[oslc-core] Comment on ResourceShape resources
Ian Green1
ian.green at uk.ibm.com
Thu May 27 11:33:55 EDT 2010
Hello Dave
I wasn't considering rdfs semantics, but you are right - if we did, we end
up in a bad place. what i was trying to do was to reuse rather than
invent - my suggestion of rdf:predicate was a bad choice and we ought to
pick a different term for the OSLC Shape vocabulary.
OSLC Shape does not want to make assertions about the property it wants to
make assertions about the _use_ of the property. That's the way OSLC is
set up - there is a tension between simple RDF semantics (no OWL) and the
desire that OSLC has to close the world.
I think the desire to close the world stems from the pragmatic view that
working with an open model is harder - less can be assumed and more
flexible reasoning is needed to get things done. I think we have the
desire to be open (decoupled, few assumptions, flexible reasoning) but
this is hard. For example, use of logical reasoning to draw conclusions
about resources. "Mixing" computation with inference is what logic
programming is about - a lot of work has been done in description logic
programming.
If we used OWL DL, for example, we could close the world and describe OSLC
resources more precisely.
best wishes,
-ian
ian.green at uk.ibm.com (Ian Green1/UK/IBM at IBMGB)
Chief Software Architect, Requirements Definition and Management
IBM Rational
oslc-core-bounces at open-services.net wrote on 26/05/2010 15:35:24:
> [image removed]
>
> Re: [oslc-core] Comment on ResourceShape resources
>
> Dave Steinberg
>
> to:
>
> oslc-core
>
> 26/05/2010 15:35
>
> Sent by:
>
> oslc-core-bounces at open-services.net
>
> Hi all,
>
> First off, apologies for barging into the conversation. I haven't
> been involved in this group at all, but I've just been digging into
> Core spec draft over the past few days and I was most puzzled by
> this particular issue (and, really, resource shapes in general). So,
> I thought I'd add my two cents in the hopes that someone might find
> it helpful.
>
> I definitely agree with Ian's comments about rdf:type being
> inappropriate there, but I'm not sure about the suggestion to use
> rdf:predicate instead. rdf:predicate is part of RDF's reification
> vocabulary, and its meaning in that context is very different from
> what you're attempting to convey here. For those who don't know,
> reification provides the ability to use RDF statements to describe
> other statements. The vocabulary includes the type rdf:Statement and
> the properties rdf:subject, rdf:predicate, and rdf:object. For
> example, consider the following statement:
>
> a:service-provider dc:title "Blogging Service" .
>
> It can be fully described by the following four statements:
>
> b:stmt rdf:type rdf:Statement .
> b:stmt rdf:subject a:service-provider .
> b:stmt rdf:predicate dc:title .
> b:stmt rdf:object "Blogging Service" .
>
> The effect of this is to introduce another node into the RDF graph
> representing the statement and identifiable, itself, by a URI. Note
> that the four reification statements are not actually equivalent to
> the statement they describe.
>
> One important thing to notice about the reification statements is
> that the subjects of them are all a statement. Indeed, RDF Schema
> declares that rdf:Statement is the domain of rdf:subject,
> rdf:predicate, and rdf:object. This allows an RDFS-aware system to
> infer that the subject of any rdf:predicate statement (such as the
> oslc:Property blank node in the proposal below) is an rdf:Statement.
> So in this proposal, we would be saying that the blank node is both
> an oslc:Property and an rdf:Statement. That's not strictly speaking
> wrong (pretty much nothing is in RDF), but it's confusing and
> probably not what was intended, since an oslc:Property appears to be
> some kind of "meta" concept, while an rdf:Statement is actually a
> statement itself (an instance).
>
> The RDFS equivalent of oslc:Property is (unsurprisingly)
> rdf:Property, but there is no equivalent relationship to what you're
> trying to establish here, since it is the URI of the rdf:Property
> itself that one uses in writing the predicate of the statement. So,
> if you wanted to follow that lead, you would assign the URI http://
> purl.org/dc/terms#modified to the oslc:Property, not leave it as a
> blank node, and make your shape statements about the property itself.
>
> But I'm not at all sure if that's really what you want to do. It
> seems to me that oslc:Property is intended to be something more like
> a particle/attribute use in XML Schema (where the rdf:Property is
> analogous to an element/attribute). That is, it restricts how the
> property can be used in a particular context. If that's the case,
> I'm not sure what to advise. It does seem rather counter to the non-
> restrictive nature of RDF as I understand it, but I'm definitely not
> an RDF expert and I can certainly see the appeal of knowing what to
> expect. At the very least, I would encourage you to be clear about
> the concepts that resource shapes introduce and not to reuse any
> vocabulary that would obscure those concepts.
>
> Cheers,
> Dave
>
> --
> Dave Steinberg
> Rational Software - IBM Toronto Lab
> mailto:davidms at ca.ibm.com
>
>
> [image removed] Dave ---05/26/2010 08:39:44 AM---Thanks for the
> excellent feedback Ian and I agree pdf:predicate seems a much better
> fit here. I capt
>
> [image removed]
> From:
>
> [image removed]
> Dave <snoopdave at gmail.com>
>
> [image removed]
> To:
>
> [image removed]
> oslc-core <oslc-core at open-services.net>
>
> [image removed]
> Date:
>
> [image removed]
> 05/26/2010 08:39 AM
>
> [image removed]
> Subject:
>
> [image removed]
> Re: [oslc-core] Comment on ResourceShape resources
>
> [image removed]
> Sent by:
>
> [image removed]
> oslc-core-bounces at open-services.net
>
>
>
>
> Thanks for the excellent feedback Ian and I agree pdf:predicate seems
> a much better fit here. I captured this issue on the issues page and
> applied your suggested change to the Resource Shape resource
> definition in appendix A.
>
> So far we are using two notions from RDFS: the notion of a resource
> definition, known as a class in the RDFS terminology, and
> rdf:predicate. Perhaps I should add RDFS to the list of references at
> the end of the spec.
>
> - Dave
>
>
> On Wed, May 19, 2010 at 8:02 AM, Ian Green1 <ian.green at uk.ibm.com>
wrote:
> > Hi all,
> >
> > the resource shape spec states that a Shape resource has zero-or-more
> > oslc:Property's
> >
> >
> > an oslc:Property has a (Resource, exactly-one) "rdf:type".
> >
> > This is a category error in the RDF model. The rdf:type is the type
of
> > the instance, not the type of the RDF predicate which is modelling the
> > oslc:Property.
> >
> > To be concrete - here is an example from the draft:
> >
> >
> >
> > <oslc:property>
> > <oslc:Property>
> > <oslc:name>modified</oslc:name>
> > <oslc:occurs>exactly-one</oslc:occurs>
> > <oslc:namespace rdf:resource="http://purl.org/dc/terms#"
/>
> > <rdf:type rdf:resource="http://purl.org/dc/terms#modified"
/>
> > <oslc:valueType rdf:resource="
> > http://www.w3.org/2001/XMLSchema#dateTime" />
> > </oslc:Property>
> > </oslc:property>
> >
> > This asserts that there is a resource, of rdf:type oslc:Property AND
also
> > of rdf:type http://purl.org/dc/terms#modified
> >
> > We don't mean this. We mean that the oslc:Property is indicated by
an
> > RDF predicate http://purl.org/dc/terms#modified.
> >
> > I suggest we change this part of the spec. to use rdf:predicate rather
> > than rdf:type. So the example becomes:
> >
> > <oslc:property>
> > <oslc:Property>
> > <oslc:name>modified</oslc:name>
> > <oslc:occurs>exactly-one</oslc:occurs>
> > <oslc:namespace rdf:resource="http://purl.org/dc/terms#"
/>
> > <rdf:predicate rdf:resource="
> > http://purl.org/dc/terms#modified" />
> > <oslc:valueType rdf:resource="
> > http://www.w3.org/2001/XMLSchema#dateTime" />
> > </oslc:Property>
> > </oslc:property>
> >
> > -ian
> >
> > ian.green at uk.ibm.com (Ian Green1/UK/IBM at IBMGB)
> > Chief Software Architect, Requirements Definition and Management
> > IBM Rational
> >
> >
> >
> >
> >
> > Unless stated otherwise above:
> > IBM United Kingdom Limited - Registered in England and Wales with
number
> > 741598.
> > Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6
3AU
> >
> >
> >
> >
> >
> >
> >
> > _______________________________________________
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> >
>
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