XML

Resources

• XML and Localization: FAQ
• XML Schema Part 2: Datatypes Second Edition
• Units of Measure
• Measurement Units in XML Datatypes
• The Unified Code for Units of Measure

Character representations

See XML and Localization :: FAQ :: Character Representation

Character encoding

See XML and Localization :: FAQ :: Encoding

Language identification

See XML and Localization :: FAQ :: Language Identification

XML Descriptions

• XML currently uses two systems for describing XML documents
• XML DTDs (Document Type Definitions) are good for describing document structure
• XML Schema are good for describing data types
• Both may be needed

XML DTDs

XML DTDs (Document Type Definitions) are good for describing document structure e.g.

• A book consists of a preface followed by one or more chapters followed by an index
• A chapter is made up of one or more sections
• A section consists of text
• The DTD for this is

  
        <!ELEMENT book (preface?, chapter+, index)>
        <!ELEMENT chapter (section+)>
        <!ELEMENT section (#PCDATA)>
        

XML Schema

• Schema are good for data type descriptions
• For example, if you want to have the data

        <age> 45 </age>
        <height> 5.9 </height>
        <color> blue </color>
        

• then you would describe the data type using the schema

        <element name="age" type="positiveInteger"/>
        <element name="height" type="float"/>
        <element name="color">
          <simpleType base="xsd:string">
            <enumeration value="green">
            <enumeration value="blue">
          </simpleType>
        </element>
        

• XML Schema also describes how to make lists and unions of other data types. e.g. for a list

        <simpleType name='sizes'>
            <list itemType='decimal'/>
        <simpleType>
        

  with example data

        <cerealSizes xsi:type='sizes'> 8 10.5 12 </cerealSizes>
        

XML Schema Standard Data Types

Java mapping of data types

Most XML standard data type are represented by a Java data type

XML	Java
boolean	boolean
byte	byte
dateTime	java.util.Calendar
double	double
float	float
int	int
integer	java.math.BigInteger
string	String

Some types can't be represented since they don't exist in Java, such as unsigned int. If a type can be "nillable" (that is, have a nil value) and it would be a Java primitive type, then it is wrapped in a Java class such as Integer.

XML datatypes and i18n

• Most of the standard datatypes are ok for i18n because they are locale independent such as
  • date
  • time
  • dateTime
  • double
  • integer
• Why is a date such as 2005-05-31 considered locale independent? Because: it contains enough information that it can be translated into a date of any calendar
• These are considered unsafe
  • duration
  • gYearMonth
  • gYear
  • gMonthDay
  • gDay
  • gMonth
• They are unsafe because they cannot be translated unambiguously into other locales. For example, the Muslim year 1426H started on 10 February 2005. The gYear 2005 is ambiguously either 1425H or 1426H. Similarly, it can be the year 2547 or 2548 in the Thai calendar (switching at the 13 May)

Defining your own datatypes

• Anyone can define their own datatypes
• The full details are at XML Schema Part 2: Datatypes Second Edition
• This defines an attribute to describe font sizes as an integer between 8 and 72, or with one of the tokens "small", "medium" or "large".

  
  <xsd:attribute name="size">
    <xsd:simpleType>
      <xsd:union>
        <xsd:simpleType>
          <xsd:restriction base="xsd:positiveInteger">
            <xsd:minInclusive value="8"/>
            <xsd:maxInclusive value="72"/>
          </xsd:restriction>
        </xsd:simpleType>
        <xsd:simpleType>
          <xsd:restriction base="xsd:NMTOKEN">
            <xsd:enumeration value="small"/>
            <xsd:enumeration value="medium"/>
            <xsd:enumeration value="large"/>
          </xsd:restriction>
        </xsd:simpleType>
      </xsd:union>
    </xsd:simpleType>
  </xsd:attribute>
        

Units in XML

• Many numbers are the value of some unit e.g. 2 inches, 3 kilometres, 5 stone, 100 kilograms
• A number without a unit description is meaningless
• An XML document is meant to be machine understood - it is accidental if it is also human understood
• Humans can often understand the context for textual documents and interpret words such as "cm". Machines cannot (easily). A phrase "20cm" is understood by a person, maybe not by a processing system
• XML documents can be made XML-machine understandable by adding tags

  
        <measure>
            <value>20</value>
            <unit>cm</unit>
        </measure>
        

Units schema

• There is no WWW Consortium recommendation on units
• A general discussion of issues is at Units of Measure
• Another proposal at Measurement Units in XML Datatypes and another paper at Simplified Measurement Units for XML Datatypes Version 0.4
• A brief scheme at Custom Units of Measure
• A schema by Roger Costello
• A non-XML proposal is at The Unified Code for Units of Measure