4. Defining Abstract Modules


This section is normative.

An Abstract Module is a definition of an XHTML module using prose text and some informal markup conventions. While such a definition is not generally useful in the machine processing of document types, it is critical in helping people understand what is contained in a module. This section defines the way in which XHTML abstract modules are defined. An XHTML conforming module is not required to provide an abstract module. However, anyone developing an XHTML module is encouraged to provide an abstraction to ease in the use of that module.

4.1. Syntactic Conventions

The abstract modules are not defined in a formal grammar. However, the definitions do adhere to the following syntactic conventions. These conventions are similar to those of XML DTDs, and should be familiar to XML DTD authors. Each discrete syntactic element can be combined with others to make more complex expressions that conform to the algebra defined here.

element name
When an element is included in a content model, its explicit name will be listed.
Content set
Some modules define lists of explicit element names called content sets. When a content set is included in a content model, its name will be listed.
expr ?
Zero or one instances of expr are permitted.
expr +
One or more instances or expr are required.
expr *
Zero or more instances of expr are permitted.
a , b
Expression a is required, followed by expression b.
a | b
Either expression a or expression b is required.
a - b
Expression a is permitted, omitting elements in expression b.
When an expression is contained within parentheses, evaluation of any subexpressions within the parentheses take place before evaluation of expressions outside of the parentheses (starting at the deepest level of nesting first).
extending pre-defined elements
In some instances, a module adds attributes to an element. In these instances, the element name is followed by an ampersand (&).
Defining the type of attribute values
When a module defines the type of an attribute value, it does so by listing the type in parentheses after the attribute name.
Defining the legal values of attributes
When a module defines the legal values for an attribute, it does so by listing the explicit legal values (enclosed in quotation marks), separated by verical bars (|), inside of parentheses following the attribute name.

4.2. Content Types

Abstract module definitions define minimal, atomic content models for each module. These minimal content models reference the elements in the module itself. They may also reference elements in other modules upon which the abstract module depends. Finally, the content model in many cases requires that text be permitted as content to one or more elements. In these cases, the symbol used for text is PCDATA. This is a term, defined in the XML 1.0 Recommendation, that refers to processed character data. A content type can also be defined as EMPTY, meaning the element has no content in its minimal content model.

4.3. Attribute Types

In some instances, it is necessary to define the types of attribute values or the explicit set of permitted values for attributes. The following attribute types (defined in the XML 1.0 Recommendation) are used in the definitions of the Abstract Modules:

Attribute Type Definition
CDATA Character data
ID A document-unique identifier
IDREF A reference to a document-unique identifier
NAME A name with the same character constraints as ID above
NMTOKEN A name composed of CDATA characters but no whitespace
NMTOKENS Multiple names composed of CDATA characters separated by whitespace
PCDATA Processed character data

4.4. An Example Abstract Module Definition

This section defines a sample abstract module as an example of how to take advantage of the syntax rules defined above. Since this exampple is trying to use all of the various syntactic elements defined, it is pretty complicated. Typical module defintions would be much simpler than this. Finally, note that this module references the attribute collection Common. This is a collection defined in the XHTML Modularization specification that includes all of the basic attributes that most elements need.

4.4.1. XHTML Skiing Module

The XHTML Skiing Module defines markup used when describing aspects of a ski lodge. The elements and attributes defined in this module are:

Elements Attributes Minimal Content Model
resort Common, href (CDATA) description , Aspen+
lodge Common description, (Aspen - lift)+
lift Common, href description?
chalet Common, href description?
room Common, href description?
lobby Common, href description?
fireplace Common, href description?
description Common PCDATA*

This module also defines the content set Aspen with the minimal content model lodge | lift | chalet | room | lobby.