Learning to RELAX
In this article, we'll explore some of the more advanced features of the RELAX schema language by using it to create a schema for the XMLNews-Story Markup Language. Although the XMLNews-Story markup language has been superseded by the News Industry Text Format, I've chosen it because it's simple, quite widely used, looks a great deal like HTML, and its RELAX specification will use most of the features we want to focus on.
A RELAX document is enclosed within a <module>
element. The opening tag looks like
<module
moduleVersion="1.2"
relaxCoreVersion="1.0"
targetNamespace=""
xmlns="http://www.xml.gr.jp/xmlns/relaxCore">
The module tag is followed by the <interface>
element which specifies the root element of the document
that this RELAX schema is intended to validate. In the
case of a news story, this is
<interface>
<export label="nitf"/>
</interface>
If you have two types of documents that are the same except for their root element, you can have one RELAX document that will validate either kind of document.
<interface>
<export label="account-receivable"/>
<export label="account-payable"/>
</interface>
After the interface is specified, you specify the types of elements to be validated.
Every element in the target markup language is described by an
<elementRule>, which describes its content, and a
corresponding <tag>.
Each <elementRule> plays
a role in the document's structure and has a
label to which other elements can refer.
An empty element, such as <br/>, is specified thus:
<elementRule role="br" label="br">
<empty/>
</elementRule>
<tag name="br"/>
If the label is omitted, it is presumed to be the same
as the role.
A news story has a <byline>, which in turn includes
a <bytag> element that declares who wrote the story.
The <bytag> element consists of string data.
RELAX specifies this as follows.
<elementRule role="byline">
<ref label="bytag"/>
</elementRule>
<tag name="byline"/>
<elementRule role="bytag" type="string"/>
<tag name="bytag"/>
The type attribute specifies an element's datatype. The valid values
for a datatype include those taken from the XML Document Type
Definition, such as NMTOKEN and ID, as well
as those introduced by XML
Schema Part 2: Datatypes, such as string,
float, nonPositiveInteger, etc.
As in HTML documents, an <nitf> news story element
contains <head> and <body>
elements respectively. In a news story, they are both required.
<elementRule role="nitf" label="nitf">
<sequence>
<ref label="head"/>
<ref label="body"/>
</sequence>
</elementRule>
On the other hand, some elements may contain sub-elements in any
order. News stories, like HTML, can have tables, and their table rows
(<tr>) can contain cells (either <td> or
<th>) in any order:
<elementRule role="tr">
<choice occurs="*">
<ref label="td"/>
<ref label="th"/>
</choice>
</elementRule>
The occurs attribute can be used with a
<choice>, <sequence>, or
<ref> tag. It has three possible values.
| * | occurs zero or more times |
| + | occurs one or more times |
| ? | occurs zero or one times |
You write a news story description list (<dl>),
containin an optional list header, followed by one or more optional
description titles, and required descriptive data entries thus:
<elementRule role="dl">
<sequence>
<ref label="lh" occurs="?"/>
<sequence occurs="+">
<ref label="dt" occurs="?"/>
<ref label="dd"/>
</sequence>
</sequence>
</elementRule>
Some information requires text that isn't between tags. For example,
a news story <location> can look like
The movie was filmed in
<location>
<city>Pekin</city>, a
small city in
<state>Illinois</state>
</location>.
The bold red text above is inside the <location> tag,
but it isn't part of any sub-element.
That makes <location> a mixed element, specified
(in part) like
<elementRule role="location">
<mixed>
<choice occurs="*">
<ref label="city"/>
<ref label="state"/>
<ref label="region"/>
<!-- etc. -->
</choice>
</mixed>
</elementRule>
If you look at the specification for a news story, you'll find that
a paragraph (<p>) is a mixed element that can
contain, among others, <br>,
<person>, <location>, and quoted
phrase (<q>) sub-elements.
Likewise a quoted phrase can contain exactly the same sub-elements. Rather than specify the common sub-elements twice, RELAX allows you to specify a hedge rule:
<hedgeRule label="common.elements">
<choice>
<ref label="br"/>
<ref label="person"/>
<ref label="location"/>
<ref label="q"/>
</choice>
</hedgeRule>
Once a hedge rule is established, other
specifications can refer to it by using the <hedgeRef> tag.
<elementRule role="p">
<mixed>
<hedgeRef label="common.elements" occurs="*"/>
</mixed>
</elementRule>
<elementRule role="q">
<mixed>
<hedgeRef label="common.elements" occurs="*"/>
</mixed>
</elementRule>
Note that the <hedgeRule> can only describe elements;
it cannot be <mixed>. That's why
each <elementRule> has its own
<mixed> tag in the example above.
You may have noticed that the specification says that a quoted phrase can contain another quoted phrase. While this may be unusual in a news story, it's technically possible, and RELAX is not bothered by this at all.
Of course, XML does not consist of elements alone, as we'll see in the next section.
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XML tags can have attributes, and RELAX allows you to specify them in
great detail. A news story, like HTML, can include an
<img> tag which has a required src and
optional width and height attributes. RELAX
treats attributes as part of the tag, so the full specification of an
image is as follows:
<elementRule role="img"> <sequence> <!-- its sub-elements --> </sequence> </elementRule> <tag name="img"> <attribute name="src" required="true" type="string"/> <attribute name="width" type="positiveInteger"/> <attribute name="height" type="positiveInteger"/> </tag>
Notice that RELAX lets you specify that an image's width and height must be positive integers.
Just as it was possible to create a re-usable element specification,
so it is possible to create a set of attributes that can be reused by
many tags. For example, both the table body
(<tbody>) and table header
(<th>) elements have identical attributes for
determining their horizontal and vertical alignment. This makes those
attributes a perfect candidate for an attribute pool.
<attPool role="alignment"> <attribute name="align" type="string"> <enumeration value="left"/> <enumeration value="center"/> <enumeration value="right"/> <enumeration value="justify"/> </attribute> <attribute name="valign" type="string"> <enumeration value="top"/> <enumeration value="middle"/> <enumeration value="bottom"/> <enumeration value="baseline"/> </attribute> </attPool>
And now this attribute pool may be used in multiple tags.
<tag name="tbody"> <ref role="alignment"/> </tag> <tag name="th"> <ref role="alignment"/> <attribute name="rowspan" type="integer"> <minInclusive value="1"/> </attribute> <attribute name="colspan" type="integer"> <minInclusive value="1"/> </attribute> </tag>
There are two things to note about this example:
<th> tag has attributes in addition to
those included via the reference to the attribute pool;Practically everything we've done up to this point is possible with DTD specifications. Now let's examine something that RELAX can do that other methods can't do.
As the definition of a news story currently stands, both a list item
(<li>) and the <body.content>
tag may contain an information block (<block>)
element. An information block may contain, among other things,
<p>, <ul>,
<ol> and <img> elements.
Let's say that we would like to set up news stories so that a block in
the main body content can contain all these elements, but a block
inside of list items may not contain images. To do this, we
first set up the element rule for a <block> in the
body content and the corresponding tag. Note that we have a
label that is different from the role.
<elementRule role="block" label="block-in-content"> <mixed> <choice occurs="*"> <ref label="p"/> <ref label="ul"/> <ref label="ol"/> <ref label="img"/> </choice> </mixed> </elementRule> <tag name="block"/>
We then add another rule for the element that plays the role of a
block, but this rule is labeled for use in a list.
<elementRule role="block" label="block-in-list"> <mixed> <choice occurs="*"> <ref label="p"/> <ref label="ul"/> <ref label="ol"/> </choice> </mixed> </elementRule>
Both of these rules are for elements that play the role of a
“block.” In the definition of the
<body.content> element, we refer to the appropriate
<block> rule.
<elementRule role="body.content"> <ref label="block-in-content"/> </elementRule>
And in the definition of a list item, we refer to the other rule.
<elementRule role="li"> <ref label="block-in-list"/> </elementRule>
The <block> tag will now be validated differently,
depending on the context in which it appears.
A news story can use the <money> tag to indicate
that a number is a monetary amount. Its RELAX definition looks
like
<elementRule role="money" type="decimal"/> <tag name="money"> <attribute name="unit" type="string"/> </tag>
Let's say we'd like to extend this definition so that there are two
types of <money> elements, one for costs and
another for balances. An item's cost is always positive; a company's
balance can be either positive or negative. In other words, we'd like
to be able to specify that
After paying <money unit="dollar" usage="cost">5</money> dollars, the account had a balance of <money unit="dollar" usage="balance">-10</money> dollars.
In this case, rather than using the context to choose the content of a tag, we want to use one of the tag's attributes to determine what content that tag may contain. To accomplish this, we'll need two element rules.
<!-- costs are greater than or equal to zero --> <elementRule role="money-cost" label="money" type="decimal"> <minInclusive value="0"/> </elementRule> <!-- balances are unrestricted --> <elementRule role="money-balance" label="money" type="decimal"/>
In the previous section, we had element rules with the same role and different labels; here we have element rules with the same label and different roles. Now we define elements that refer to the different roles:
<tag name="money" role="money-cost"> <attribute name="unit" type="string"/> <attribute name="usage" type="string"> <enumeration value="cost"/> </attribute> </tag> <tag name="money" role="money-balance"> <attribute name="unit" type="string"/> <attribute name="usage" type="string"> <enumeration value="balance"/> </attribute> </tag>
Thus, the <money> tag plays the
money-cost role when the usage attribute is
cost; and it plays the money-balance role
when its usage is balance.
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RELAX is a powerful markup language that permits you to specify how other XML documents are to be validated. You may, as with other specification methods
Additionally, RELAX gives you the power to
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