W3C home > Mailing lists > Public > www-rdf-interest@w3.org > April 2001

RE: RDF in XHTML

From: Danny Ayers <danny@panlanka.net>
Date: Wed, 18 Apr 2001 12:51:01 +0600
To: "Murray Altheim" <altheim@eng.sun.com>, "Seth Russell" <seth@robustai.net>
Cc: "Dan Brickley" <danbri@w3.org>, "Joshua Allen" <joshuaa@microsoft.com>, "Sean B. Palmer" <sean@mysterylights.com>, "RDF Interest" <www-rdf-interest@w3.org>
Message-ID: <EBEPLGMHCDOJJJPCFHEFEEJIDCAA.danny@panlanka.net>
Just a thought - what we are more or less saying by embedding RDF in a doc
is that 'here is some metadata, process it if you can', so might not a PI
play a role here? (presumably in combination with NOTATIONs)

---
Danny Ayers
http://www.isacat.net

<- -----Original Message-----
<- From: altheim@mehitabel.eng.sun.com
<- [mailto:altheim@mehitabel.eng.sun.com]On Behalf Of Murray Altheim
<- Sent: 18 April 2001 06:18
<- To: Seth Russell
<- Cc: Dan Brickley; Joshua Allen; Sean B. Palmer; Danny Ayers; RDF
<- Interest
<- Subject: Re: RDF in XHTML
<-
<-
<- Seth Russell wrote:
<- >
<- > From: "Murray Altheim" <altheim@eng.sun.com>
<- >
<- > > I've never offered to solve world hunger, even for RDF
<- flesh. I don't know
<- > > how to solve that one. The *only* way I can imagine (that
<- wouldn't involve
<- > > an act of Congress) would be to have CDATA section nodes
<- containing RDF be
<- > > notation-marked as RDF, such that they get passed off to an
<- RDF schema
<- > > processor for *appropriate* processing. This isn't
<- technically all that
<- > > difficult, but it's religiously and politically unlikely. IMO.
<- >
<- > Actually this solution is rather attractive to me ... it has
<- the smell of
<- > the right way to do it.  But I'm a babe in the woods where it comes to
<- > swimming in these waters.  Could you (or somebody) sketch the
<- theological
<- > implications for us?
<-
<- Theologically, it seems to me that few people in the W3C like the SGML
<- approach to dealing with non-XML content, which would be to use
<- notations.
<- Their way would be using XML namespaces, which unfortunately
<- don't provide
<- the features that XML notations do. You'd use XML Schema datatypes, which
<- is a might bit more complex.
<-
<- The XML Schema approach is also markedly different, which is to
<- *validate*
<- the content. Notation-based approaches simply indicate what previously-
<- declared notation a specific entity is considered to be, "entity" in our
<- case being a CDATA-wrapped DOM node.
<-
<- XML 1.0 got halfway there in supporting SGML notations, in that one can
<- indicate the notation of element content, but one cannot do this for
<- attribute content. Given that most theologians believe that element and
<- attribute content are both "document content" this was an unfortunate
<- oversight that would have allowed DTDs to compete with XML Schemas on a
<- more level playing field. I'd like to see any update of XML include
<- notations on attributes, but I'm a bit cynical given the W3C's dislike
<- of DTDs.
<-
<- But for our purposes here, what we have will do just fine. Check out the
<- following if you want to follow along in the XML spec as to I'm talking
<- about:
<-
<-    http://www.xml.com/axml/target.html#Notations
<-
<- Basically, we can't simply put any markup that contains angle brackets
<- into any XML document without breaking validity. XHTML is not special
<- in this regard. But we can wrap such markup (such as RDF) in a CDATA
<- section. This means that it doesn't get well-formedness checking, which
<- would have to occur in the processor that receives the CDATA section
<- DOM node. But if this was an understood part of the process, we could
<- proceed.
<-
<- In the DTD we'd have something akin to:
<-
<-    <!NOTATION dc PUBLIC
<-        "-//DCMI//NOTATION Dublin Core Metadata Element Set V1.0//EN""
<-        "http://dublincore.org/">
<-    <!NOTATION rdf SYSTEM "http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<-    <!NOTATION blat PUBLIC "-//doctypes.org//NOTATION Blat 1.0//EN"
<-        "http://www.doctypes.org/blat/1.0/">
<-    ...
<-    <!ELEMENT  metadata  ( #PCDATA ) >  <!-- really, a CDATA section -->
<-    <!ATTLIST  metadata
<-        type  NOTATION  (dc|rdf|blat)
<-    >
<-    ]><!-- end of DTD -->
<-    ...
<-    <head>
<-    <metadata type="rdf">
<-    <![CDATA[
<-      {rdf content}
<-    ]]></metadata>
<-
<- The "(dc|rdf|blat)" list can't unfortunately be an open-ended list.
<- Each of the tokens in the list must be a declared NOTATION in the DTD,
<- otherwise you'll get an error. But given the purpose of DTDs is to create
<- constraints, this isn't too bad; this is essentially a contract to
<- everyone on what we'd all accept in terms of available public notations.
<- We could add a empty parameter entity to allow it to be extended in a
<- document's internal subset for custom or development use, such as:
<-
<-    <!ENTITY % Metadata.ext "">
<-    <!ATTLIST  metadata
<-        type  NOTATION  (dc|rdf|blat %Metadata.ext;)
<-    >
<-
<- Then, as I mentioned above, the CDATA section DOM node (ie., the content
<- of the metadata element) would be passed off to processor which would
<- strip off the CDATA section wrapper and pass it to another XML parser
<- process, which would first well-formedness check it before sending it
<- off to the RDF processor. If the RDF was of a particular known
<- application,
<- it would then process the content appropriately. Any decent
<- engineer could
<- whip this up in fairly short order from commonly available tools. [Okay,
<- so I've outlined the process...]
<-
<- > > Why do you need to have the RDF be *in* the XHTML file?
<- Honestly, without
<- > > trying to sell you an XTM solution, this is precisely what
<- XTM is good
<- > > for: mapping resources within XML files. [..good stuff snipped...]
<- >
<- > Well for the general solution to describing resources with RDF
<- we need to be
<- > able to read and write it ... were working on the writing ...
<- the reading
<- > (first level) will need to be just as simple.  Ideally a
<- browser plug-in or
<- > the browser itself can pop up a surfable user friendly window of the
<- > metadata.  Knowing the XTM tags and retrieving other resources
<- is going to
<- > complicate that application an order of magnitude.  Imho, it's a deal
<- > breaker.
<-
<- I wasn't trying to make it a bar to entry, only pointing out a possible
<- solution. RDF could be used for this too, but one would need a specific
<- application of RDF to standardize the semantics of the mapping, and you'd
<- have something *like* XTM then. And I'm not particularly interested in
<- creating a custom RDF language to use for mapping web documents. I'll
<- bet somebody else is, and I'm not one to rein on anyone's parade here.
<-
<- > > As has been mentioned in other threads and by other people, creating
<- > > external documents means more document management, less document
<- > portability,
<- > > the likelihood of metadata-document mismatch, etc. People
<- already spend
<- > too
<- > > much time managing (or not managing) their document sets.
<- I'd hate to add
<- > > to their burden.
<- >
<- > I agree.  I seem to smell a consensus that the embedded way is
<- the best ...
<- > it's just that there is this theological problem with it,  and it's
<- > currently politically incorrect.   Well shucks .. we stopped
<- the war, didn't
<- > we, so this should be an easy piece.
<-
<- Embedding DC won't be too bad. Embedding RDF will require also a software
<- solution that's a tad bit custom currently in the XML world: actually
<- paying attention to notation declarations and passing content off to
<- notation processors. Oh, and in having an RDF notation process know to
<- cast off the CDATA wrapper prior to beginning processing. There's no
<- spec that spells out this process; perhaps there should be. But I do
<- maintain that the solution I've described above is likely unpalatable
<- to the W3C. I'd be happy to be shown wrong, though.
<-
<- Murray
<-
<- PS. I'm cc'ing myself as I just did a substantial amount of the
<- DTD work. :-)
<- BTW, isn't everyone on the cc list also on RDF Interest? Why not
<- kill the cc
<- and rely on the listserver? I'm getting an awful lot of duplicates.
<- .................................................................
<- ..........
<- Murray Altheim, SGML/XML Grease Monkey
<mailto:altheim&#64;eng.sun.com>
XML Technology Center
Sun Microsystems, 1601 Willow Rd., MS UMPK17-102, Menlo Park, CA 94025

      the wood louse sits on a splinter and sings to the rising sap
      ain't it awful how winter lingers in springtimes lap -- archy
Received on Wednesday, 18 April 2001 02:55:17 GMT

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