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What does a rel value really mean?
The rel values in HTML follow a consistent pattern which at a minimum implies a deliberate design. rel describes the relationship of a hyperlink in terms of what the destination (href) is, to the source, or from the source's perspective. rel="stylesheet" is perhaps the easiest example to understand in this manner, because it is saying that this resource over there (indicated with the href) is a "stylesheet" for the current document. Similarly, rel="help" link states that the resource indicated by the href is "help" (e.g. a help file or FAQ) for the current document. All the rest of the values defined in HTML4 also follow this pattern of being a noun which labels the resource indicated by the href, from the perspective of the current document.
Then what does "rev" mean?
"rev" is the precise opposite (or "reverse") of the "rel" attribute. E.g. a rev="help" link indicates that the current document is "help" for the resource indicated by the href.
Is rel single or bi directional?
Though the HTML4 specification is not specific on this point, all the rel values in the HTML4 specification follow a very consistent pattern that indicate a specific directionality to the value of the rel attribute. The only exception to this is the value "alternate", which, is actually inherently symmetric, so rel="alternate" means pretty much the same thing as rev="alternate". I say pretty much because in theory (and in practice) one of a pair of alternates is likely to be more definitive, and thus one is more a primary resource, with the other being an alternative. In theory you could extend rel with a new value, e.g. "original" which could be used to point back the original version of a document. This could be used for example with translations, where translations of a document would point back to the original with rel="original".
"lower-case semantic web" is:
semantic (x)html examples:
- Not "Uppercase Semantic Web"
- RDF, OWL, "The Semantic Web" etc.
- plenty of sessions on them
- out of scope for tonight's discussion
- but can work together
- simple semantics with microformats
- don't try to "define the world"
- small pieces loosely joined
- evolutionary not revolutionary
- add semantics to today's web
- rather than create a future web
- user centric design
- humans first, machines second
- "people are helping to create metadata" - david sifry
- GeoURL by Joshua Schachter
- also uses simple <meta> tags
- social software
- xhtml friends network (xfn)
- easy way to personalize hyperlinks with relationship information
- <link rel="license" href="http://creativecommons.org/licenses/by/1.0/" />
- extensible open xhtml outlines (xoxo)
- use XOXO to list, plus CSS styling to display
- easy to embed in a website
- e.g. http://diveintomark.org/public/2004/01/xo-with-style.xo
- vote links
The Haystack Project and Piggy are two Semantic Web projects from MIT. Haystack seems to be some sort of universal information client, working provide a client to objectise information. [Piggy Bank (category)] says it provides via a Firefox extension and a Java server (Bank) a method for collecting, sharing. RDFs are generate via screen scraping.
(nothing here yet)
http://www.w3.org/2004/01/rdxh/spec Gleaning Resource Descriptions from Dialects of Languages (GRDDL)
http://www.w3.org/TR/2006/WD-grddl-primer-20061002/ GRDDL Primer
GRDDL Data Views: Getting Started, Learning More GRDDL is a technique for gleaning resource descriptions from dialects of languages. It's a way of extracting Semantic Web data in RDF from XML formats (especially XHTML dialects or microformats) via transformations identified by URIs and typically expressed in XSLT. ... 2. If you're using an XHTML dialect (Dublin Core encoding, GeoURL markup, etc.) that has an existing transformation (dc-extract.xsl, grokGeoURL.xsl) ... 1. Make links from your XHTML data to the transformation, using the transformation link type:<html xmlns="http://www.w3.org/1999/xhtml"> <head profile="http://www.w3.org/2003/g/data-view"> <title>Some Document</title> <link rel="transformation" href="http://www.w3.org/2000/06/dc-extract/dc-extract.xsl" /> <meta name="DC.Subject" content="ADAM; Simple Search; Index+; prototype" /> ... </head> ... </html>
2. Reference the GRDDL profile to make it clear what that transformation link type means:<html xmlns="http://www.w3.org/1999/xhtml"> <head profile="http://www.w3.org/2003/g/data-view"> <title>Some Document</title> ...
- Q: How about an example?
Sure: a home page for Joe Lambda shows DC, geoURL, RSS as above plus markup for FOAF (using grokFOAF.xsl) and Creative Commons Licenses (using grokCC.xsl) that you can probably figure out if you view source.
- Q: Where can I find out about existing
'ways' rather than duplicate similar work?
A: The CustomRdfDialects wiki topic provides an informal directory of existing conventions.
This document, http://www.w3.org/2003/g/data-view, is a metadata profile in the sense of the HTML specification, in section 22.214.171.124 Meta data profiles.
The following term is introduced here as an XHTML link relationship name and RDF property name:
Modelling questions Besides modelling the tagging relationship, modelling tags is itself a challenge. Simple literals are inadequate — e.g., we cannot use them as subjects of triples. Instead, I suggest that tags are modelled through URIs. This seems to agree with some other usages; e.g. WordNet, SKOS, Annotea…? ... Fundamental design The fundamental design decisions so far are thus: 1. Taggers are foaf:Agents. 2. Taggings reify the n-ary relationship between a tagger, a tag, a resource, and a date. Relationships exist for each of these roles. 3. Tags are members of a Tag class. Tags have names. We do not attempt to implement plurals (as in the labels schema) or synonymity at this stage, as these are subjective assessments of a tag
[Question: What if I want to remove the agent/tagger from the equation and just have one set of tags used by all users?]
Using a tag namespace to allow for repeated references to the same tag (not just different tags with the same label):ex:post tags:taggedWithTag tag:great , tag:interesting . # Label the tags — once is enough. tag:great a tags:Tag ; tags:tagName "great" . tag:interesting a tags:Tag ; tags:tagName "interesting" .
With an identified Tagging event, so we can refer to the event itself in future:ex:post tags:tag tagging:abcde . tagging:abcde a tags:Tagging ; tags:associatedTag tag:great , tag:interesting ; tags:taggedBy [ foaf:mbox <mailto:email@example.com> ] ; tags:taggedOn "2005-12-03T21:15:00.000Z"^^xsd:dateTime .
Relationships between tags
It is desirable to provide some way to interrelate tags; e.g. to describe synonymity, or more human relations like simple association. Of course, these relations are not absolute; statingtag:ruby :relatedTag tag:gemstone
will be useless or wrong to a significant proportion of users. more accurate is to reify:_:x a :TagRelation ; rdf:subject tag:ruby ; rdf:object tag:programming ; rdf:predicate skos:broader ; :relater ex:Richard . _:y a :TagRelation ; rdf:subject tag:ruby ; rdf:object tag:gemstone ; rdf:predicate :relatedTag ; :relater ex:John .
These terms are experimental, and there are probably better ways to do this, but you get the idea.
Note, of course, that this vocabulary does nothing to stop you tagging tags…
It's sort of like a tree, but since it can have loops, I think it's even more closely represented by a vertex-labeled directed (hierarchy) multigraph/pseudograph. (See http://en.wikipedia.org/wiki/Glossary_of_graph_theory)
Just like how RDF data in general "represents a labeled, directed pseudo-graph".
Or, to put it another way, a collection of tags/categories and their assignments can be represented by a bunch of triples...
article1 hasTag tag1 article7 hasTag tag3 tag1 hasTag tag3 tag2 hasTag tag22
A collection of RDF statements intrinsically represents a labeled, directed pseudo-graph. As such, an RDF-based data model is more naturally suited to certain kinds of knowledge representation than the relational model and other ontological models traditionally used in computing today. However, in practice, RDF data is often stored in relational database representations sometimes also called triple stores. As RDFS and OWL demonstrate, additional ontology languages can be built upon RDF.
Example 1: The postal abbreviation for New York
Certain concepts in RDF are taken from logic and linguistics, where subject-predicate and subject-predicate-object structures have meanings similar to, yet distinct from, the uses of those terms in RDF. This example demonstrates:
In the English language statement 'New York has the postal abbreviation NY' , 'New York' would be the subject, 'has the postal abbreviation' the predicate and 'NY' the object.
Encoded as an RDF triple, the subject and predicate would have to be resources named by URIs. The object could be a resource or literal element. For example, in the N-Triples form of RDF, the statement might look like:<urn:states:New%20York> <http://purl.org/dc/terms/alternative> "NY" .
In this example, "urn:states:New%20York" is the URI for a resource that denotes the U.S. state New York, "http://purl.org/dc/terms/alternative" is the URI for a predicate (whose human-readable definition can be found at ), and "NY" is a literal string. Note that the URIs chosen here are not standard, and don't need to be, as long as their meaning is known to whatever is reading them.
N-Triples is just one of several standard serialization formats for RDF. The triple above can also be equivalently represented in the standard RDF/XML format as:<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:terms="http://purl.org/dc/terms/"> <rdf:Description rdf:about="urn:states:New%20York"> <terms:alternative>NY</terms:alternative> </rdf:Description> </rdf:RDF>
However, because of the restrictions on the syntax of QNames (such as terms:alternative above), there are some RDF graphs that are not representable with RDF/XML.
Statement reification and context
The body of knowledge modeled by a collection of statements may be subjected to reification, in which each statement (that is each triple subject-predicate-object altogether) is assigned a URI and treated as a resource about which additional statements can be made, as in "Jane says that John is the author of document X". Reification is sometimes important in order to deduce a level of confidence or degree of usefulness for each statement.
The Decentralized Information Group explores technical, institutional, and public policy questions necessary to advance the development of global, decentralized information environments.
Aliases: Decentralization of information