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Re: meta-specs (was RE: A few things I noticed about w3c's xml-schema)


I am discussing a type system that is independent of schema languages.

> Then in a data-centric design, I would concentrate on one
> schema language, and more particularly, I would
> concentrate on the instances I wish to produce.

Sure, but the question is in the context of "how do I make sense of these
different systems"?

> Yes?  Or what does it mean to say "instances are equal"?

An "Instance set" is the set of all possible documents which are valid with
respect to a particular schema, for example a specific DTD.

This says that two DTDs dtd-a and dtd-b are "equal" if the set of all
possible documents that are valid with respect to dtd-a is equal to the set
of all possible documents that are valid with respect to dtd-b.

(note that i don't provide a mechanism for actually computing this set, i
don't have too as this is a formal definition).

> In a relational system, it is as if one designed
> the report data, then designed the tables, then designed
> the queries that produce the reports.  Then they
> discover the business rules. Then they create the
> GUI.

Excellent analogy! In SQL, you don't tell the system _how_ to compute the
data, rather you tell the system _what set_ of data you want, and the
implementation (e.g. query engine) does the work.

> Naive yes, but I want an explanation I could give
> to a naive person.  If I have to resort to explaining
> QNames, the explanation is DOA.

Well then don't try to explain XML Schema datatypes, as they are defined by
QNames :-)

Think of a schema as a box with a light on top and a door on the side. The
Instance Set of a Schema is the collection of all the documents for which
the light turns on when you stick the document inside the door of the box.

Now suppose you have 4 of these boxes, all painted black. You are told that
there are two types of Schema boxes, and you are assigned the task of
sorting the schema boxes into two groups.

As you stick each of your documents into the boxes you find that for two of
the boxes, half the documents cause the box to light up and for the other
two boxes the other half of the documents cause the box to light up. You now
have your two groups of documents, and two groups of schemata. We say each
of the schema in the group is "equal" because they each have the same set of
instance documents.