[xquery-talk] Accessing empty namespaced elements in non-empty contexts

[Joe Wicentowski]

Hi all,

A common question for beginners is how to access empty namespaced elements
in non-empty contexts.  The easiest answer is to use a wildcard.  For
example, we use one here inside the curly braces:

  let $x := <x><y/></x>
  return
    <z xmlns="foo">{ $x/*:y }</z>

As expected, this query returns:

  <z xmlns="foo"><y xmlns=""/></z>

The other method I know of is to use the URI-qualified name:

  let $x := <x><y/></x>
  return
    <z xmlns="foo">{ $x/Q{}y }</z>

A third method occurred to me, but it does not work, and I haven't been
able to pin down the reason despite reading the spec.  The idea is to bind
the empty namespace URI to a namespace prefix, e.g., "my":

  declare namespace my="";

  let $x := <x><y/></x>
  return
    <z xmlns="foo">{ $x/my:y }</z>

In BaseX, eXist, and Saxon, this returns err:XPST0081 (
https://www.w3.org/TR/xquery-31/#ERRXPST0081).  The location of the error
points to the step, "my:y" on the final line.

Can anyone enlighten me on the reason this 3rd method is invalid?  Bonus
points for an explanation that includes why I cannot bind a namespace
prefix to the empty namespace, but I can access it via *: and Q{}?

Thanks,
Joe
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