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Re: I18N Issue alternative: collapsing plain and xml literals

From: Graham Klyne <gk@ninebynine.org>
Date: Tue, 09 Sep 2003 10:10:52 +0100
Message-Id: <5.1.0.14.2.20030909095250.02e3c000@127.0.0.1>
To: Brian McBride <bwm@hplb.hpl.hp.com>, rdf core <w3c-rdfcore-wg@w3.org>
Cc: Martin Duerst <duerst@w3.org>, "Richard Ishida" <ishida@w3.org>, i18n <w3c-i18n-ig@w3.org>

Brian,

As I understand it, this proposal could break existing RDF data.  E.g.

   <rdf:Description>
     <eg:prop>foo &lt; bar</eg:prop>
   </rdf:Description>

would no longer be valid RDF.  Currently, I think it describes the graph:

   _:a eg:prop "foo < bar" .

[[[Hmmm... The RDF validator service says it maps to:

   _:a eg:prop "foo &lt; bar" .

That doesn't seem right to me.  Did we really decode that?  If I add the 
parsetype=literal, I get the same thing with the XML literal datatype 
added.  RDF validator bug report submitted.]]]

A similar proposal I have is to make the effect of parseType=Literal purely 
syntactic, in that it modifies the handling of literal test in RDF/XML, so 
that '&' and tags are effectively uninterpreted in the translation to graph 
form.  (I had thought of proposing something similar earlier, but was wary 
of adding more strands to the debate, but now we're there...)

Then:

<rdf:Description>
   <eg:prop>foo &lt;br /&gt; bar</eg:prop>
</rdf:Description

parses to:

   _:a eg:prop "foo <br /> bar" .

and

   <rdf:Description>
     <eg:prop rdf:parseType="Literal"><br /></eg:prop>
   </rdf:Description>

parses to:

   _:a eg:prop "foo <br></br> bar" .

I think there remains a question:  can parseType=Literal be used in 
conjunction with rdf:datatype=...?   I see no reason why not.  Then the 
current functionality of XML literals is possible without making it part of 
the RDF core specification;  e.g.

   <eg:prop rdf:parseType="Literal" rdf:datatype="foo:XMLLiteral">
      The <em>best</em> solution?
   </eg:prop>

(Note:  I expect that rdf:datatype continues to ignore language 
information, so the above example would not be sensitive to language tagging.)

I think this approach has similar advantages to what you propose, without 
some of the disadvantages, also separates XML syntax issues from datatype 
issues, which I think is a distinct improvement, and remains fully backward 
compatible with (my understanding of) existing RDF.


At 12:08 08/09/03 +0100, Brian McBride wrote:
>After discussing this informally over lunch, Danbri asked me to send it to 
>the list to make our consideration of it explicit.
>
>This is an alternative design for literals.  The  idea is to drop the 
>rdf:XMLLiteral datatype and allow plain literals to contain markup.  Two 
>test cases illustrate:
>
><rdf:Description>
>   <eg:prop>foo &lt;br /&gt; bar</eg:prop>
></rdf:Description
>
>parses to:
>
>_:a eg:prop "foo &lt;br /&gt; bar" .
>
><rdf:Description>
>   <eg:prop rdf:parseType="Literal"><br /></eg:prop>
></rdf:Description>
>
>parses to:
>
>_:a eg:prop "foo <br></br> bar" .
>
>The definition of a plain literal changes.  The lexical space of plain 
>literal becomes the lexical space of rdf:XMLLiteral, i.e. is restricted to 
>(the unicode representation of) canonicalised well formed balanced xml 
>markup.  The denotation of a plain literal remains - it is a sequence of 
>unicode characters - permitting string comparison for equality testing.

Testing this proposal on your criteria:

>Advantages:
>
>I think this provides everything that Martin has been asking for:
>
>   - no discontinuity between plain and xml literals

Yes (where s/xml literals/literals containing XML markup/

>   - lang on mixed content

Yes (where that content is textual, as opposed to some datatype)

>   - no use of datatypes

Yes (though remains compatible with use datatypes)


>Disadvantages:
>
>- a bigger change than alternatives

Yes, though I think mine is a less-big change

>- builds XML into the core of the RDF model

My proposal does not:  parseType=Literal applies only to the XML syntax.

>- breaks current implementations (but see below)

I believe my proposal does not.

>Ameliorating the Disadvantages - implementation strategy
>
>The above design says that e.g. "<" is not in the lexical space of plain 
>literals, and many (all?) current implementations will store
>"<" in their representation of a graph.  The point to note is that 
>implementations are free to represent literals any way they please. Thus 
>"<" is just the way this implementation represents the literal "&lt;".

I think this is a significant (maybe fatal) problem.

>The implementation does need to distinguish between markup and plain 
>text.  To do this, it adds a single bit to literals to indicate whether 
>they are stored in escaped or unescaped form.  The above example was in 
>unescaped form, which cannot represent markup.  To represent markup, the 
>literal must be be stored in escaped form.

This seems to be purely an implementation choice.  I don't believe an 
implementation *needs* to do anythiong of the kind, though it may make life 
easier in some cases.

>Literal comparison becomes more complex - literals stored in unescaped 
>form should first be escaped and then canonicalized.  Various optimization 
>strategies can be employed here.

Again, implementation detail.  By comparison, though, I think my proposal 
doesn't introduce this complication.

>By this strategy, It may be possible to argue that this approach does not 
>break current implementations of plain literals.  It simply makes clearer 
>what xml literals are.

Hmmm... sounds more like redefinition of lirterals to me.

#g


------------
Graham Klyne
GK@NineByNine.org
Received on Tuesday, 9 September 2003 05:37:01 EDT

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