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- From: firstname.lastname@example.org (Ron Bourret)
- To: email@example.com
- Date: Wed, 23 Sep 1998 01:04:29 +0200
Here is the long rumored section 4, which discusses the interaction of XSchema documents and
DTDs and how to convert between the two. An HTML version will be available in a
day or two. Please send comments to the list or directly to me
-- Ron Bourret
4 XSchema Documents and DTDs
An XSchema document is related to two different DTDs: the DTD of the XSchema
document itself and the DTD of the document described by the XSchema document.
This section discusses the relationship of XSchema documents to these DTDs and
describes what conversions are possible between the XSchema document and the
latter DTD. There is no requirement that either DTD actually exist.
4.1 DTDs in XSchema Documents
An XSchema document may include a DTD as an internal subset, external subset, or
both. If included, this DTD must include all of the markup declarations in
Appendix B, "XSchema DTD." It may also include additional markup declarations,
such as declarations of elements to be used under the More element.
The main reason to include a DTD in an XSchema document is so an XSchema-unaware
XML parser can supply default attribute values and determine the system and
public identifiers of notations and unparsed general entities. Default attribute
values are used in the XSchema DTD defined in Appendix B. Notations and unparsed
general entities can be used by user-defined elements under the More element.
Secondary reasons for including a DTD in an XSchema document are to declare
parsed entities (see Section 5.2.1, "Parsed Entities in XSchema Documents") and
to allow the document to be validated by XSchema-unaware software.
4.2 DTDs in Documents Described by XSchema Documents
A document described by an XSchema document may include a DTD as well as
processing instructions that refer to XSchema documents (see section 5.1.1,
"XSchema Processing Instruction"). This DTD can describe the same information as
the XSchema documents as well as additional information.
The main reason to include a DTD in a document described by an XSchema document
is so an XSchema-unaware XML parser can supply default attribute values and
determine the system and public identifiers of notations and unparsed general
entities. Secondary reasons are so that the document can be used with both
XSchema-aware and -unaware software, to define the root element in the document,
and to declare parsed general entities.
If an XML document includes both a DTD and processing instructions that refer to
XSchema documents, it is the responsibility of the document author to ensure
that the information common to both is the same. If the common information is
different, it might not be possible to use the document with both XSchema-aware
and -unaware software. For example, it might not be possible to validate the
document against both the DTD and the XSchema documents.
If an XSchema processor is built on top of an XML parser, the XSchema processor
is not required to process the DTD of the XML document. If an XSchema processor
also functions as an XML parser, it is required to process the DTD only to the
extent required of a non-validating parser.
4.3 Converting Between XSchema Documents and DTDs
Schema information can be converted between XSchema documents and DTDs, although
some information may be lost. Most logical information (such as element and
attribute declarations) can be converted from DTDs to XSchema documents, while
some logical information (such as attribute declarations not assigned to
elements) cannot be converted from XSchema documents to DTDs. In general,
physical information (such as parsed entity declarations and use, the order of
declarations, and the distribution of declarations among different files) either
cannot be converted or is converted only at the option of the converter.
4.3.1 Converting DTDs to XSchema Documents
The following DTD structures must be converted to the corresponding XSchema
* Element, attribute, notation, and unparsed entity declarations. The order of
the resulting elements, including whether attribute declarations are placed
inside element declarations, is the choice of the converter.
* Namespace prefixes in element and attribute declarations. These must be
stripped from the element or attribute name and stored in the appropriate
attribute (prefix or ElementPrefix). The converter may prompt the user for the
URI of the namespace to be stored in the corresponding ns or ElementNS
The following DTD structures may be converted to the corresponding XSchema
structures or discarded:
* Comments. These may be converted to Doc elements. The position of resulting
Doc elements in the XSchema document is the choice of the converter. For
example, a converter might place comments in a Doc element inside the following
element, attribute, notation, or unparsed entity declaration.
* Parameter entity declarations and use. These may be converted to parsed
general entity declarations and use.
The following DTD structures cannot be converted to XSchema structures because
such structures do not exist:
* Duplicate attribute and unparsed entity declarations.
* Parsed general entity declarations and use.
* Conditional sections in external parameter entities.
QUESTION: What happens to the following:
* Processing instructions
* Text encoding declarations
4.3.2 Converting XSchema Documents to DTDs
The following XSchema structures must be converted to the corresponding DTD
* All information in element, notation, and unparsed entity declarations and
attribute declarations that apply to a particular element, except as noted
elsewhere. The order of the resulting declarations is the choice of the
* Namespace prefixes declared in prefix and ElementPrefix attributes. These must
be prepended to the element or attribute name.
The following XSchema structures may be converted to the corresponding DTD
structures or discarded:
* Doc elements. These may be converted to comments. The position of resulting
comments in the DTD is the choice of the converter.
* Parsed entities declared in the DTD of the XSchema document. These may be
converted to parsed general entities or parameter entities as appropriate.
The following XSchema structures cannot be converted to DTD structures because
such structures do not exist:
* More elements, AttDef and AttGroup elements that do not apply to a particular
element, and Model and Enumeration elements nested directly beneath an XSchema
* All id attributes, all attributes of the XSchema element except for prefix,
all ns and ElementNS attributes, and the Root attribute of the ElementDecl
* Nesting of schema information provided by nested XSchema elements.
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