W3C home > Mailing lists > Public > semantic-web@w3.org > January 2010

Re: Turtle null prefix and namespaces

From: Richard Light <richard@light.demon.co.uk>
Date: Tue, 19 Jan 2010 09:09:27 +0000
Message-ID: <Z42OeYFHbXVLFwv3@light.demon.co.uk>
To: Peter Ansell <ansell.peter@gmail.com>
Cc: semantic-web@w3c.org
In message 
<a1be7e0e1001181249t23397d9bh3259b3dbc66abfd2@mail.gmail.com>, Peter 
Ansell <ansell.peter@gmail.com> writes
>
>Why can't you make up a namespace prefix in the RDF/XML document that
>hasn't been used in the turtle document or the RDF/XML document and
>apply it to all of the null prefixed items that you get out of turtle?

Neat.  Thank you.

>The fact that the prefix represents a namespace in Turtle has no
>bearing on the RDF model in general, you are just lucky that the
>RDF/XML document has support for namespaces.

That bears on the whole issue of the degree of equivalence between 
Turtle/SPARQL prefixes and XML namespace prefixes.  They look as though 
they do the same job, but I have been harbouring a suspicion that there 
might be differences that matter.  The "null prefix" case seemed to me 
to be an example of this.

You say I am "just lucky", but in fact RDF/XML itself could not work as 
currently defined without namespace support.  Since predicates are 
expressed as element names in RDF/XML, and since they are URIs, the use 
of a namespace prefix is necessary for an absolute URI even to have a 
chance of being a valid XML element name.  All the non-NAME characters 
can go into the namespace URI.  (Personally I would prefer predicates to 
be expressed as attributes in a revised XML serialization of RDF, so as 
to remove this dependency/limitation.)

>What is the advantage of doing a straight translation without
>representing the triples in memory as abstract RDF by the way? Have
>you tried translating the turtle document to NTriples first?

This whole exercise started when a draft framework of interest to me 
(VMF [1]) was offered only in Turtle form.  What started as a quick hack 
has become a reasonably serious attempt to deal with the full Turtle 
syntax.

I didn't see any value in going via intermediate forms, since my aim was 
to get machine-processible XML to drop into a triple store.  I could 
have done this job more easily with a custom XML target format, but I 
wanted to improve my understanding of RDF/XML, and also to produce 
something which might be of use to others.  In fact I load the parsed 
data directly into an XML DOM, and just serialize it when parsing is 
completed.  An extension which would be easy to add would be to apply a 
user-supplied XSLT transform to this document, allowing output in any 
required format.

A couple of points about Turtle also dropped out of this exercise. 
First, a literal-minded reading of the EBNF spec would suggest that 
white space is only allowed where specified.  There ought to be a 
statement somewhere that white space is allowed freely between tokens 
(or did I miss it?). Second, the Turtle VMF resource didn't use UTF-8 
encoding (accented characters were in ISO-8859-1), and despite this it 
imported without problems into Protege 4.  This in turn suggests that 
Protege's Turtle support doesn't deal correctly with encoding issues.

>Would the following do what you want?
>
><rdf:Description rdf:about="http://example.org/a1"><a2
>xmlns="http://example.org/"
>rdf:resource="http://example.org/a3"/></rdf:Description>

Only if the rdf:resource is an absolute URI, I think.

Richard

[1] http://cdlr.strath.ac.uk/VMF/
-- 
Richard Light
Received on Tuesday, 19 January 2010 09:10:03 UTC

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