the annotations either before performing RELAX NG validation or after performing RELAX NG validation or without performing RELAX NG validation at all. It should also be possible for an implementation of this specification not to modify the infoset at all and instead provide the application with a description of the modifications implied by the annotations, independently of any particular instance.
This specification does not provide any support for features of XML 1.0 DTDs, such as entity declarations, that cannot be cleanly separated from validation.
In an XML 1.0 document that is valid with respect to a DTD, each element or attribute in the instance has a unique corresponding element or attribute declaration in the DTD. With RELAX NG this is not always the case: it may be ambiguous which element or attribute pattern any particular element or attribute in the instance matches. In addition, it is non-trivial to determine when a RELAX NG schema is ambiguous. A further complication is that even when cases where it is not ambiguous, it may require multiple passes or lookahead to determine which element or attribute pattern a particular element or attribute matches. Detecting this situation is also non-trivial.
Some features of XML 1.0 DTDs, in particular default attribute values and ID/IDREF/IDREFS validation, depend crucially on this unambiguous correspondence between elements or attributes in the instance and their corresponding declarations. In order to support these features by means of annotations on RELAX NG patterns, it is therefore necessary to impose restrictions on the use of such annotations. The goals in framing these restrictions were as follows:
The elements and attributes defined in this specification have the namespace URI.
Examples in this specification follow the convention of using the prefix a to refer to this namespace URI.
The following DTD
<!DOCTYPE employees [ <!-- A list of employees. --> <!ELEMENT employees (employee*)> <!-- An individual employee. --> <!ELEMENT employee (#PCDATA)> <!ATTLIST employee id ID #REQUIRED manages IDREFS #IMPLIED managedBy IDREF #IMPLIED country (US|JP) "US" > ]>
could be translated to the following RELAX NG schema with annotations:
<element name="employees" xmlns="http://relaxng.org/ns/structure/0.9" xmlns:a="http://relaxng.org/ns/annotation/0.9"> <a:documentation>A list of employees.</a:documentation> <zeroOrMore> <element name="employee"> <a:documentation>An individual employee.</a:documentation> <attribute name="id" a:attributeType="ID"> <data type="token"/> </attribute> <optional> <attribute name="manages" a:attributeType="IDREFS"> <list> <oneOrMore> <data type="token"/> </oneOrMore> </list> </attribute> </optional> <optional> <attribute name="managedBy" a:attributeType="IDREF"> <data type="token"/> </attribute> </optional> <optional> <attribute name="country" a:defaultValue="US"> <choice> <value>US</value> <value>JP</value> </choice> </attribute> </optional> <text/> </element> </zeroOrMore> </element>
This specification defines three features:
Conformance is defined separately for each feature. A conformant implementation can support any combination of features. There are also two levels of conformance.
A conformant implementation may support different features at different levels.
A conformant implementation may be an integral part of a RELAX NG validator or may be a separate software module.
An a:defaultValue attribute on a RELAX NG attribute element specifies the default value for the attribute.
If an attribute element has an a:defaultValue attribute, then, after schema simplification,
<define name="ln1"> <element> nc1 p1 </element> </define>
competes with a definition
<define name="ln2"> <element> nc2 p2 </element> </define>
if there is a name n that belongs to both nc1 and nc2.
The a:defaultValue annotation implies a modification of the infoset that adds attribute information items for omitted attributes.
Define this more precisely.
An a:attributeType attribute on a RELAX NG attribute element must have the value ID, IDREF or IDREFS. It specifies the XML 1.0 attribute type of the attribute and corresponds to the [attribute type] infoset property.
Leading and trailing whitespace is ignored in the value of the a:attributeType element during schema simplification.
If an attribute element has an a:attributeType attribute, then, after schema simplification,
<attribute> nc1 p1 </attribute>
<attribute> nc2 p2 </attribute>
compete if and only if the containing definitions compete and there is a name n that belongs to both nc1 and nc2. Note that a definition competes with itself.
An instance is valid with respect to the a:attributeType attributes in the schema if the attribute values in the instance declared by a:attributeType attributes in the schema to be of type ID, IDREF or IDREFS meet the validity constraints specified in [XML 1.0] for values of that type, after normalizing the values by applying the normalizeWhiteSpace function defined in [RELAX NG].
The a:attributeType annotation implies a modification of the infoset that changes the [attribute type] property of attribute information items to ID, IDREF or IDREFS and modifies the [normalized value] by applying the normalizeWhiteSpace function.
Define this more precisely.
The a:documentation element can be used to specify human-readable documentation. The functionality provided by an a:documentation element in a RELAX NG schema would be provided by a comment in an XML 1.0 DTD. An a:documentation element must not contain any elements. It can have any attributes whose namespace URI is neither the empty string, the RELAX NG namespace URI nor the RELAX NG annotation namespace URI. In particular, it may have an xml:lang attribute.
The documentation specified in an a:documentation element applies to the parent of the a:documentation element. To apply documentation to a value element, wrap the value element in a group element. To apply documentation to a name element, wrap the name element in a choice element.
A RELAX NG element may have multiple a:documentation child elements, but all a:documentation child elements must precede all child elements from the RELAX NG namespace.
James Clark, Makoto MURATA, editors. RELAX NG Specification. OASIS, 2001.
Tim Bray, Jean Paoli, and C. M. Sperberg-McQueen, Eve Maler, editors. Extensible Markup Language (XML) 1.0 Second Edition. W3C (World Wide Web Consortium), 2000.
John Cowan, Richard Tobin, editors. XML Information Set. W3C (World Wide Web Consortium), 2001.