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The answers to these questions completely depend on what
you are trying to do. For example,
1) A schematron schema might be used to apply
_additional_ constraints on top of an XSD schema, i.e. to
express constraints not expressible in XSDL (this is a
great use for Schematron imho). In such cases I presume
that we are dealing with instances that _actually are_
2) Of course if an instance is not schema valid, making
the assumption that the schema constraints are met e.g.
that the type "aaa/bbb" has a null instance set, would be
incorrect -- that is the whole point of _not being_
schema valid, that the constraints expressed in the
schema are not met. Shrug. If we are talking about using
Schematron for XML documents that aren't valid w.r.t a
particular schema, then certainly we would not want to
import the schema into the Schematron "compilation" (if
that makes sense).
My point is that considering schema constraints as
defining a set of nodes (an empty set if the constraint
is that the XPath not exist) _is actually what the
XSLT/XPath model does_ and this is actually how
Schematron itself works, i.e. when Schematron "assert"s
an XPath, it asserts that the instance set defined by the
XPath is "not null". This is exactly why I say that XPath
1.0 defines a "type" system in which a type is a set of
nodes, such type or more properly class heirarchies are
exactly how OWL (the language formerly known as DAML+OIL)
*** the reason I quote "type" and prefer "class" is that
in set theory a type has a different meaning than "class"
whereas in programming, type and class have essentially
the same meaning, albeit they are used in slightly
different ways (e.g. Java, the relationship between "int"
and "class Integer")
> > A Schematron assertion (that the node list is non-null) would then always
> > fail, even at "compile" time.
> > I think this behavior would be _better_ for Schematron.
> How can that be better for schematron? It completely breaks the whole
> purpose of the schematron schema, which is to find the "bad" markup and
> to report the occurrences to the user. Having your schematron schema
> fail to compile because it is looking for illegal cases means that
> almost all schematron schema would not compile.
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