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   Re: What are schemata

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  • From: Paul Prescod <papresco@technologist.com>
  • To: xml-dev@ic.ac.uk
  • Date: Mon, 01 Jun 1998 08:16:58 -0400

Tim Bray wrote:
> >> | So is a schema a function that maps a document to a truth value?
> >>
> >> I'd rather say that it is a definition of a set of documents, just as
> >> a formal language is usually considered to be the set of sentences
> >> that are well-formed in that language.
> Hmm, this line of thought may be perpetuating what I see as one of
> the shortcomings of DTDs, namely that the DTD has to describe the
> whole document, i.e. a class of languages.  What about partial
> validation/constraints?  I think it's important that child-of-DTD
> support compond documents & partial validation.  So in the terms above,
> maybe these things define sets of elements and attributes, rather
> than whole documents. -Tim

This is all true. But at the end of the day, a complete document either
conforms to a schema or it doesn't, even if only half of its elements or
attributes are verfied, and even if they are only verified piece-wise. So
a schema definitely maps a document to a truth value. (You could also say
that it defines a set of documents...the statements are equivalent)

The question is: should it also map sub-document fragments to truth
values? And what should the granularity of those fragments be? Must they
be well-formed? Should it also define a set of "XML fragments"?

I am completely comfortable with the simple notion of piece-wise
verification (e.g. ignore what you don't understand). I am not yet
comfortable with the more complex notion that a schema should be able to
map an arbitrary XML fragment to a truth-value. We would have to define
"XML fragment" so that it makes sense. We would also have to discuss the
interaction between the application and the schema language that would
give rise to this situation.

 Paul Prescod  - http://itrc.uwaterloo.ca/~papresco

Three things it is far better that only you should know:
How much you're paid, the schedule pad, and what is just for show

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