Hi Folks,

The XML Schema working group is in the process of incorporating rules
(assertions) into the XML Schema language:
 
      "... one of the things we had to decide when putting 
       XPath-based assertions into Schema 1.1" [Noah Mendelsohn]

Thus, the XML Schema language will become both a grammar-based language
as well as a rule-based language.

Up till this date, grammar-based and rule-based languages have been
kept separate:

    Grammar-based Languages: XML Schema, Relax NG, DTD

    Rule-based Languages: Schematron, RuleML

What do you think about XML Schema working group incorporating
rule-based capabilities into the language?

Here are some potential advantages and disadvantages:

ADVANTAGES

1. Need only one language to express all data validation requirements.

2. Possible performance improvement (as compared to separate languages
with separate validations).

DISADVANTAGES

1. XML Schemas is already quite large and complex.  This will make it
larger and more complex.

2. Discourages the use of a pipeline of validations for implementing
data validation requirements.

3. Possible performance degradation since, for example, validation
can't be halted when grammar requirements fail.

4. Replacing one grammar language with another becomes prohibitive
(example: you may want to replace XML Schemas with Relax NG)

5. Discourages competition.  Today there is a competition among the
schema languages.  A single language that does everything may reduce
the competition.

QUESTIONS

1. Can you add to the above list?  What other advantages and
disadvantages are there?

2. Is grammar validation of a fundamentally different nature than rule
validation?

3. If so, is it reasonable to merge two fundamentally different things?

4. Is it in the best interest of the marketplace to have a single,
all-encompassing data validation language, or is it better to have
multiple data validation languages that work together?

/Roger