XSchema Spec - Attribute Declarations (Section 2.4), Draft 6
SimonStL at classic.msn.com
Fri Jul 24 20:51:50 BST 1998
Following is the latest draft of Section 2.4, Attribute Declarations. This
section appears to be the magnet for the most criticism.
The XSC:EnumerationValue element is back, and can be used for both enumerated
types and notation enumerations. (Thanks to Carl Hage and Ron Bourret for
feedback on this one.)
I haven't taken up Carl's other suggestion, that the default value be given
its own element. I can see where it _might_ be worth documenting, but I'm not
sure it's worth the inconvenience. As always, I'm open to convincing.
As always, a prettier HTML version of this will be posted shortly at
Dynamic HTML: A Primer / XML: A Primer / Cookies
2.4 Attribute Declarations
Attribute declarations are made with XSC:AttDef elements. XSC:AttDef elements
may be nested inside of XSC:ElementDecl element declarations or linked to
element. 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 XSC:AttDef (XSC:Doc?, XSC:More?, XSC:EnumerationValue*)>
Name NMTOKEN #REQUIRED
Element NMTOKEN #IMPLIED
id ID #IMPLIED
Type (CData |
Required (Yes | No) "No"
Fixed (Yes | No) "No"
AttValue CDATA #IMPLIED>
<!ELEMENT XSC:EnumerationValue (XSC:Doc?, XSC:More?)>
Value CDATA #REQUIRED>
In XSchema 1.0, an attribute declaration (XSC:AttDef element) may be nested
within the element declaration (XSC:ElementDecl element) for the element to
which the attribute belongs. If the XSC:AttDef element appears nested inside
an XSC:ElementDecl element, the Element attribute must be ignored. If the
XSC:AttDef element appears nested under the XSC: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 XSC:AttDef declaration applies to all elements within the declaration's
parent XSC:XSchema element and any child XSC:XSchema elements.
The Name attribute of the XSC: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.
<XSC:AttDef Name="status" ...additionalAttributeInformation.../>
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.
<XSC:AttDef Name="status" Element="Species"
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.
Applications may also use the id attribute to provide unique identifiers for
attribute declarations using values that are unique within the XSchema.
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 XSC: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.)
<XSC:AttDef name="Latin" Type="CData"/>
This attribute would then be available for use in instances of the Species
<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
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
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:
<XSC:AttDef Name="status" Type="Enumerated">
A Species element created conforming to this declaration might look like:
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 and
fixed or not fixed, 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 XSC:AttDef element.
For instance, if the Latin attribute described above was required by the
Species element, the XSC:AttDef element would contain a Required attribute
with a value of "Yes":
<XSC:AttDef name="Latin" Required="Yes"/>
Optional attributes (identified in XML 1.0 by #IMPLIED) are identified
assigning the value "No" to the Required attribute of an XSC: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 XSC: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.)
<XSC:AttDef name="Latin" Required="No"/>
Fixed attributes (identified in XML 1.0 by #FIXED AttValue) are identified
through the use of the Fixed 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 Fixed attribute has a value of "Yes", the AttValue attribute
must be present. A Fixed value without an AttValue must be treated as an
For example, to declare a planet attribute for the Species element, a Fixed
attribute given the value of "Yes" would identify the fixed nature of the
attribute and the AttValue attribute would provide the value.
<XSC:AttDef Name="planet" Fixed="Yes" AttValue="Earth"/>
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:
<XSC:AttDef Name="status" Type="Enumerated" Enumeration="extinct endangered
protected non-threatened" AttValue="non-threatened"/>
Any default (required, fixed, etc.) may be used with any attribute type,
though default values should always correspond to acceptable values for the
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 Fixed AttValue XML 1.0 Value
Yes Yes <value> This does not occur in XML. It means that the instance file
must include the value and the value must be the fixed value. At best this
Yes Yes -- error
Yes No <value> This does not occur in XML. It means that the instance file
must include a value and the default happens to be whatever is given with
AttValue. This should be treated as simply AttValue for maximum compatibility.
Yes No -- #REQUIRED
No Yes <value> #FIXED <value>
No Yes -- error
No No <value> AttValue
No No -- #IMPLIED
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