# RDF::RDFa reader/writer
[RDFa][RDFa 1.1 Core] parser for RDF.rb.
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## DESCRIPTION
RDF::RDFa is an RDFa reader and writer for Ruby using the [RDF.rb][RDF.rb] library suite.
## FEATURES
RDF::RDFa parses [RDFa][RDFa 1.1 Core] into statements or triples.
* Fully compliant RDFa 1.1 parser.
* Template-based Writer to generate XHTML+RDFa.
* Writer uses user-replaceable [Haml][Haml] -based templates to generate RDFa.
* If available, uses [Nokogiri][] for parsing HTML/SVG, falls back to REXML otherwise. For HTML5, include the [Nokogumbo][] gem for a pure-HTML5 parser with better error detection.
Install with `gem install rdf-rdfa`
### Pure Ruby
In order to run as pure ruby (not requiring any C modules), this gem does not directly depend on [Nokogiri][] or [Nokogumbo][]
and falls back to using REXML. [Nokogumbo][] uses the [Gumbo pure-C HTML5 parser](https://github.com/google/gumbo-parser#readme).
As REXML is not really an HTML parsing library, the results will only be useful if the HTML is well-formed.
For best performance, install the [Nokogiri][] and [Nokogumbo][] gems as well.
### Important changes from previous versions
RDFa is an evolving standard, undergoing some substantial recent changes partly due to perceived competition
with Microdata. As a result, the RDF Webapps working group is currently looking at changes in the processing model for RDFa. These changes are now being tracked in {RDF::RDFa::Reader}:
#### RDFa 1.1 Lite
This version fully supports the limited syntax of [RDFa Lite 1.1][]. This includes the ability to use `@property` exclusively.
#### Vocabulary Expansion
One of the issues with vocabularies was that they discourage re-use of existing vocabularies when terms from several vocabularies are used at the same time. As it is common (encouraged) for RDF vocabularies to form sub-class and/or sub-property relationships with well defined vocabularies, the RDFa vocabulary expansion mechanism takes advantage of this.
As an optional part of RDFa processing, an RDFa processor will perform limited
[OWL 2 RL Profile entailment](https://www.w3.org/TR/2009/REC-owl2-profiles-20091027/#Reasoning_in_OWL_2_RL_and_RDF_Graphs_using_Rules),
specifically rules prp-eqp1, prp-eqp2, cax-sco, cax-eqc1, and
cax-eqc2. This causes sub-classes and sub-properties of type and property IRIs to be added
to the output graph.
{RDF::RDFa::Reader} implements this using the `#expand` method, which looks for `rdfa:usesVocabulary` properties within the output graph and performs such expansion. See an example in the usage section.
#### Experimental support for rdfa:copy template expansion
RDFa 1.1 is just about an exact super-set of microdata, except for microdata's
`@itemref` feature. Experimental support is added for `rdfa:copy` and `rdfa:Pattern` to get a similar effect using expansion. To use this,
reference another resource using `rdfa:copy`. If that resource has the type
`rdfa:Pattern`, the properties defined there will be added to the resource
containing the `rdfa:copy`, and the pattern and `rdfa:copy` will be removed
from the output.
For example, consider the following:
<div>
<div typeof="schema:Person">
<link property="rdfa:copy" resource="_:a"/>
</div>
<p resource="_:a" typeof="rdfa:Pattern">Name: <span property="schema:name">Amanda</span></p>
</div>
if run with vocabulary expansion, this will result in the following Turtle:
@prefix schema: <http://schema.org/> .
[a schema:Person; schema:name "Amanda"] .
#### RDF Collections (lists)
One significant RDF feature missing from RDFa was support for ordered collections, or lists. RDF supports this with special properties `rdf:first`, `rdf:rest`, and `rdf:nil`, but other RDF languages have first-class support for this concept. For example, in [Turtle][Turtle], a list can be defined as follows:
[ a schema:MusicPlayList;
schema:name "Classic Rock Playlist";
schema:numTracks 5;
schema:tracks (
[ a schema:MusicRecording; schema:name "Sweet Home Alabama"; schema:byArtist "Lynard Skynard"]
[ a schema:MusicRecording; schema:name "Shook you all Night Long"; schema:byArtist "AC/DC"]
[ a schema:MusicRecording; schema:name "Sharp Dressed Man"; schema:byArtist "ZZ Top"]
[ a schema:MusicRecording; schema:name "Old Time Rock and Roll"; schema:byArtist "Bob Seger"]
[ a schema:MusicRecording; schema:name "Hurt So Good"; schema:byArtist "John Cougar"]
)
]
defines a playlist with an ordered set of tracks. RDFa adds the @inlist attribute, which is used to identify values (object or literal) that are to be placed in a list. The same playlist might be defined in RDFa as follows:
<div vocab="http://schema.org/" typeof="MusicPlaylist">
<span property="name">Classic Rock Playlist</span>
<meta property="numTracks" content="5"/>
<div rel="tracks" inlist="">
<div typeof="MusicRecording">
1.<span property="name">Sweet Home Alabama</span> -
<span property="byArtist">Lynard Skynard</span>
</div>
<div typeof="MusicRecording">
2.<span property="name">Shook you all Night Long</span> -
<span property="byArtist">AC/DC</span>
</div>
<div typeof="MusicRecording">
3.<span property="name">Sharp Dressed Man</span> -
<span property="byArtist">ZZ Top</span>
</div>
<div typeof="MusicRecording">
4.<span property="name">Old Time Rock and Roll</span>
<span property="byArtist">Bob Seger</span>
</div>
<div typeof="MusicRecording">
5.<span property="name">Hurt So Good</span>
<span property="byArtist">John Cougar</span>
</div>
</div>
</div>
This basically does the same thing, but places each track in an rdf:List in the defined order.
#### Magnetic @about/@typeof
The @typeof attribute has changed; previously, it always created a new subject, either using a resource from @about, @resource and so forth. This has long been a source of errors for people using RDFa. The new rules cause @typeof to bind to a subject if used with @about, otherwise, to an object, if either used alone, or in combination with some other resource attribute (such as @href, @src or @resource).
For example:
<div typeof="foaf:Person" about="https://greggkellogg.net/foaf#me">
<p property="name">Gregg Kellogg</span>
<a rel="knows" typeof="foaf:Person" href="https://manu.sporny.org/#this">
<span property="name">Manu Sporny</span>
</a>
</div>
results in
<https://greggkellogg.net/foaf#me> a foaf:Person;
foaf:name "Gregg Kellogg";
foaf:knows <https://manu.sporny.org/#this> .
<https://manu.sporny.org/#this> a foaf:Person;
foaf:name "Manu Sporny" .
Note that if the explicit @href is not present, i.e.,
<div typeof="foaf:Person" about="https://greggkellogg.net/foaf#me">
<p property="name">Gregg Kellogg</span>
<a href="knows" typeof="foaf:Person">
<span property="name">Manu Sporny</span>
</a>
</div>
this results in
<https://greggkellogg.net/foaf#me> a foaf:Person;
foaf:name "Gregg Kellogg";
foaf:knows [
a foaf:Person;
foaf:name "Manu Sporny"
].
### Support for embedded RDF/XML
If the document includes embedded RDF/XML, as is the case with many SVG documents, and the RDF::RDFXML gem is installed, the reader will add extracted triples to the default graph.
For example:
<?xml version="1.0" encoding="UTF-8"?>
<svg width="12cm" height="4cm" viewBox="0 0 1200 400"
xmlns:dc="http://purl.org/dc/terms/"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xml:base="http://example.net/"
xmlns="http://www.w3.org/2000/svg" version="1.2" baseProfile="tiny">
<desc property="dc:description">A yellow rectangle with sharp corners.</desc>
<metadata>
<rdf:RDF>
<rdf:Description rdf:about="">
<dc:title>Test 0304</dc:title>
</rdf:Description>
</rdf:RDF>
</metadata>
<!-- Show outline of canvas using 'rect' element -->
<rect x="1" y="1" width="1198" height="398"
fill="none" stroke="blue" stroke-width="2"/>
<rect x="400" y="100" width="400" height="200"
fill="yellow" stroke="navy" stroke-width="10" />
</svg>
generates the following turtle:
@prefix dc: <http://purl.org/dc/terms/> .
<http://example.net/> dc:title "Test 0304" ;
dc:description "A yellow rectangle with sharp corners." .
### Support for embedded N-Triples or Turtle
If the document includes a `<script>` element having an `@type` attribute whose value matches that of a loaded RDF reader (text/ntriples and text/turtle are loaded if they are availble), the data will be extracted and added to the default graph. For example:
<html>
<body>
<script type="text/turtle"><![CDATA[
@prefix foo: <http://www.example.com/xyz#> .
@prefix gr: <http://purl.org/goodrelations/v1#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
foo:myCompany
a gr:BusinessEntity ;
rdfs:seeAlso <http://www.example.com/xyz> ;
gr:hasLegalName "Hepp Industries Ltd."^^xsd:string .
]]></script>
</body>
</html>
generates the following Turtle:
```
@prefix foo: <http://www.example.com/xyz#> .
@prefix gr: <http://purl.org/goodrelations/v1#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
foo:myCompany
a gr:BusinessEntity ;
rdfs:seeAlso <http://www.example.com/xyz> ;
gr:hasLegalName "Hepp Industries Ltd."^^xsd:string .
```
### Support for Role Attribute
The processor will generate RDF triples consistent with the [Role Attr][] specification.
<div id="heading1" role="heading">
<p>Some contents that are a header</p>
</div>
generates the following Turtle:
@prefix xhv: <http://www.w3.org/1999/xhtml/vocab#> .
<#heading1> xhv:role xhv:heading.
### Support for microdata
The RDFa reader will call out to `RDF::Microdata::Reader`, if an `@itemscope` attribute is detected, and the microdata reader is loaded. This avoids a common problem when pages contain both microdata and RDFa, and only one processor is run.
### Support for value property
In an [RDFA+HTML Errata](https://www.w3.org/2001/sw/wiki/RDFa_1.1._Errata#Using_.3Cdata.3E.2C_.3Cinput.3E_and_.3Cli.3E_along_with_.40value), it was suggested that the `@value` attribute could be parsed to obtain a numeric literal; this is consistent with how it's treated in microdata+rdfa. This processor now parses the value of an `@value` property to determine if it is an `xsd:integer`, `xsd:float`, or `xsd:double`, and uses a plain literal otherwise. The datatype can be overriden using the `@datatype` attribute.
## Usage
### Reading RDF data in the RDFa format
graph = RDF::Graph.load("etc/doap.html", format: :rdfa)
### Reading RDF data with vocabulary expansion
graph = RDF::Graph.load("etc/doap.html", format: :rdfa, vocab_expansion: true)
or
graph = RDF::RDFa::Reader.open("etc/doap.html").expand
### Reading Processor Graph
graph = RDF::Graph.load("etc/doap.html", format: :rdfa, rdfagraph: :processor)
### Reading Both Processor and Output Graphs
graph = RDF::Graph.load("etc/doap.html", format: :rdfa, rdfagraph: [:output, :processor])
### Writing RDF data using the XHTML+RDFa format
require 'rdf/rdfa'
RDF::RDFa::Writer.open("etc/doap.html") do |writer|
writer << graph
end
Note that prefixes may be chained between Reader and Writer, so that the Writer will
use the same prefix definitions found during parsing:
prefixes = {}
graph = RDF::Graph.load("etc/doap.html", prefixes: prefixes)
puts graph.dump(:rdfa, prefixes: prefixes)
### Template-based Writer
The RDFa writer uses [Haml][Haml] templates for code generation. This allows
fully customizable RDFa output in a variety of host languages. The [default
template]({RDF::RDFa::Writer::DEFAULT_HAML}) generates human readable HTML5
output. A [minimal template]({RDF::RDFa::Writer::MIN_HAML}) generates HTML,
which is not intended for human consumption.
To specify an alternative Haml template, consider the following:
require 'rdf/rdfa'
RDF::RDFa::Writer.buffer(haml: RDF::RDFa::Writer::MIN_HAML) << graph
The template hash defines four Haml templates:
* _doc_: Document Template, takes an ordered list of _subject_s and yields each one to be rendered. From {RDF::RDFa::Writer#render_document}:
{include:RDF::RDFa::Writer#render_document}
This template takes locals _lang_, _prefix_, _base_, _title_ in addition to _subjects_
to create output similar to the following:
<!DOCTYPE html>
<html prefix='xhv: http://www.w3.org/1999/xhtml/vocab#' xmlns='http://www.w3.org/1999/xhtml'>
<head>
<base href="http://example/">
<title>Document Title</title>
</head>
<body>
...
</body>
</html>
Options passed to the Writer are used to supply _lang_ and _base_ locals.
_prefix_ is generated based upon prefixes found from the default profiles, as well
as those provided by a previous Reader. _title_ is taken from the first top-level subject
having an appropriate title property (as defined by the _heading\_predicates_ option).
* _subject_: Subject Template, take a _subject_ and an ordered list of _predicate_s and yields
each _predicate_ to be rendered. From {RDF::RDFa::Writer#render_subject}:
{include:RDF::RDFa::Writer#render_subject}
The template takes locals _rel_ and _typeof_ in addition to _predicates_ and _subject_ to
create output similar to the following:
<div resource="http://example/">
...
</div>
Note that if _typeof_ is defined, in this template, it will generate a textual description.
* _property\_value_: Property Value Template, used for predicates having a single value; takes
a _predicate_, and a single-valued Array of _objects_. From {RDF::RDFa::Writer#render_property}:
{include:RDF::RDFa::Writer#render_property}
In addition to _predicate_ and _objects_, the template takes _inlist_ to indicate that the
property is part of an `rdf:List`.
Also, if the predicate is identified as a _heading predicate_ (via _:heading\_predicates_ option),
it will generate a heading element, and may use the value as the document title.
Each _object_ is yielded to the calling block, and the result is rendered, unless nil.
Otherwise, rendering depends on the type of _object_. This is useful for recursive document
descriptions.
Creates output similar to the following:
<div class='property'>
<span class='label'>
xhv:alternate
</span>
<a property='xhv:alternate' href='http://rdfa.info/feed/'>http://rdfa.info/feed/</a>
</div>
Note the use of methods defined in {RDF::RDFa::Writer} useful in rendering the output.
* _property\_values_: Similar to _property\_value_, but for predicates having more than one value.
Locals are identical to _property\_values_, but _objects_ is expected to have more than one value. Described further in {RDF::RDFa::Writer#render_property}.
In this case, and unordered list is used for output. Creates output similar to the following:
<div class='property'>
<span class='label'>
xhv:bookmark
</span>
<ul rel='xhv:bookmark'>
<li>
<a href='http://rdfa.info/2009/12/12/oreilly-catalog-uses-rdfa/'>
http://rdfa.info/2009/12/12/oreilly-catalog-uses-rdfa/
</a>
</li>
<a href='http://rdfa.info/2010/05/31/new-rdfa-checker/'>
http://rdfa.info/2010/05/31/new-rdfa-checker/
</a>
</li>
</ul>
</div>
If _property\_values_ does not exist, repeated values will be replecated
using _property\_value_.
* Type-specific templates.
To simplify generation of different output types, the
template may contain a elements indexed by a URI. When a subject with an rdf:type
matching that URI is found, subsequent Haml definitions will be taken from
the associated Hash. For example:
{
document: "...",
subject: "...",
:property\_value => "...",
:property\_values => "...",
RDF::URI("http://schema.org/Person") => {
subject: "...",
:property\_value => "...",
:property\_values => "...",
}
}
## Dependencies
* [Ruby](https://ruby-lang.org/) (>= 2.4)
* [RDF.rb](https://rubygems.org/gems/rdf) (~> 3.1)
* [Haml](https://rubygems.org/gems/haml) (~> 5.1)
* [HTMLEntities](https://rubygems.org/gems/htmlentities) (>= 4.3)
* Soft dependency on [Nokogiri](https://rubygems.org/gems/nokogiri) (>= 1.10)
* Soft dependency on [Nokogumbo](https://github.com/rubys/nokogumbo) (>= 2.0)
## Documentation
Full documentation available on [Rubydoc.info][RDFa doc]
### Principle Classes
* {RDF::RDFa::Format}
* {RDF::RDFa::Reader}
* {RDF::RDFa::Reader::Nokogiri}
* {RDF::RDFa::Reader::REXML}
* {RDF::RDFa::Context}
* {RDF::RDFa::Expansion}
* {RDF::RDFa::Writer}
## TODO
* Add support for LibXML and REXML bindings, and use the best available
* Consider a SAX-based parser for improved performance
## Resources
* [RDF.rb][RDF.rb]
* [Distiller](http://rdf.greggkellogg.net/distiller)
* [Documentation][RDFa doc]
* [History](file:History.md)
* [RDFa 1.1 Core][RDFa 1.1 Core]
* [XHTML+RDFa 1.1][XHTML+RDFa 1.1]
* [RDFa-test-suite](https://rdfa.info/test-suite/ "RDFa test suite")
## Author
* [Gregg Kellogg](https://github.com/gkellogg) - <https://greggkellogg.net/>
## Contributors
* [Nicholas Humfrey](https://github.com/njh) - <https://njh.me/>
## Contributing
This repository uses [Git Flow](https://github.com/nvie/gitflow) to mange development and release activity. All submissions _must_ be on a feature branch based on the _develop_ branch to ease staging and integration.
* Do your best to adhere to the existing coding conventions and idioms.
* Don't use hard tabs, and don't leave trailing whitespace on any line.
* Do document every method you add using [YARD][] annotations. Read the
[tutorial][YARD-GS] or just look at the existing code for examples.
* Don't touch the `.gemspec`, `VERSION` or `AUTHORS` files. If you need to
change them, do so on your private branch only.
* Do feel free to add yourself to the `CREDITS` file and the corresponding
list in the the `README`. Alphabetical order applies.
* Do note that in order for us to merge any non-trivial changes (as a rule
of thumb, additions larger than about 15 lines of code), we need an
explicit [public domain dedication][PDD] on record from you,
which you will be asked to agree to on the first commit to a repo within the organization.
Note that the agreement applies to all repos in the [Ruby RDF](https://github.com/ruby-rdf/) organization.
## License
This is free and unencumbered public domain software. For more information,
see <https://unlicense.org/> or the accompanying [UNLICENSE](UNLICENSE) file.
## FEEDBACK
* gregg@greggkellogg.net
* <https://rubygems.org/rdf-rdfa>
* <https://github.com/ruby-rdf/rdf-rdfa>
* <https://lists.w3.org/Archives/Public/public-rdf-ruby/>
[RDF.rb]: https://rubygems.org/gems/rdf
[YARD]: https://yardoc.org/
[YARD-GS]: https://rubydoc.info/docs/yard/file/docs/GettingStarted.md
[PDD]: https://unlicense.org/#unlicensing-contributions
[RDFa 1.1 Core]: https://www.w3.org/TR/2012/REC-rdfa-core-20120607/ "RDFa 1.1 Core"
[RDFa Lite 1.1]: https://www.w3.org/TR/2012/REC-rdfa-lite-20120607/ "RDFa Lite 1.1"
[XHTML+RDFa 1.1]: https://www.w3.org/TR/2012/REC-xhtml-rdfa-20120607/ "XHTML+RDFa 1.1"
[HTML+RDFa 1.1]: https://www.w3.org/TR/rdfa-in-html/ "HTML+RDFa 1.1"
[RDFa-test-suite]: https://rdfa.info/test-suite/ "RDFa test suite"
[Role Attr]: https://www.w3.org/TR/role-attribute/ "Role Attribute"
[RDFa doc]: https://rubydoc.info/github/ruby-rdf/rdf-rdfa/frames
[Haml]: https://haml-lang.com/
[Turtle]: https://www.w3.org/TR/2011/WD-turtle-20110809/
[Nokogiri]: https://www.nokogiri.org
[Nokogumbo]: https://github.com/rubys/nokogumbo/#readme