RE: Why Are Schemas Hard? (WAS RE: "Uh, what do I need this for" (wasRE: XML.COM: How I Learne d t o Love daBomb))

[Murali Mani <mani@CS.UCLA.EDU>]

I think one of my biggest problems with xml schema is 1-unambiguity, and
the difficulty (or rather impossibility?) of using xml-schemas for type
inferencing.

in the recently held conference, we gave a talk on 1-unambiguity, and came
to know that xml-schema allows different set of key, uniqueness
constraints to be imposed on the two b's in a content model such as (b, c
| b, d) -- this definitely made me think more about 1-unambiguity and
whether they might be needed for xml-schema.

my opinions at this point are -- 1-unambiguity constraints are still very
bad (in other words, we should still remove the 1-unambiguity constraint,
and if any, allow for different definition of 1-ambiguity for xml-schema),
and of course several design choices of xml-schema seem quite bad..??

and we are using relax ng for our applications... type inferencing, and
document processing are quite important for our applications, so is the
necessity that our schema language be closed under union.

regards - murali.

On Wed, 22 Aug 2001, Michael Brennan wrote:

> > From: Bullard, Claude L (Len) [mailto:clbullar@ingr.com]
>
> <snip/>
>
> > 1.  What about Schemas is hard?
> >
> > 2.  Are some of the issues about things Schemas cannot
> >     represent (eg, the co-occurrence constraints)?
> >
> > I'm missing something here.  With a product like
> > XML Spy for sanity checking, I don't seem to find
> > the Schemas hard to develop.  That doesn't say
> > that implementing a system around a schema isn't
> > hard, but I am curious what others are struggling with.
> > Yes, the spec is tough (they all are), but the primer isn't, there
> > are dozens of web articles on learning schemas, and
> > a week or so with a beta of an IDE seems to cover
> > the sanity checks.
>
> I strongly suspect that most of the criticisms of XSD's complexity are
> coming from those who are trying to write tools that leverage schemas, such
> as those trying to leverage schemas for data-binding.
>
> I also think that issues about things Schemas cannot represent are probably
> a frustration to those trying to use it as a validation language. I know
> that we have been adapting some of our integration interfaces to use XML
> structures that can be adequately expressed with XML Schema. I think you
> have to question the value of a validation language when you find that you
> are redesigning your XML structures to accomodate the weaknesses of the
> schema language. In our case, this means changing our structures such that
> an element's content model is not identified by an attribute value. RELAX NG
> can accomodate this, but XSD cannot. The lack of support for co-occurence
> constraints is a frustration, as well. We are investigating incorporating
> support for Schematron-like assertions into our integration mapping tools.
> Up to now, developers have simply written code to handle these cases.
>
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