There are two primary
points of contention I have with Chris's post. The
first is the assumption that W3C XML Schema is
descriptive enough to replace prose descriptions of
the structure of an XML document. Many who've
worked with W3C XML Schema can list several
limitations inherent in the language that occur in
XML document formats that cannot be expressed,
here's five off the top of my head- Specifying that an
element or attribute cannot be from a particular
namespace.
- Specifying that a choice
of attributes that can appear on an
element.
- Specifying that content
models vary based on values of certain attributes
or elements. For example, if the online attribute
is true then the element should have a
time-online child element otherwise should have a
time-offline child element.
- Specifying that two
elements at the same scope may have different
types.
- Specify that certain
elements can appear in any order without
limitation on the how many times a particular
element can appear.
Now let's assume that you have a document format
that is simple enough that it doesn't stretch the
capabilities of W3C XML Schema. Does this mean that
one can use the having a schema makes a
specification redundant? Why don't you be the judge
of that, compare
Jorgen Thelin's schema for RSS 2.0 to the
RSS 2.0
specification. Which would you rather read if
you wanted to create an application that processed
RSS?
Specifically which would you trust about what to do
if an RSS feed's
item had two
link elements? The prose specification
implies that this shouldn't happen but the schema
specifies that any child element of the
item element can be repeated zero or
more times. This reminds me some of the problems I
face in my day job dealing with the discrepencies
between the
schema for Schema (sForS) and the
actual prose of the specification. As Don said,
folks are
trying to make my hobby look like my day
job.
A schema/grammar is a good companion to a
specification but it isn't a replacement for
one.
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