[DFDL-WG] Action 274 - on qnames in path expressions in DFDL

[Mike Beckerle]

I did some experiments with the XML validation built into eclipse, and the
key constraint, which has paths (in the selector and field attributes) that
refer to other parts of the XML infoset/document and so involve the names
of elements.

I also have been reading the XPath 2.0 specs including the formal
semantics. To my interpretation XPath 2.0 is pretty explicit:

An element or type QName consisting only of a local part NCName expands to
the default element/type namespace and the local part.

They don't qualify this in any way that I can find for the names used in
path steps. The formal semantics is pretty hard to understand, but this
detail seems to be swept under the rug. There would need to be specific
treatment of this in the naming environment, but I cannot find this. There
is discussion of name resolution in some of their judgements, but nothing I
can see that creates an exception for names in path steps. (Called name
tests)

What I was looking for was language that specifies something about a
non-prefixed name in a path step and that it is NOT augmented by the
default namespace. I could find no such exception.

But if I am right and this detail isn't there, i.e., If there is no such
exception, then the implementation of XPath in eclipse (which is xerces I
believe) is incorrect in the way it resolves names that appear in path
steps.

Here's how it works according to eclipse:

In xpath expressions in the selectors and fields of unique and key
constraints, xmlns (the default namespace) is not considered as implicitly
augmenting unqualified names that appear in path steps.

An identifier in a path step that has no prefix (e.g., 'foo' in the path
'./p1:bar/foo/p2:baz') is always interpreted as identifying either

1) a child element defined by a local element declaration with form
"unqualified".
or
2) this corner case: a child element defined by an element reference to a
global element declaration but only when that global element declaration
has no effective target namespace. (Use of 'effective' here rules out an
element decl inside a schema document that itself has no target namespace
but is included into a schema that has one. Such an element declaration
effectively has a target namespace, so this corner case doesn't apply.)

Case (2) here is a rather obscure corner case, but there nevertheless.

In no case (that I could construct) does the name in a path step get
augmented by a schema's default namespace (defined by
xmlns="...whatever...").

The above is true for names without prefix appearing in path steps of
expressions.

The QNames that are used in an XML schema to reference a global declaration
or definition (in the ref of an element ref, type attribute in a type
reference, base in a simple type derivation, etc.) are different. Those
names DO pickup a default namespace if one is defined and they have no
explicit prefix themselves. This creates a corner case for these references
(to global decls/defs) which is if a definition/declaration appears in no
effective target namespace, then there is no way to reference it at all
from a schema that has a default namespace definition surrounding the point
of reference. There is no syntactic way to turn-off use of a default
namespace. (The semantics of xpath allows for a "none" default namespace
definition, but there is no xml syntax for expressing explicitly this
"none". It is simply what you have if there is no prefix and no default
namespace). You can change which namespace the default is, but there's no
way to say explicitly "I meant this QName reference to be to something not
in any namespace.

Unless/until we can find language in some XML Schema spec indicating how
path step names (name tests) are specifically resolved, then the above is
just "how one system does it." But we may end up having to follow precedent
here from implementations.

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
www.tresys.com
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