Re: Comments on RELAX NG 2001-06-08

[Murata Makoto <mura034@attglobal.net>]

Rick Jelliffe wrote:

> Issue 21. duplicateAttributes: prohibiting duplicate attributes
> --------------------------------------------------------------------------
> 
> Why not make duplicate attribute declaration operate in parallel?
> 
> Since it is impossible to have duplicates in XML, any regular expression can
> only act as if it were a logical expression w.r.t. attributes. So any
> operation that is not an OR will be an AND: sequence should AND, so ( @a,
> @a ) would be perfectly fine:
> @a AND @a which = @a.

In an early version of RELAX Core, I did the same thing.  But this makes 
type assignment for simple types ambiguous, since two <attribute> for the 
same attribute name can have different datatypes.  In particular, Relaxer 
had a big problem about such duplicated declarations.  Thus, I later dropped 
this feature from RELAX Core.

Furthermore, this appoarch makes validation of RELAX NG very difficult or even 
impossible.  As an example, consider 

<group>
  <choice><group><attribute name="foo"/><ref name="bar11"/></group>
          <ref name="bar12"/>
  </choice>
  <choice><group><attribute name="foo"/><ref name="bar21"/></group>
          <ref name="bar22"/>
  </choice>
</group>

In your approach, when the attribute "foo" is present, this is equivalent 
to (bar11, bar21) | (bar12, bar21) | (bar12, bar22) .  If it is absent, 
this is equivalent to (bar12, bar22).  However, I do not think we can 
implement this semantics.

When we have <... foo="">, what is our derivative?  If we simply consume 
both <attribute>s, we have 

<group>
  <choice><ref name="bar11"/>
          <ref name="bar12"/>
  </choice>
  <choice><ref name="bar21"/>
          <ref name="bar22"/>
  </choice>
</group>, 

but this incorrectly permits (bar12, bar22).

If we consume one of the two <attribute>s, we do not have the 
AND interpretation in other cases (e.g., <group><attribute name="foo"/>
<attribute name="foo"/></group>).

Suppose we consume both <attribute>s after introducing <doesNotExist>.
(More about this, see http://lists.oasis-open.org/archives/relax-ng/200106/msg00118.html)

Then, we first have 

<group>
  <choice><group><attribute name="foo"/><ref name="bar11"/></group>
          <group><doesNotExist><attribute name="foo"/></doesNotExist>
		<ref name="bar12"/></group>
  </choice>
  <choice><group><attribute name="foo"/><ref name="bar21"/></group>
          <group><doesNotExist><attribute name="foo"/></doesNotExist>
		<ref name="bar22"/></group>
  </choice>
</group>

and then have 

<group>
  <choice><ref name="bar11"/>
          <group><doesNotExist><attribute name="foo"/></doesNotExist>
		<ref name="bar12"/></group>
  </choice>
  <choice><ref name="bar21"/>
          <group><doesNotExist><attribute name="foo"/></doesNotExist>
		<ref name="bar22"/></group>
  </choice>
</group>

This allows only (bar11, bar21), and is thus incorrect.

Cheers,

Makoto