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From: "Chris Wilper" <cwilper@cs.cornell.edu>
> Well, this looks really promising. I had a quick read of
> the spec and a question came up, re: Concurrent Validation
> and defaults.
> I couldn't discern what happens with default values
> in this case. Say I'm doing WXS validation and
> schematron validation on an instance doc. The WXS
> schema provides a default value for some item in
> the instance. Would the schematron validation
> phase pick up the default value?
For NRL, these would happen in parallel. The Schematron schema
would not pick up any new PSVI items. Notionally, ISO DSDL
Part 4 Validation Candidate Selection Language works by
selecting sections of documents and diverting them through
various schemas (or subsequent processes). NRL does provide
in itself anyway to apply, say, Schematron as a function on the
output of a WXS transformation/validation.
If you wanted to transform a document, e.g. by adding default
values taken from an XML Schema, this would have to be
done using some prior transformation of the document.
The general ISO DSDL framework is expected to provide
various kinds of transformation operations, which should
cope with Chris' request.
ISO DSDL is based on modularity, achieved by the simple
pipeline architecture of "XML-in, XML-out". There is no
PSVI (in the sense of information items of kinds other
than those in the XML infoset) and all validation results
are not specified by the framework (in the sense that
a schema language may specify various kinds of validation
results, or the implementations may do so, but there is not
expected to be a PSVI-style node-by-node outcome
specified at the schema level.)
The general reason for this is a belief (shared by most
of the instigators of the inidividual schema languages
that are being standardized in ISO DSDL) that
document transformation (such as adding defaults)
and validation are different things and should be disentangled.
The problem with defaults values as part of schema languages
is that they mean that in order to be sure of the infoset of
a document, you have to be sure that the document has been
validated. This is the same problem that XML DTDs have;
its sounds good to have defaults, but it is so impractical
to tie them to validation (i.e. validation of the kind where
you have to download a schema as well as a document)
for data transmission that people won't use them.
Furthermore, I think some of the schema language creators
suspect that a schema language can be good for providing the
information for closely-coupled systems (i.e. for data binding,
where typing is important) or for loosely-coupled systems
(i.e. where the information set needs to be independent of
validation, where validation may occur at any point in
the processing chain, and where information items are
handled generically as elements, attributes, etc ) but probably
not for both.
NRL looks really nice. It is the third generation of implemented
languages for this use, each a good advance on the last:
Murata-san's RELAX Namespaces, James' NLM and now
James' NRL. I expect ISO VCSL will look pretty much like
NRL, if it proves useful enough.
What amuses me is how much this all breaks the cliche that
Asians are only good at taking Western ideas and making them
into concrete technologies: with both RELAX and RELAX
Namespaces we have an Asian's ideas being implemented by
a Westerner. Japan and other Asian countries are now crammed
full of XML expertise and overflowing with creativity: can the
West's regional NIH syndrome be overcome?
On a more reasonable note (i.e. not attempting to substitute one
cliche for another), it is interesting that Schematron too was
developed in Asia in part as a response to non-Western requirements
(in the case of Schematron, the suggestion from an IT professor that
many Taiwanese students found the idea of grammars for natural language
ludicrous, because of the non-tokenized nature of Chinese.)
Cheers
Rick Jelliffe
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