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   Re: Common XML (was Re: Document Feature Requirements) [REPOST]

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  • From: Sean McGrath <sean@digitome.com>
  • To: xml-dev@xml.org
  • Date: Sun, 09 Apr 2000 09:17:27 +0100

Hit the send key half way through re-reading. This edit needed
to fix a "howler" or two. Apologies.


[Rick Jelliffe]
>I think the name "Common XML" doesn't capture where the Common XML
>conventions are most appropriate. Some name like "Exchange XML" or
>"Round-trippable XML"
>would be more appropriate. 

True. My first cut at a spec for it over on SML-DEV used
the phrase "Interchange XML". After a while though, the
group settled on "Common XML".

>I welcome Common XML. We need to keep a profile to let people know which
>features have been implemented well or are appropriate at any time.

Yup. In SGML-land years ago Wayne Wohler used the term "Monastic
SGML" for what we are calling "Common XML".

>Does anyone else detect a running fallacy throughout some of the other
>sections that goes "if you want to use an apple as an orange you cannot,
>therefore you
>should use only oranges and we don't need to provide apples"?  
>For example, comments should not be used becuase it is impossible to
>guarantee roundtripping or to transport data in them.  PIs are
>"because they are not part of the document's character data" and "many
>simple applications ignore or discard them".  CDATA section tags don't 
>have semantic meaning. 
>However, it is the nature of a comment that it is not data -- it is 
>an annotation for the benefit of human readers on the state of the text
>of a document.

The above interpretatioon of a comment is the result of a human
being attaching semantics to a construct that of itself, has
no such semantics.

What is this:-

	<!-- bgcolour=#00AA00 -->

Syntactically, it is a comment. Semantically it is something
else - probably. There is nothing in the nature of a comment
that makes it not data - it is all data at the end of the
day and processing systems will attach arbitrary semantics
to chunks of it as they see fit.

What *is* different between a comment and PCDATA is that
the syntax demarcates them differently and the XML type
system assigns different handles to them.

In the ontology of XML types, PCDATA and COMMENT are
at the same level

                - PCDATA
                - ELEMENT

It could be argumed that the XML element type system is
powerful enough to handle COMMENTS and PROCESSING INSTRUCTIONS
without adding extra syntax and ontology entries.

I.e. elements have and associated element type name. If the
element type name is "xml:pi" the element contents can have
the general semantics of a processing instruction as defined in
the XML 1.0 spec. If the element type name is "xml:comment"
the element contents can have the general semantics of a comment
as defined in the XML 1.0 spec.

The result is the use of "element syntax" for comments and
PIs. If this is done in the syntax of the XML documents
there is no need for comments or PIs and issues of
round-tripping just go away. Of course they are round-tripped
because they are cosher elements.
An alernative from a developers point of view is to
leave them syntactially different in serialized
XML form but treat them homogenously at the
API level.

In Pyxie[1] for example, I read comments and PIs in their native
form but transform PIs to element nodes internally.
Thus in a Pyxie tree, there is no API for manipulating
PIs - you use the element API restricting your
attention to element nodes of type "xml:pi".

When Pyxie serializes XML, it recreates the PI
syntax from XML 1.0.

[1] http://www.pyxie.org



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