15 elementary truths about XML

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

Below I start with the most basic concepts and build upon them. Do you agree with each statement?  /Roger

1. An XML file is a sequence of zeros and ones called bits.

2. A sequence of 8 bits is called a byte.

3. Thus, an XML file is a sequence of bytes.

4. Here is an example of a byte: 00110001

5. That byte may be interpreted in various ways by software applications. For example, it may be interpreted as:

    - corresponding to an integer in base two. 
      In base 10 it represents the number 49.

    - corresponding to a character.
      In the ASCII character encoding scheme it 
     represents the character 1.  

6. XML processors always interpret the bytes in XML files as characters.

7. Thus, XML processors interpret XML files as a sequence of characters.

8. There are various character encoding schemas, such as ASCII and UTF-8.

9. An application may identify the character encoding scheme used by an XML file either by its encoding attribute in the XML declaration or by some out-of-band means.

10. An XML application is software that processes the bytes in an XML file.

11. An XML processor is an XML application. As discussed above, it interprets the bytes in XML files as a sequence of characters.

12. An XML Schema validator is an XML application.

13. XML applications may interpret the bytes in XML files differently than how an XML processor interprets the bytes.

14. For example, consider the XML Schema file that declares an element A with a Boolean data type:

    <element name="A" type="boolean" />

    Suppose the value of <A> is the above byte. 
    The element declaration informs the XML Schema validator 
    and the XML Schema validator interprets the byte as the 
    Boolean value "true."

15. Thus, an XML processor interprets the byte as representing the character 1, and an XML Schema validator interprets the byte as representing the Boolean value "true."