Re: Fresnel: Lenses: summary and unsolved issues

From: Jacco van Ossenbruggen <>
Date: Mon, 25 Apr 2005 16:25:41 +0200

Emmanuel Pietriga wrote:

> 1. Jacco's wondering about the possibility to define lenses as
> refinments of other lenses. We do have such a mechanism. See section
> 5.2 Lens Inheritance [2]. Does this meet your needs?
Yes, this is what I meant. I missed section 5.2, sorry.
Technical question: why not use RDFS built-in semantics for inheritance
using subclass and subproperty hierarchies?
I assume you will have good reasons for it - just wondering.

> 2. How do we resolve conflicts between different lenses that apply to
> the same instance? This is indeed still underspecified, and we need to
> work on this.

Related thought (Lloyd deserves the credits): you could view fresnel
lens and style info as "just" another type of metadata, which you can
attach to any web resource like other RDF. Note that currenty, the lens
specifies to what instances (say, instances of foaf:Person) it applies
by attaching a fresnel:lensDomain property to the lens. You could also
imagine you attach a fresnel:lens property to the foaf:Person class to
specify a suitable lens. This would also allow use of rdfs
subclass/subproperty inheritance: instances of subclasses of foaf:Person
would inherit the lens, and a subproperty of fresnel:lens, say,
fresnel:defaultLens, would return a valid result even when foaf:Person
is queried only on fresnel:lens.

Where the current approach is similar to a CSS style rule that uses a
selector to specify to which HTML classes it applies, the other approach
would be similar to, for example, an HTML element that defines its own
style. But thanks to the properties of RDF, without many of the
drawbacks of the HTML case!

In HTML, this approach is a maintance problem because it becomes hard to
change the style when it is defined inline in the HTML source. But in
RDF, the direction of the property does not have any relation with the
files in which the property is specified. For example, the triple that
relates the foaf:Person class with the fresnel lens does not need to be
in the same file as the definition of foaf:Person or the lens.

A second drawback of the inline HTML approach is that you need to
duplicate the same style specification into every element that uses it.
But in RDFS, you can make use of the instance/class relation (a style
defined on the class level applies to all instances) and of the subclass
hierarchy (a style defined on the class is inherited by the subclasses).

Comments welcome,

Received on Mon Apr 25 2005 - 14:24:52 EDT

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