Re: representing hypermedia controls in RDF

Martynas:

No "by extension" needed here. Affordance is a quality of a thing that
allows action.

RDF alone (w/o an added ontology) affords "data interchange."

And yes, RDF _classes_ afford things, too. For example, when implementing
the FOAF ontology a whole host of "Friend of a Friend" affordances are
available. The Hydra ontology affords HTTP-based hypermedia. So does
RDF:Forms.





mamund
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On Tue, Nov 26, 2013 at 7:24 AM, Martynas Jusevičius
<martynas@graphity.org>wrote:

> Mike,
>
> You wrote
>
> > Yes,
> >
> > <http://example.com/xxxxx> a foaf:Image .
> >
> > is an affordance.
>
> Then, by extension, RDF classes are affordances, and vocabularies are
> specifications of them.
> So this gives me the impression that Linked Data applications can
> solve the affordance issue using the standard components that have
> always been there - RDF and vocabularies/ontologies?
>
> Martynas
> graphityhq.com
>
>
> >
> > of course, that affordance (like HTML.IMG) relies a number of
> expectations
> > which most all of us recognize when we see it.
> >
> > From the network perspective, the expectations are (to keep it simple):
> > - this is a "safe"[1] and "idempotent"[2] operation
> > - an image media type to be returned.
> >
> > From the client application perspective, the expectations are:
> > - use an HTTP.GET when executing this operation
> > - take the results of the locator and "transclude" it into the existing
> view
> >
> > if any these expectations are not fulfilled, the affordance is usually
> > considered "broken" (the network failed) or "mis-used" (the client did
> > something else).
> >
> > <snip>
> > By the way, nothing stops me from having <a href="isbn:343-224122">
> either.
> > It will probably be clickable, but won't work.
> > </snip>
> > I would restate this as "it is *possible* to have..." since there is,
> > actually something that stops you - the expectations of so many others
> who
> > recognize this affordance.
> >
> > Think of the HTML.A as a door in a room. If i go up to a door, turn the
> > handle, and nothing happens (I don't "navigate" to a new room), I
> consider
> > the door broken or (as Donald Norman might say) that the door is "lying
> to
> > me." If I encounter a home where many of the doors act in this unexpected
> > way, I find the experience off-putting. I might even judge the architect
> > incompetent to, at the least, perverse.
> >
> > Now consider a hypermedia representation where many of the affordances
> are
> > either not working as expected (as in your case) or are actually
> inscrutable
> > to me; ones that just don't tell me enough to be usable.
> >
> > There is an advantage to using affordances in that they offer a shared
> > understanding without the need for narrative or instructions. That's why
> we
> > can drive most vehicles if we've already learned to drive one. Why we can
> > operate most telephones, etc.  Devices that have unfamiliar or
> > counter-intuitive affordances are frustrating and, in some rare cases,
> can
> > be dangerous.
> >
> >
> >
> > [1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1
> > [2] http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.2
> >
> >
> > mamund
> > +1.859.757.1449
> > skype: mca.amundsen
> > http://amundsen.com/blog/
> > http://twitter.com/mamund
> > https://github.com/mamund
> > http://www.linkedin.com/in/mamund
> >
> >
> > On Fri, Nov 22, 2013 at 12:16 PM, Martynas Jusevičius
> > <martynas@graphity.org> wrote:
> >>
> >> Mike,
> >>
> >> so if RDF representation includes a triple such as
> >>
> >>   <http://example.com/xxxxx> a foaf:Image .
> >>
> >> is that an affordance? Because that gives me enough information to
> >> render it as <img src="http://example.com/xxxxx"/>.
> >>
> >> By the way, nothing stops me from having <a href="isbn:343-224122">
> >> either. It will probably be clickable, but won't work.
> >>
> >> Martynas
> >>
> >> On Fri, Nov 22, 2013 at 4:42 PM, mike amundsen <mamund@yahoo.com>
> wrote:
> >> > <snip>
> >> > A browser for example doesn't render the string
> >> > http://example.com/343-224122 as a clickable link unless you mark it
> up
> >> > as
> >> > one using the <a> tag.
> >> > </snip>
> >> >
> >> > Yep, the A element is the thing that _affords_ clicking. it is the A
> >> > element
> >> > which is the affordance.
> >> >
> >> > Affordances don't just supply addresses, they supply information about
> >> > what
> >> > you can _do_ with that address (navigate, transclude, send arguments,
> >> > write
> >> > data, remove data, etc.). The appearance of a URL alone provides very
> >> > little
> >> > affordance.
> >> >
> >> > For example:
> >> > - http://example.com/xxxxx
> >> > - http://example.com/yyyyy
> >> > one of the two URLs points to a blog page to which the user can
> >> > navigate,
> >> > the other points to a logo which should be displayed inline. which is
> >> > which?
> >> >
> >> > Now this:
> >> > - <a href="...">blog</a>
> >> > - <img href="..."  />
> >> > one of the two URLs points to a blog page, the other points to a logo.
> >> > which
> >> > is which?
> >> >
> >> > Note it is not the URL that provides the information (which is for
> >> > navigation, which is for transclusion), but the element in which the
> URL
> >> > appears. The element is the affordance. These are HTML affordances.
> >> > There
> >> > are a couple more hypermedia affordances in HTML. Other message models
> >> > (media types) contain their own affordances.
> >> >
> >> > It is the appearance of affordances within the response representation
> >> > that
> >> > is a key characteristic of hypermedia messages.
> >> >
> >> >
> >> >
> >> > mamund
> >> > +1.859.757.1449
> >> > skype: mca.amundsen
> >> > http://amundsen.com/blog/
> >> > http://twitter.com/mamund
> >> > https://github.com/mamund
> >> > http://www.linkedin.com/in/mamund
> >> >
> >> >
> >> > On Fri, Nov 22, 2013 at 10:13 AM, Markus Lanthaler
> >> > <markus.lanthaler@gmx.net> wrote:
> >> >>
> >> >> Hi Martynas,
> >> >>
> >> >> On Friday, November 22, 2013 3:12 PM, Martynas Jusevičius wrote:
> >> >> > Markus,
> >> >> >
> >> >> > in the Linked Data context, what is the difference between
> >> >> > "identifier" and "hyperlink"? Last time I checked, URIs were opaque
> >> >> > and there was no such distinction.
> >> >>
> >> >> These things quickly turn into philosophical discussions but simply
> >> >> speaking
> >> >> the difference lies in the expectations of a client. In XML for
> >> >> example,
> >> >> namespaces are just identifiers. There's no expectation that you can
> go
> >> >> and
> >> >> dereference that namespace identifier (even though in most cases they
> >> >> use
> >> >> HTTP URIs). The same is true about RDF. All URIs are just
> identifiers.
> >> >> From
> >> >> an RDF point of view, there's no difference between isbn:343-224122
> and
> >> >> http://example.com/343-224122. As you say, they are opaque.
> >> >>
> >> >> But if you build applications, it is important to distinguish between
> >> >> identifiers and hyperlinks. A browser for example doesn't render the
> >> >> string
> >> >> http://example.com/343-224122 as a clickable link unless you mark
> it up
> >> >> as
> >> >> one using the <a> tag.
> >> >>
> >> >> Linked Data advocates that all URIs are dereferenceable. But that's
> >> >> communicated out of band. Apart from JSON-LD, which states that URIs
> >> >> SHOULD
> >> >> be dereferenceable, no other RDF media type makes such a statement.
> >> >> Thus
> >> >> you
> >> >> need to use constructs such as hydra:Link and hydra:Resource to make
> >> >> the
> >> >> distinction explicit.
> >> >>
> >> >> Hope this helps. If not, let me know.
> >> >>
> >> >>
> >> >> --
> >> >> Markus Lanthaler
> >> >> @markuslanthaler
> >> >>
> >> >>
> >> >
> >
> >
> >
>