xml-dev - XSchema Spec - Content Model Declarations (Section 2.3), Draft 7

["Simon St.Laurent" <simonstl@simonstl.com>]

Here is what I hope to be the last round of revisions for the content model
section.  The Model element can now include Model subelements, as can Seq
and Choice.

As always, a prettier HTML version of this will be posted shortly at
http://purl.oclc.org/NET/xschema.

Simon St.Laurent
Dynamic HTML: A Primer / XML: A Primer / Cookies 

2.3 Content Model Declarations

Content model declarations are made within Model sub-element of the
declaration for the element to which they apply. 
Reference, Mixed, Choice, and Sequence models may appear inside XSchema
elements for reusability, documentation, and reference, but will need to be
linked to particular element declarations through mechanisms not yet
defined (most likely XLink). All content model declarations have an
optional id value for reference.

The Model element holds the content model for an element.

<!ELEMENT Model (Doc?, More?, (Ref | Choice | Seq | Empty | Any | PCData |
Mixed | Model))>
<!ATTLIST Model 
       id ID #IMPLIED>

Model elements are pure containers, and act much like parentheses in XML
1.0 DTD declarations.  If a Model element used as a part of a declaration
contains content that is inappropriate to that declaration (for instance, a
Mixed model in a Choice or Seq model), the processor should halt and signal
a fatal error.

2.3.1 Empty Content Model

The simplest content model is empty, which indicates that the parent
element has no sub-elements and no character data content. The Empty
element indicates that an element is empty.

<!ELEMENT Empty EMPTY>
<!ATTLIST Empty
    id ID #IMPLIED>

For example, to declare the Species element shown in the previous section
empty, use the following XSchema declaration:

<ElementDecl name="Species">
  <Model>
    <Empty/>
  </Model>
</ElementDecl>
This would not allow the Species element to contain any text or sub-elements.

2.3.2 Any Content Model

The Any content model, which allows the element to contain parsed character
data or any other elements as content, is equally simple:

<!ELEMENT Any EMPTY>
<!ATTLIST Empty
    id ID #IMPLIED>

Using the Any content model is much like using the Empty content model. To
declare that the Species element had a content model of any, use the
following declaration:
<ElementDecl name="Species">
  <Model>
    <Any/> 
  </Model>
</ElementDecl>

This allows the Species element to contain text and any sub-elements an
author desired.

2.3.3 PCData Content Model

The PCData content model, which allows the element to contain only parsed
character data, is also represented by a single empty element.

<!ELEMENT PCData EMPTY>
<!ATTLIST Empty
    id ID #IMPLIED>

Using the PCData content model is much like using the Empty and Any content
models. For example, to assign the Species element a content model of
PCData, use the following declaration:

<ElementDecl name="Species">
  <Model>
    <PCData/> 
  </Model>
</ElementDecl>

This allows the Species element to contain text, but no sub-elements.

2.3.5 Reference Content Model

The Reference content model allows an element to specify other elements
which it may contain, as well as their quantity. Ref elements identify the
element to be contained, as well as the frequency with which it must appear:

<!ELEMENT Ref EMPTY>
<!-- Element references the name in an ElementDecl element -->
<!ATTLIST Ref
    id ID #IMPLIED
    Element NMTOKEN #REQUIRED
    Frequency (Required | Optional | ZeroOrMore | OneOrMore) 'Required'>

The Element attribute must refer to the Name attribute of an ElementDecl
element elsewhere in the XSchema document. An ElementDecl element may
contain at most one Ref element. 
The Frequency attribute controls the number of referenced elements that may
occur. To define content models that permit or require the use of more
elements, the Any, Mixed, Choice, or Sequence content models should be used
as appropriate.

To declare that the Species element may contain a single CommonName
element, and nothing else, use the following declaration:

<ElementDecl name="Species">
  <Model>
    <Ref Element="CommonName" Frequency="Required"/> 
  </Model>
</ElementDecl>

This requires the Species element to contain a single CommonName element.
To make the CommonName element optional - though it may still only appear
once, set the Frequency attribute to 'Optional':

<ElementDecl name="Species">
  <Model>
    <Ref Element="CommonName" Frequency="Optional"/> 
  </Model>
</ElementDecl>

Optional is the equivalent of the ? occurrence indicator in XML 1.0 DTDs.

To require the Species element to contain at least one but possibly
multiple CommonName elements, set the Frequency attribute to 'OneOrMore':

<ElementDecl name="Species">
  <Model>
    <Ref Element="CommonName" Frequency="OneOrMore"/> 
  </Model>
</ElementDecl>

OneOrMore is the equivalent of the + occurrence indicator in XML 1.0 DTDs.

Finally, to allow the Species element to contain any number (including
zero) of CommonName elements, set the Frequency attribute to 'ZeroOrMore':

<ElementDecl name="Species">
  <Model>
    <Ref Element="CommonName" Frequency="ZeroOrMore"/> 
  </Model>
</ElementDecl>
ZeroOrMore is the equivalent of the * occurrence indicator in XML 1.0 DTDs.

2.3.6 Mixed Content Model

Mixed content model allows the unordered use of different element types and
character data. Content within an element that uses a mixed declaration
must be PCData or one or more of the elements referenced by Ref elements
nested within the Mixed declaration. Only Ref elements can be nested under
an Mixed element; the PCData content is inherent in the Mixed content model.

<!ELEMENT Mixed (Ref+)>
<!ATTLIST Mixed
    id ID #IMPLIED
    Frequency (ZeroOrMore) #FIXED "ZeroOrMore">

To declare that the Species element may contain a mix of PCData, CommonName
elements, LatinName elements, and PreferredFood elements in any order, use
the following declaration:

<ElementDecl name="Species">
  <Model>
    <Mixed>
         <Ref Element="CommonName"/>
         <Ref Element="LatinName"/>
         <Ref Element="PreferredFood"/>
    </Mixed>
  </Model>
</ElementDecl>

The XSchema processor should ignore any frequency attributes in Ref
elements that appear as subelements of the Mixed element.

2.3.7 Choice Content Model

The Choice content model allows for either-or inclusions of elements and
groups of elements. The Choice content model represents groups of element
content possibilities and must contain at least two sub-elements.
Situations where only one element is needed should use the Ref content
model instead of Choice. The Choice element may indicate a frequency,
allowing the content model defined by the Choice model to appear one, one
or zero, one or more, or zero or more times.
 
<!-- A Choice must have two or more children -->
<!ELEMENT Choice ((Seq | Ref | Model), (Seq | Ref | Model)+)>
<!ATTLIST Choice 
    id ID #IMPLIED
    Frequency (Required | Optional | ZeroOrMore | OneOrMore) 'Required'>

The simplest Choice element will contain two Ref elements and a frequency
attribute. By default, the Choice element's content model is required to
appear once.
To declare that a Species element may contain either a common name or a
Latin name, but not both, use the following declaration:

<ElementDecl name="Species">
  <Model>
    <Choice Frequency="Required">
         <Ref Element="CommonName"/>
         <Ref Element="LatinName"/>
    </Choice>
  </Model>

</ElementDecl>
The Ref elements in an Choice element may also specify the frequency with
which they appear, as may the Seq elements described in section 2.3.8. The
Choice element is the equivalent of the choice group (element | element) in
XML 1.0 DTDs. The ordering of the sub-elements within an Choice element has
no effect.

2.3.8 Sequence Content Model

The Sequence content model allows for the sequential appearance of
sub-elements. Elements, if they are required to appear, must appear in the
order of the Choice and Ref sub-elements in the Seq element. The Seq
element may also indicate a frequency, allowing the content model defined
by the Seq model to appear one, one or zero, one or more, or zero or more
times.

<!-- A Seq must have two or more children -->
<!ELEMENT Seq ((Choice | Ref | Model),(Choice | Ref | Model)+)>
<!ATTLIST Seq 
    id ID #IMPLIED
    Frequency (Required | Optional | ZeroOrMore | OneOrMore) 'Required'>

The simplest Seq element will contain two Ref elements in the order in
which they should appear and a frequency attribute. By default, the Seq
element's content model is required to appear once.

To declare that the Species element requires a common name and a Latin
name, in that order, use the following declaration:

<ElementDecl name="Species">
  <Model>
    <Seq Frequency="Required">
         <Ref Element="CommonName"/>
         <Ref Element="LatinName"/>
    </Seq>
  </Model>
</ElementDecl>

The Ref elements in an Seq element may also specify the frequency with
which they appear, as may the Choice elements. The Seq element is the
equivalent of the sequence group (element, element) in XML 1.0 DTDs.





Simon St.Laurent
Dynamic HTML: A Primer / XML: A Primer / Cookies 

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