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Re: RDF 1.1 Lite Issue # 2: property vs rel

From: Gregg Kellogg <gregg@kellogg-assoc.com>
Date: Mon, 24 Oct 2011 02:19:39 -0400
To: Ivan Herman <ivan@w3.org>
CC: Gregg Kellogg <gregg@kellogg-assoc.com>, Stéphane Corlosquet <scorlosquet@gmail.com>, Ramanathan V Guha <guha@google.com>, Manu Sporny <msporny@digitalbazaar.com>, HTML Data Task Force WG <public-html-data-tf@w3.org>
Message-ID: <DE388754-476D-4C84-BEE9-DFC8AFE3F179@greggkellogg.net>
On Oct 23, 2011, at 10:52 PM, Ivan Herman wrote:

> Gregg,
> 
> just for my understanding, because your answer was not fully clear to me (it is still early morning and I have not yet have my breakfast...): do you say that the mechanical transformation I outline is enough, and the fact that leads to the chaining artifact is not a problem in practice? I am happy if we all agree on that...
> 
> The good thing of the rule:
> 
> "If, on an element, there is an @href/@src, there is a @property, but there is no @rel/@rev, then @property behaves like a @rel with @href in RDFa terms"
> 
> is that, conceptually, this could be considered for an RDFa processor as some sort of a preprocessing step. This is the way I would implement it. Which can also be a way to spec it, without upsetting the balance of the current processing step description in the spec.

I can see why this might lead to a simpler implementation, but I think there are a number of issues with @rel and @typeof that we can't deal with in RDFa 1.1 due to backwards compatibility issues, that we have an opportunity to fix in HTML+RDFa 1.1 (Lite) with specific @property behavior.

A problem with @rel and @typeof (and one of the common errors I sited in my blog post) is that if they are both used on the same element, a new subject is created before the @rel is applied, so the @rel is applied to the new subject, rather than being a property of the current subject referencing the newly created subject. AFIK, this was necessary so you could say:

<img src="http://foo" rel="license" resource="http://creativecommons.org/licenses/by/3.0/us/"/>

Of course, with @src changing to be like @href, this wouldn't work anymore, but we're still left with @rel being processed after creating a new subject, which could happen because of @about or @typeof.

I'm suggesting that RDFa 1.1 Lite use different semantics for @property along with @about and/or @typeof on the same subject. This allows you to say the following:

<div vocab="http://schema.org/" about="#me" typeof="Person">
  <div property="address" typeof="PostalAddress>
    <span property="streetAddress">...</span>
  </div>
</div>

which requires an extra element using @rel.

The chaining rule I suggested was the following:

	If an element has an @property attribute and either or both of @about or @typeof, create a new subject _s_.
	For each IRI reference _p_ obtained from the values of @property emit the following triple:
		*subject*: current subject
		*predicate*: _p_
		*object*: _s_

This does pretty much the same thing as Microdata does with an element having both @itemprop and @itemscope, and I think is the way that people naturally think @property should work, as evidenced by the problems people have using @rel for chaining.

The advantage is, we do all this without breaking backwards compatibility (there is no HTML+RDFa 1.0, only XHTML+RDFa 1.0), we're still fully compatible with existing RDFa Core 1.1 processing rules, but allow for a limited subset of the language, with HTML-specific extensions, that has characteristics that are actually closer to what is expected for schema.org than the current Microdata spec does, vis-a-vis property URI generation from non-URI @itemprop values.

It's certainly worth a but more effort on the part of processor maintainers to support this pattern, if it is actually easier for publishers to get right.

Gregg

> Ok, I have go to go now, and I will be on a different workshop for the whole day... Ie, I can react sporadically only!
> 
> Cheers
> 
> Ivan
> 
> 
> On Oct 24, 2011, at 07:33 , Gregg Kellogg wrote:
> 
>> On Oct 23, 2011, at 10:15 PM, Ivan Herman wrote:
>> 
>>> Hey Gregg et al,
>>> 
>>> (Guha, there is an explicit question to you...)
>>> 
>>> On Oct 23, 2011, at 19:16 , Gregg Kellogg wrote:
>>> 
>>>> On Oct 23, 2011, at 12:42 AM, Stéphane Corlosquet wrote:
>>>> 
>>>>> (removing public-vocabs)
>>>>> 
>>>>> On Sun, Oct 23, 2011 at 12:34 AM, Ivan Herman <ivan@w3.org> wrote:
>>>>> Gregg,
>>>>> 
>>>>> 
>>>>> just for my understanding and concentrate on the most frequent @href/@src cases (the others are of a secondary importance compared to @href/@src). Can your rules be summarized as:
>>>>> 
>>>>> - If, on an element, there is an @href/@src, there is a @property, but there is no @rel/@rev, then @property behaves like a @rel with @href in RDFa terms
>>>>> - In RDFa 1.1 Lite we advise users not to use @rel. Alternatively, @rel is not recognized in RDFa 1.1 Lite though still possible. (I would prefer the former, b.t.w.)
>>>>> 
>>>>> I can see the value of this but, to be able to move ahead, we have to analyze the technical issues. Some that come to my mind immediately (I am at a conference now, unfortunately, so I have to divide my attention...):
>>>>> 
>>>>> - If this is a general rule, I am not sure how we could use the textual content of a <a> element as a literal property. Well, it can be done by putting it in a separate <span> with all kind of other things.
>>>>> 
>>>>> I don't see this as a problem. Microdata has the same caveat, and <span> has to be used inside <a> as well.
>>>> 
>>>> Agreed, I think this removes some ambiguity for users; the general advice would be to be as specific as possible when using @property.
>>> 
>>> So the problem is chaining, as I realized (again:-(. If I say
>>> 
>>> <a href="blah" property="yup"><span property="foo">something</span></a>
>>> 
>>> and I mechanically apply the rule I have outlined above then, per chaining, I would get
>>> 
>>> <inherited_subject> <yup> <blah> .
>>> <blah> <foo> "Something" .
>>> 
>>> Ie, if one wants to reproduce the 'old' behaviour, then we have, I think, two options
>>> 
>>> 1. the author is supposed to add an explicit @about on the span. Probably error prone.
>>> 2. the rule I outline above should be expanded with something like @property behaves like the 'proper' property in terms of chaining, ie, it does not set the new subject, but behaves like @rel in terms of the triples generated
>>> 
>>> The problem with #2 is how to spec it properly without distorting the current RDFa 1.1 spec too much. Otherwise we really get into some sort of a spaghetti code. Any good ideas there?
>> 
>> I think we can over-complicate and over-think this considering all the different behaviors that _might_ happen. If, as Guha said, that chaining isn't recommended, then I really don't think we want to introduce a new subject without some explicit markup. Using @about or @typeof makes this explicit, much in the way @itemscope does for Microdata. In fact, the more like Microdata we can make this behavior, the more Microdata can be considered just a transformation of RDFa 1.1 Lite, which I think would be a good direction (@itemid => @about, @itemtype => @typeof, @itemprop => @property, ...).
>> 
>> The one time where chaining is necessary is when the object doesn't have a URI, as is commonly the case for schema.org examples. In this case, chaining is necessary to avoid needing to explicitly use a named blank node.
>> 
>> Gregg
>> 
>>> Guha, do you have any experience, based on the rich snippets, how frequent is the situation when the content of the <a> element is also used to generate additional triples, or is it so that users usually 'stop' at <a>, so we should not worry about that too much? 
>>> 
>>>> 
>>>>> Steph.
>>>>> 
>>>>> A possible, hack-style approach is to put a @rel="" on the element, which would push @property back on its traditional role.
>>>> 
>>>> Yes, that can still work, as it is still RDFa 1.1, and if used, an @rel would have it's original intent, but this should be discouraged in the spec/note/primer.
>>>> 
>>> 
>>> Agreed.
>>> 
>>>>> - I know I will sound as a broken record, but I am forced to beat this issue because it is still open. This works in microdata because the microdata spec introduced a special treatment for <link> (and <meta>) insofar as it allows <link> being part of the body if it uses microdata attributes. Until the same happens with RDFa attributes, the model above means that users can encode non-literal links (using RDF terms) only with clickable links (forget about the <head> now). Current HTML5 parser would move <link> to the head, unless I am mistaken (and I hope I am!), ie, it will not work.
>>>> 
>>>> As HTML+RDFa is an HTML spec, not an RDFA WG spec, we can make it explicit there that <meta> and <link> are in the body if used with a @property. Or, the HTML WG could just make it simpler and remove restrictions on <meta> and <link> in the body altogether, although they would have no purpose but to express metadata. It would allow it's use for other specs, such as Microformats, though.
>>> 
>>> Yes. But how can we convince the HTML5 WG to really _do_ this change?
>>> 
>>> Ivan
>>> 
>>> 
>>> 
>>> ----
>>> Ivan Herman, W3C Semantic Web Activity Lead
>>> Home: http://www.w3.org/People/Ivan/
>>> mobile: +31-641044153
>>> PGP Key: http://www.ivan-herman.net/pgpkey.html
>>> FOAF: http://www.ivan-herman.net/foaf.rdf
>>> 
>>> 
>>> 
>>> 
>>> 
>> 
>> 
> 
> 
> ----
> Ivan Herman, W3C Semantic Web Activity Lead
> Home: http://www.w3.org/People/Ivan/
> mobile: +31-641044153
> PGP Key: http://www.ivan-herman.net/pgpkey.html
> FOAF: http://www.ivan-herman.net/foaf.rdf
> 
> 
> 
> 
> 
Received on Monday, 24 October 2011 06:20:22 GMT

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