STEP 5: Dividing large modules

$Id: step5e.html 1.4 2000/02/29 12:03:19 murata Exp $

text by MURATA Makoto

html by NAMBA Ryosuke

Large modules are hard to maintain. STEP 5 introduces a mechanism for dividing a module into several pieces, which can be maintained easier.

1. Why divide modules?

Suppose that we rewrite a DTD containing 200 element types in RELAX. This size is a fairly large, but is not uncommon. For each element type, RELAX needs an elementRule and a tag. If each elementRule and tag requires three lines, the total is 1200 lines. If we write extensive documentation, the total may become 3000 lines or even more. This size is too large to put in a single file.

Even DTD provides external parameter entities so as to divide large DTDs into modules and maintain each module independently. RELAX strongly requires some mechanism for dividing large modules.

2. The include element

In RELAX, a module can reference to another module by the include element. The include element is replaced with the content of the referenced module.

We shall examine an example of include. First, a module to be included is as below:

<module
    moduleVersion="1.2"
    relaxCoreVersion="1.0"
    targetNamespace=""
    xmlns="http://www.xml.gr.jp/2000/relaxCore">

  <interface/>

  <elementRule pred="bar" type="emptyString"/>

  <tag name="bar"/>

</module>

This module contains an elementRule and tag for the element type bar. The interface element is empty. Suppose that this module is stored in bar.rlx.

Next, a module which references to and includes this module is as below:

<module
    moduleVersion="1.2"
    relaxCoreVersion="1.0"
    targetNamespace=""
    xmlns="http://www.xml.gr.jp/2000/relaxCore">

  <interface>
    <export labels="foo"/>
  </interface>

  <elementRule pred="foo">
    <ref label="bar"/>
  </elementRule>

  <tag name="foo"/>

  <include moduleLocation="bar.rlx" />

</module>

This module contains an elementRule and tag for the element type foo. The include at the end of this this module references to bar.rlx via the moduleLocation attribute.

The include element is replaced by the body of the referenced module, which the content of the module element except the interface element. In this example, replacement is done as below:

<module
    moduleVersion="1.2"
    relaxCoreVersion="1.0"
    targetNamespace=""
    xmlns="http://www.xml.gr.jp/2000/relaxCore">

  <interface>
    <export labels="foo"/>
  </interface>

  <elementRule pred="foo">
    <ref label="bar"/>
  </elementRule>

  <tag name="foo"/>

  <elementRule pred="bar" type="emptyString"/>

  <tag name="bar"/>

</module>

3. Non-empty interface elements

In the above example, the interface element of the referenced module is empty. Suppose that an export element is supplied in the interface element.

<module
    moduleVersion="1.2"
    relaxCoreVersion="1.0"
    targetNamespace=""
    xmlns="http://www.xml.gr.jp/2000/relaxCore">

  <interface>
    <export labels="bar"/>
  </interface>

  <elementRule pred="bar" type="emptyString"/>

  <tag name="bar"/>

</module>

In this case, the children of the interface element in the referenced module are attached to the interface element in the referencing module. In this example, the result of replacement is as below:

<module
    moduleVersion="1.2"
    relaxCoreVersion="1.0"
    targetNamespace=""
    xmlns="http://www.xml.gr.jp/2000/relaxCore">

  <interface>
    <export labels="foo"/>
    <export labels="bar"/>
  </interface>

  <elementRule pred="foo">
    <ref label="bar"/>
  </elementRule>

  <tag name="foo"/>

  <elementRule pred="bar" type="emptyString"/>

  <tag name="bar"/>

</module>

4. Summary

STEP 5 makes it easy to maintain large modules. Enjoy and RELAX!