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Re: operationally, ResourceSets are sets of URIs not resources

From: Phil Archer <parcher@icra.org>
Date: Fri, 21 Dec 2007 11:39:37 +0000
Message-ID: <476BA5F9.50802@icra.org>
To: Jeremy Carroll <jjc@hpl.hp.com>
CC: public-powderwg@w3.org

Thanks Jeremy, some comments inline..

Jeremy Carroll wrote:
> 
> 
> On my reading of the POWDER WDs, the intent with the definition of a 
> ResourceSet is that, given a URI u, you can use each part of the 
> definition in turn, to test whether u is in the resource set or not.
> However, it is not the intent that you can use each test in the 
> description of a ResourceSet independently of the other tests.

Correct.

> 
> Thus, a ResourceSet operationally defines a set of URIs, and we can then 
> consider the set of resources, which are identified by those URIs.
> 
> ===
> 
> Now, this seems straight forward, but is less so, when we consider a 
> resource which has two URIs identifying it.
> 
> Suppose R is a resource
>   identified by http://example.org/r and by http://example.com/s
> 
> 
> <wdr:ResourceSet rdf:ID="RS1">
>   <wdr:includeHosts>example.org</wdr:includeHosts>
>   <wdr:includePathsStartsWith>/s</wdr:includePathsStartsWith>
> </wdr:ResourceSet>
> 
> The intent seems to be that R is not in this resource set.

True

> 
> The operational rules when processing both http://example.org/r and 
> http://example.com/s do, indeed reject both.
> 
> But the descriptions of both in the vocabulary document, permit R to be 
> in the resourceset
> 
> http://www.w3.org/TR/2007/WD-powder-voc-20070925/#includeHosts
> [[
> This property defines a set of resources, that have a URI host component 
> matching at least one of the values given in a white space separated 
> list, that is to be included when interpreting a Resource Set definition.
> ]]
> 
> So that R has a URI http://example.org/r and is included.
> 
> http://www.w3.org/TR/2007/WD-powder-voc-20070925/#includePathStartsWith
> [[
> This property defines a set of resources, that have a URI path component 
> starting with at least one of the values given in a white space 
> separated list, that is to be included when interpreting a Resource Set 
> definition.
> ]]
> 
> So that R has a URI http://example.com/s and matches.

I'm not sure I agree. Yes, each triple in the Resource Set defines a 
set. The ResourceSet is the intersection of those sets so that the set 
of things with a host component of example.org is a super set of RS1, as 
is the set of things that have a path component that starts with /s. 
This, of course, goes back to your initial observation that each set is 
not to be taken in isolation - well, it can be, and this is reflected in 
the vocabulary document definitions, but the ResourceSet must be 
considered in the round.

Personally, I am a little reluctant to let go completely of resource set 
definitions based on resource properties (cf. URI component properties) 
as it is not hard to conceive of real-world situations where this is of 
potential usage. For example, an English teacher might publish an 
opinion that if the language used in a particular reference document is 
en-us then both the spelling and usage of nouns as verbs is not to be 
repeated when answering exam questions; if the number of images in a 
document exceeds x then the page in unlikely to be suitable for display 
on mobile etc. These apply whatever the URI(s) of the resource may be.

That said, I do take the point that a single resource can have any 
number of URIs and that one way to reduce the ambiguity and potential 
logical inconsistencies would be to forget about trying to define 
resource sets and concentrate on URI sets.

The RS definition by resource property has vexed us mightily in the past 
and my guess is that the WG would not object to its removal if that 
helps us to make progress.

> 
> I suggest this can be addressed by aligning the semantics with the 
> operational definition, and having a resource set being a set of URIs, 
> with the mapping to resources being done elsewhere (e.g. in the 
> definition of
> http://www.w3.org/TR/2007/WD-powder-voc-20070925/#hasScope
> 
> i.e. change
> [[
> This property links a Description Resource to the set of resources to 
> which it applies. It MUST be included in Description Resources.
> ]]
> 
> to
> [[
> This property links a Description Resource to the set of URIs of the 
> resources to which it applies. It MUST be included in Description 
> Resources. Any resource which has a URI in the set of URIs is in scope 
> for the Description Resource.
> ]]
> 
Received on Friday, 21 December 2007 11:40:01 GMT

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