STEP 10: tag embedded in elementRule

$Id: step10.sdoc 1.8 2000/11/01 13:46:38 murata Exp $

In this section, we consider embedding of tag elements in elementRule elements.

Describing attributes and hedge models together

In STEPs 0 thru 9, attributes and tag names are separated from hedge models. Attributes and tag names are described by tag and attPool elements, while hedge models are described by elementRule and hedgeRule elements. An elementRule references to a tag via a role, and the tag may in turn reference to attPool elements.

When an elementRule and a tag is so closely related, it may be convenient to merge them into a single element rather than separating them.

As an example of elementRule-tag separation, we duplicate an example in STEP 8 below.

<!-- Case 1: type="integer" -->

<tag name="val" role="val-integer">
  <attribute name="type" type="NMTOKEN" required="true">
    <enumeration value="integer"/>
  </attribute>
</tag>

<elementRule role="val-integer" label="val" type="integer"/>

<!-- Case 2: type="string" -->

<tag name="val" role="val-string">
  <attribute name="type" type="NMTOKEN" required="true">
    <enumeration value="string"/>
  </attribute>
</tag>

<elementRule role="val-string" label="val" type="string"/>

Suppose that roles val-integer and val-string are referenced from these two elementRule elements only. Rather than introducing two names val-integer and val-string for referencing, authors might want to directly embed tag elements within elementRule elements.

<!-- Case 1: type="integer" -->

<elementRule label="val" type="integer">
  <tag>
    <attribute name="type" type="NMTOKEN" required="true">
      <enumeration value="integer"/>
    </attribute>
  </tag>
</elementRule>

<!-- Case 2: type="string" -->

<elementRule label="val" type="string">
  <tag>
    <attribute name="type" type="NMTOKEN" required="true">
      <enumeration value="string"/>
    </attribute>
  </tag>
</elementRule>

An advantage of this style is that roles do not need names. Before this rewrite, we needed names which are different from the tag names or labels. Omission of these names enhance readability.

Some people find it attractive to describe attributes and hedge models together. For example, points with the x-coordinate and y-coordinate can be represented in two alternative manners. The first example uses attributes, while the second uses elements. Their differences are minor and can be easily rewritten from each other.

<elementRule label="point" type="emptyString">
  <tag>
    <attribute name="x" type="integer"/>
    <attribute name="y" type="integer"/>
  </tag>
</elementRule>

<elementRule label="point">
  <tag/>
  <sequence>
    <element name="x" type="integer"/>
    <element name="y" type="integer"/>
  </sequence>
</elementRule>

An elementRule containing a tag may not have the role attribute. The label attribute is mandatory, instead.

An embedded tag may not have the role attribute. The name attribute is permitted, but it is not present in this example.

Handling of embedded tag elements

An embedded tag element is moved from the elementRule and placed as a sibling element. We show how the first example in this STEP is handled.

<elementRule label="val" type="integer">
  <tag>
    <attribute name="type" type="NMTOKEN" required="true">
      <enumeration value="integer"/>
    </attribute>
  </tag>
</elementRule>

First, we generate a role that does not conflict with any other role. In this example, we generate role val$1.

Next, we move the embedded tag element from the elementRule and place as a sibling element. We then add the role attribute and specify the generated role as the attribute value.

Only when this tag element does not have the name attribute, we introduce this attribute. As the attribute value, we use the value of the label attribute of the elementRule element. In this example, we specify "val" as the value of the name attribute.

Finally, we add the role attribute to the elementRule and specify the role generated above.

<elementRule label="val" type="integer" role="val$1">
</elementRule>

<tag name="val" role="val$1">
  <attribute name="type" type="NMTOKEN" required="true">
    <enumeration value="integer"/>
  </attribute>
</tag>

Summary

To describe elements and attributes together, embedded tag elements provides concise and comprehensible description. Enjoy and RELAX!