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Re: Agenda Topic / Issue: Clarify the meaning of "ignore" with respect to attributes that have no legal value

From: Jeni Tennison <jeni@jenitennison.com>
Date: Fri, 11 Sep 2009 19:32:55 +0100
Message-Id: <184851DB-7C42-460D-8BC1-2D5BB77B78AB@jenitennison.com>
To: "public-rdf-in-xhtml-tf.w3.org list" <public-rdf-in-xhtml-tf@w3.org>

On 11 Sep 2009, at 16:36, Shane McCarron wrote:
> It is my belief that the following are NOT equivalent from a  
> processing perspective (assume the foaf prefix is declared, the blah  
> prefix is not):
>
> 	 <span about="#shane" property="foaf:name" datatype="">Shane</span>
> 	 <span about="#shane" property="foaf:name"  
> datatype="blah:blah">Shane</span>
> The first item explicitly requests a text literal.  The second item  
> requests a datatype that is undefined (illegal, invalid, whatever).   
> I maintain that in a conforming processor the second item would NOT  
> generate any triple.

I don't think that's what the spec currently says. An illegal value as  
above causes an *attribute* that is present (ie there is a datatype  
attribute there) and a value that is empty (because illegal CURIEs are  
ignored). Step 9 says that a typed literal is created if:

   "@datatype is present, and does not have an empty value, and is not  
set to rdf:XMLLiteral."

In this case, datatype is present but it has an empty value, so we  
don't create a typed literal.

The next possibility is that it's a plain literal, if it meets any of  
the conditions:

   	 @content is present;
	 or all children of the [current element] are text nodes;
	 or there are no child nodes (in which case the literal value is the  
empty string);
	 or the body of the [current element] does have non-text child nodes  
but @datatype is present, with an empty value.

In this case, all children of the current element are text nodes, so  
the object is a plain literal and the triple should be:

   <#shane> foaf:name "Shane" .

In the case where the element did contain elements:

   <span about="#shane" property="foaf:name"  
datatype="blah:blah"><em>Shane</em></span>

I think the fourth of the clauses -- "or the body of the [current  
element] does have non-text child nodes but @datatype is present, with  
an empty value." -- comes into action, because there is a datatype  
attribute present but it has an empty value (because its value is  
illegal). So we again get the triple:

   <#shane> foaf:name "Shane" .

I don't think there's any language in the specification that indicates  
that the entire triple will be omitted if the datatype attribute is  
illegal. Creating a plain literal seems to be a reasonable fallback  
when the datatype is unknown.

> I think the same is true for our treatment of @property, @about, and  
> @resource.  It is explicitly NOT true for @rel, @rev, and (maybe)  
> @typeof because of their ability to cause bnodes to be created.  Do  
> you disagree?  And can you answer in one page or less? :P


FWIW, I disagree about property, because it also has the power  
(through the setting of [skip element] in step 4) to determine whether  
or not bnodes are created to complete any hanging triples. This is  
Philip's example. With no property attribute:

   <p xmlns:ex="http://example.org/" about="http://example.com/"  
rel="ex:rel1">
     <span content="Content 1">
       <span about="http://example.net/">Test 1</span>
     </span>
   </p>

we should generate:

   <http://example.com/> <http://example.org/rel1> <http://example.net/ 
 > .

With a legal property attribute:

   <p xmlns:ex="http://example.org/" about="http://example.com/"  
rel="ex:rel2">
     <span property="ex:prop" content="Content 2">
       <span about="http://example.net/">Test 2</span>
     </span>
   </p>

we should generate:

   <http://example.com/> <http://example.org/rel2> _:bnode1 .
   _:bnode1 <http://example.org/prop> "Content 2" .

The final example being an illegal property attribute which means that  
the property is still *present* (causing the creation of a bnode) but  
does not create triples of its own:

   <p xmlns:ex="http://example.org/" about="http://example.com/"  
rel="ex:rel3">
     <span property="bogus:bogus" content="Content 3">
       <span about="http://example.net/">Test 3</span>
     </span>
   </p>

we should generate:

   <http://example.com/> <http://example.org/rel2> _:bnode2 .


For @about, @resource, @href and @src, the wording is "by using the  
URI from @{attribute}, if present, obtained according to the section  
on CURIE and URI Processing;". I think the "if present" here refers to  
"the URI from @{attribute}" rather than just "@{attribute}", such that  
if @about does not resolve to a URI there is no value present and it  
is as if the entire attribute had not been there. So:

   <p xmlns:ex="http://example.org/" about="http://example.com/">
     <span about="[bogus:bogus]" property="ex:prop">Test</span>
   </p>

results in the triple:

   <http://example.com/> <http://example.org/prop> "Test" .


It would make things clearer all round if the specification used  
phraseology that specifically included the possibility of illegal  
values and made it explicit where it's talking about *values* being  
present and where about *attributes* being present in relevant places,  
such as:

   "_the_ @datatype _attribute_ is present, and does not have an empty  
_or illegal_ value, and is not set to rdf:XMLLiteral."

   "or the body of the [current element] does have non-text child  
nodes but _the_ @datatype _attribute_ is present, with an empty _or  
illegal_ value."

   "Additionally, if _the_ @property _attribute_ is not present then  
the [skip element] flag is set to 'true';"

Cheers,

Jeni
-- 
Jeni Tennison
http://www.jenitennison.com
Received on Friday, 11 September 2009 18:33:30 GMT

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