Combining XML Languages
Summary: This module explains how to use XML namespaces and DTDs to combine multiple XML languages in the same document.
XML allows you to
create documents in custom markup languages. But what if you
want to combine markup from multiple languages in the same
document? What if there are one or more tags that exist in both
languages, but with different meanings? You could, for example, have
a
<table> tag in HTML and one
in a language describing office furniture as well. How do you
use these tags unambiguously, without losing functionality?
The solution is to use an extension to XML called
namespaces (See the W3C's recommendation, Namespaces in
XML). A namespace associates a unique global identifier
(usually a URI) with a particular set of tags and their usage
rules. To declare a namespace for a particular tag, set the
xmlns attribute to the value of the unique
identifier.
You can also define a namespace prefix for use in
your document. To do this, use a modified version of the
xmlns attribute. For example, you would
use the attribute
xmlns:foo="http://somewhere.org/foo" to
associate the prefix foo with the namespace
identifier http://somewhere.org/foo. You
can then indicate which tags come from that namespace by adding
the appropriate prefix to each tag. Thus, the
bar tag in foo's namespace would be written
as <foo:bar> and
</foo:bar>.
When you use the default namespace any children of that tag
lacking an explicit prefix will be assumed to have come from the
same namespace. This allows you to define a default namespace
for all of the children of a tag. This is especially useful
when used on the root node, which is the outermost
tag in a document.
Example 1
The namespace identifier for version 0.5 of CNXML is
http://cnx.rice.edu/cnxml. Thus to
declare CNXML as the default namespace for a document, you would
add the attribute
to the
document tag. Similarly, the namespace
identifier for MathML is
http://www.w3.org/1998/Math/MathML, so to
associate the MathML namespace with the prefix
m, add the attribute
xmlns:m="http://www.w3.org/1998/Math/MathML"
to the document tag.
Namespaces allow you to write documents in multiple languages at
the same time.
However, in order for these documents to be valid, they must
have a Document Type Definition (DTD) that allows
tags from one language to exist inside tags from another
language. So if you want to include MathML in a CNXML
document, you would need a DTD that allowed MathML to exist
inside certain CNXML tags. Once that DTD existed, you would
declare the DOCTYPE the same as normal.
Example 2
For just CNXML, the doctype declaration is:
<!DOCTYPE document PUBLIC "-//CNX//DTD CNXML 0.5//EN"
"http://cnx.rice.edu/cnxml/0.5/DTD/cnxml_plain.dtd">
For CNXML and MathML, the doctype declaration is:
<!DOCTYPE document PUBLIC "-//CNX//DTD CNXML 0.5 plus MathML//EN"
"http://cnx.rice.edu/cnxml/0.5/DTD/cnxml_mathml.dtd">
For CNXML and QML, the doctype declaration is:
<!DOCTYPE document PUBLIC "-//CNX//DTD CNXML 0.5 plus QML//EN"
"http://cnx.rice.edu/cnxml/0.5/DTD/cnxml_qml.dtd">
For CNXML, QML, and MathML, the doctype declaration is:
<!DOCTYPE document PUBLIC "-//CNX//DTD CNXML 0.5 plus
MathML plus QML//EN"
"http://cnx.rice.edu/cnxml/0.5/DTD/cnxml_mathml_qml.dtd">
Here is a simple document with both CNXML and MathML. Notice
that the CNXML namespace is the default and the MathML namespace
has the
m prefix.
<?xml version="1.0" standalone="no"?>
<!DOCTYPE document PUBLIC "-//CNX//DTD CNXML 0.5 plus MathML//EN"
"http://cnx.rice.edu/cnxml/0.5/DTD/cnxml_mathml.dtd">
<document xmlns="http://cnx.rice.edu/cnxml"
xmlns:m="http://www.w3.org/1998/Math/MathML"
. . .>
<metadata xmlns:md="http://cnx.rice.edu/mdml/0.4">
<md:version>2.14</md:version>
<md:created>2001/06/29</md:created>
<md:revised>2004-04-19T19:09:52Z</md:revised>
<md:authorlist>
<md:author id="selc">
<md:firstname>Sarah</md:firstname>
<md:surname>Coppin</md:surname>
<md:email>coppin@alumni.rice.edu</md:email>
</md:author>
<md:author id="brentmh">
<md:firstname>Brent</md:firstname>
<md:othername>Michael</md:othername>
<md:surname>Hendricks</md:surname>
<md:email>brentmh@rice.edu</md:email>
</md:author>
</md:authorlist>
<md:maintainerlist>
<md:maintainer id="mizar">
<md:firstname>Christine</md:firstname>
<md:surname>Donica</md:surname>
<md:email>mizar@alumni.rice.edu</md:email>
</md:maintainer>
<md:maintainer id="selc">
<md:firstname>Sarah</md:firstname>
<md:surname>Coppin</md:surname>
<md:email>coppin@alumni.rice.edu</md:email>
</md:maintainer>
<md:maintainer id="jenn">
<md:firstname>Jenn</md:firstname>
<md:othername>A.</md:othername>
<md:surname>Drummond</md:surname>
<md:email>jenn@rice.edu</md:email>
</md:maintainer>
<md:maintainer id="brentmh">
<md:firstname>Brent</md:firstname>
<md:othername>Michael</md:othername>
<md:surname>Hendricks</md:surname>
<md:email>brentmh@rice.edu</md:email>
</md:maintainer>
</md:maintainerlist>
<md:keywordlist>
<md:keyword>XML</md:keyword>
<md:keyword>namespace</md:keyword>
<md:keyword>DTD</md:keyword>
<md:keyword>tutorial</md:keyword>
</md:keywordlist>
<md:abstract>This module explains how to use XML namespaces and DTDs to combine multiple XML languages in the same document.</md:abstract>
</metadata>
<content>
<para id='p1'>
This math says "3y".
<m:math display='inline'>
<m:apply>
<m:times/>
<m:cn>3</m:cn>
<m:ci>y</m:ci>
</m:apply>
</m:math>
</para>
</content>
</document>
So, by combining the use of namespaces and carefully written DTD's,
you can write documents that use multiple XML languages at the same time.
More about this content: Metadata | Version History | Cite This Content
This work is licensed by Sarah Coppin and Brent Hendricks. See the Creative Commons License about permission to reuse this material.
Last edited by Brent Hendricks on Apr 19, 2004 2:09 pm GMT-5.