XSchema Spec - Attribute Declarations (Section 2.4), Draft 6

Wed Aug 19 02:11:12 BST 1998

This section is large and has absorbed significant changes.

1) It has been updated for namespace handling.

2) The AttDefs are now contained in an AttGroup container element.

3) The attributes used to identify #REQUIRED, #FIXED, #IMPLIED, and default 
values for attributes have been reduced to two from three.  Many thanks to 
Jeni Tennison for her insightful analysis on this, and to Ron Bourret for 
helping develop it.

Hopefully, we're about there.

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

2.4 Attribute Declarations

2.4.1 Overview

Attribute declarations are made with AttDef elements nested inside of AttGroup 
container elements. AttGroup elements may be nested inside of ElementDecl 
element declarations or XSchema elements. The type of an attribute is defined 
with an attribute, as is a declaration of whether or not it is required and a 
possible default value. Values for enumerated types are provided with 
subelements.

<!ELEMENT AttGroup (Doc?, More?, AttDef*)>
<!ATTLIST AttGroup
    Name NMTOKEN #IMPLIED
    Element NMTOKEN #IMPLIED
    id ID #IMPLIED
    prefix NMTOKEN #IMPLIED
    ns CDATA #IMPLIED>

<!ELEMENT AttDef (Doc?, More?, EnumerationValue*)>
<!ATTLIST AttDef
    Name NMTOKEN #REQUIRED
    Type (CData |
    ID |
    IDRef |
    IDRefs |
    Entity |
    Entities |
    Nmtoken |
    Nmtokens |
    Notation |
    Enumerated) "CData"
    Required (Yes | No) "No"
    AttValue CDATA #IMPLIED>
<!ELEMENT EnumerationValue (Doc?, More?)>
<!ATTLIST EnumerationValue
    Value CDATA #REQUIRED>

In XSchema 1.0, attribute declarations (AttDef elements inside of an AttGroup 
element) may be nested within the element declaration (ElementDecl element) 
for the element to which the attribute belongs, or outside of that structure 
in an XSchema element. If the AttGroup element appears nested inside an 
ElementDecl element, the Element attribute must be ignored. Similarly, if an 
AttDef element appears nested inside an AttGroup element, its Element 
attribute must be ignored in favor of that of the AttGroup. If an AttGroup or 
AttDef element appears nested under the XSchema element, the Element attribute 
may contain a name token corresponding to the Name attribute of the element to 
which this attribute applies. If the Element attribute is missing, that AttDef 
or AttGroup declaration may only be used by reference. 

AttGroup elements are container elements. All of their attributes, except for 
id, apply to the child AttDef elements and may be overridden by the attributes 
of those child elements. The Name attribute of the AttGroup element is 
provided for convenient reference within the XSchema. This attribute should be 
ignored in conversions to DTDs.

The Name attribute of the AttDef element provides the name by which the 
attribute will be identified. Attribute names must be unique within the 
element in which they are declared. A nested declaration is shown below.

<ElementDecl Name="Species">
  ...additionalElementInformation... 
  <AttGroup>
    <AttDef Name="status" ...additionalAttributeInformation.../>
  </AttGroup>
</ElementDecl>

This declares an element with the name Species that has an attribute named 
status. If the status attribute was declared outside of the Species element 
declaration, the declarations would appear as shown below.

<ElementDecl Name="Species">
    ...additionalElementInformation... 
</ElementDecl>
...
<AttDef Name="status" Element="Species" ...additionalAttributeInformation.../>

Merely naming an attribute may be adequate. Attribute declarations may 
identify types and provide information about whether the attribute is 
required. By default, attributes will be assumed to contain character data 
(CData), not be required, and have no default value. This information is 
declared using additional attributes. The simplest attribute declaration 
possible identifies an attribute as containing character data (CData) and 
allows the attribute to be optional, as shown below.

<AttDef Name="sampleAttribute"/>

Applications may also use the id attribute to provide unique identifiers for 
attribute declarations using values that are unique within the XSchema.
The prefix attribute identifies the prefix that will be applied to this 
attribute during conversion to DTDs. The ns attribute identifies the URI which 
functions as the namespace name for this attributes. Namespace processing is 
covered further in Section 3.0, "XSchema and Namespaces". 

2.4.2 Attribute Types

XSchema 1.0 provides equivalents for all of the XML 1.0 DTD attribute types. 
All of them are declared using attribute values within the AttDef element.
The CData attribute type is one of the most common, permitting an attribute to 
contain character data as defined by the XML 1.0 specification. If the Species 
element were to contain an attribute providing the Latin name of the species, 
the declaration could look like the following. (The Type attribute could 
actually be omitted in this case, as CData is the default type.)

<ElementDecl Name="Species">
...additionalElementInformation... 
    <AttGroup>
        <AttDef name="Latin" Type="CData"/>
    </AttGroup>
</ElementDecl>

This attribute would then be available for use in instances of the Species 
element:

<Species Latin="Passerina cyanea">...additionalContent...</Species>

The ID attribute type is used to uniquely identify elements in a document for 
application processing. IDRef and IDRefs attribute types are used to refer to 
a single ID value in the same document or multiple ID values in the same 
document, separated by whitespace, respectively. These attribute declarations 
must be used with the same constraints as apply to ID, IDREF, and IDREFS 
attribute types in XML 1.0.

The Entity and Entities attribute types identify the names of unparsed 
entities. The use of these attribute types must be made with the same 
constraints as apply to the ENTITY and ENTITIES attribute types in XML 1.0. If 
a document is checked directly against an XSchema without a conversion to a 
DTD, information regarding unparsed entities must be available from the parser 
for these attribute types to be meaningful.

The Nmtoken and Nmtokens attribute types are used to declare attributes that 
must contain information conforming to the Nmtoken and Nmtokens productions in 
XML 1.0.

The Notation and Enumerated attribute types are more complex, requiring 
Enumeration subelements to identify their possible content. These two 
declarations use similar syntax, but the allowed values of Notation 
declarations must match the Notations declared elsewhere in the XSchema 
document.
If the status attribute of the Species element were to allow the values of 
extinct, endangered, protected, and non-threatened, an appropriate enumerated 
type declaration would look like:

<ElementDecl Name="Species">
...additionalElementInformation... 
    <AttGroup>
        <AttDef Name="status" Type="Enumerated">
            <EnumerationValue Value="extinct"/>
            <EnumerationValue Value="endangered"/>
            <EnumerationValue Value="protected"/>
            <EnumerationValue Value="non-threatened"/>
        </AttDef>
    </AttGroup>
</ElementDecl>

A Species element created conforming to this declaration might look like:

<Species status="extinct">...additionalContentAboutDodos...</Species>

2.4.3 Attribute Defaults

XSchema requires attribute declarations to provide information about the 
default value of a given attribute. XSchema provides for the four cases 
supported by XML 1.0: #REQUIRED, #IMPLIED, #FIXED AttValue, and AttValue, 
though they are expressed as choices between required and not required with an 
optional default value. There may be only one default value declaration per 
attribute.

Required attributes (identified in XML 1.0 by #REQUIRED) are identified by 
assigning the value "Yes" to the Required attribute of an AttDef element. For 
instance, if the Latin attribute described above was required by the Species 
element, the AttDef element would contain a Required attribute with a value of 
"Yes":

<ElementDecl Name="Species">
...additionalElementInformation... 
    <AttGroup>
        <AttDef name="Latin" Required="Yes"/>
    </AttGroup>
</ElementDecl>

Optional attributes (identified in XML 1.0 by #IMPLIED) are identified 
assigning the value "No" to the Required attribute of an AttDef element and 
not assigning a value to the AttValue attribute. Implied indicates that there 
is no default value provided, and also that no value is required. If the Latin 
attribute is optional, the AttDef element would contain a "No" value for the 
Required attribute. (Note that this is the default status and the Required 
declaration does not need to be made explicitly.)

<ElementDecl Name="Species">
...additionalElementInformation... 
    <AttGroup>
        <AttDef name="Latin" Required="No"/>
    </AttGroup>
</ElementDecl>

Fixed attributes (identified in XML 1.0 by #FIXED AttValue) are identified 
through the use of the Required attribute in combination with the AttValue 
attribute, which must contain the fixed value for the attribute. Attributes 
declared using fixed value cannot declare a different value for that 
attribute. Fixed effectively hard codes attribute values into particular 
elements. If the Required attribute has a value of "Yes", and the AttValue 
attribute is present, the attribute value should be treated as a #FIXED value 
in XML 1.0.

For example, to declare a planet attribute for the Species element, a Required 
attribute given the value of "Yes" would identify the fixed nature of the 
attribute and the AttValue attribute would provide the value.

<ElementDecl Name="Species">
...additionalElementInformation... 
    <AttGroup>
        <AttDef Name="planet" Required="Yes" AttValue="Earth"/>
    </AttGroup>
</ElementDecl>

Attributes may also be provided with a default value that may be overridden by 
other declarations. These default values are identified through the use of the 
AttValue attribute. The status attribute of species elements described above 
would be an appropriate target for such a default value, especially if most 
species being described fell into a particular category:

<ElementDecl Name="Species">
...additionalElementInformation... 
    <AttGroup>
        <AttDef Name="status" Type="Enumerated" Enumeration="extinct 
endangered protected non-threatened" AttValue="non-threatened"/>
    </AttGroup>
</ElementDecl>

Any default (required, fixed, etc.) may be used with any attribute type, 
though default values should always correspond to acceptable values for the 
attribute type.

2.4.4 Combinations of Types, Defaults, and Default Values

This notation also permits the declaration of certain attributes (IDs with 
defaults, for instance) that are prohibited by the standard XML 1.0 DTD 
syntax. Developers who use these combinations should test that their documents 
will behave as expected in DTD-only environments as well as XSchema 
environments. Additional processing of document instances may be necessary to 
produce normalized-for-DTD use documents if they included such attributes as 
default values. The attribute type should always be considered more important 
than its default values in XSchema to DTD conversion.

The table below summarizes the possible combinations of XSchema attribute 
defaults and their XML 1.0 DTD equivalents.

Required	AttValue	XML 1.0 Equivalent
Yes	      <value>	#FIXED <value>
Yes	       --	      #REQUIRED
No	      <value>	AttValue
No	      --	      #IMPLIED
(-- indicates an empty or undeclared value)

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