# RDF::Turtle reader/writer

[Turtle][] reader/writer for [RDF.rb][RDF.rb] .

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## Description
This is a [Ruby][] implementation of a [Turtle][] parser for [RDF.rb][].

## Features
RDF::Turtle parses [Turtle][Turtle] and [N-Triples][N-Triples] into statements or triples. It also serializes to Turtle.

Install with `gem install rdf-turtle`

* 100% free and unencumbered [public domain](https://unlicense.org/) software.
* Implements a complete parser for [Turtle][].
* Compatible with Ruby >= 3.0.
* Optional streaming writer, to serialize large graphs
* Provisional support for [RDF 1.2][].

## Usage
Instantiate a reader from a local file:

    graph = RDF::Graph.load("etc/doap.ttl", format:  :ttl)

Define `@base` and `@prefix` definitions, and use for serialization using `:base_uri` an `:prefixes` options.

Canonicalize and validate using `:canonicalize` and `:validate` options.

Write a graph to a file:

    RDF::Turtle::Writer.open("etc/test.ttl") do |writer|
       writer << graph
    end

## RDF 1.2

Both reader and writer include provisional support for [RDF 1.2][] quoted triples.

Both reader and writer include provisional support for [RDF 1.2][] directional language-tagged strings, which are literals of type `rdf:dirLangString` having both a `language` and `direction`.

Internally, an `RDF::Statement` is treated as another resource, along with `RDF::URI` and `RDF::Node`, which allows an `RDF::Statement` to have a `#subject` or `#object` which is also an `RDF::Statement`.

**Note: This feature is subject to change or elimination as the standards process progresses.**

### Serializing a Graph containing quoted triples

    require 'rdf/turtle'
    statement = RDF::Statement(RDF::URI('bob'), RDF::Vocab::FOAF.age, RDF::Literal(23))
    graph = RDF::Graph.new << [statement, RDF::URI("ex:certainty"), RDF::Literal(0.9)]
    graph.dump(:ttl, validate: false, standard_prefixes: true)
    # => '<<<bob> foaf:age 23>> <ex:certainty> 9.0e-1 .'

### Reading a Graph containing quoted triples

By default, the Turtle reader will reject a document containing a subject resource.

    ttl = %(
      @prefix foaf: <http://xmlns.com/foaf/0.1/> .
      @prefix ex: <http://example.com/> .
      <<<bob> foaf:age 23>> ex:certainty 9.0e-1 .
    )
    graph = RDF::Graph.new do |graph|
      RDF::Turtle::Reader.new(ttl) {|reader| graph << reader}
    end
    # => RDF::ReaderError

Readers support a boolean valued `rdfstar` option; only one statement is asserted, although the reified statement is contained within the graph.

    graph = RDF::Graph.new do |graph|
      RDF::Turtle::Reader.new(ttl, rdfstar: true) {|reader| graph << reader}
    end
    graph.count #=> 1

### Reading a Graph containing statement annotations

Annotations are introduced using the `{| ... |}` syntax, which is treated like a `blankNodePropertyList`,
where the subject is the the triple ending with that annotation.

    ttl = %(
      @prefix foaf: <http://xmlns.com/foaf/0.1/> .
      @prefix ex: <http://example.com/> .
      <bob> foaf:age 23 {| ex:certainty 9.0e-1 |} .
    )
    graph = RDF::Graph.new do |graph|
      RDF::Turtle::Reader.new(ttl) {|reader| graph << reader}
    end
    # => RDF::ReaderError

Note that this requires the `rdfstar` option to be set.

## Documentation
Full documentation available on [Rubydoc.info][Turtle doc]

### Principle Classes
* {RDF::Turtle::Format}
* {RDF::Turtle::Reader}
* {RDF::Turtle::Writer}

### Variations from the spec
In some cases, the specification is unclear on certain issues:

* The LC version of the [Turtle][] specification separates rules for `@base` and `@prefix` with closing '.' from the SPARQL-like `BASE` and `PREFIX` without closing '.'. This version implements a more flexible syntax where the `@` and closing `.` are optional and `base/prefix` are matched case independently.
* Additionally, both `a` and `A` match `rdf:type`.

### Freebase-specific Reader
There is a special reader useful for processing [Freebase Dumps][]. To invoke
this, add the `freebase:  true` option to the {RDF::Turtle::Reader.new}, or
use {RDF::Turtle::FreebaseReader} directly. As with {RDF::Turtle::Reader},
prefix definitions may be passed in using the `:prefixes` option to
RDF::Turtle::FreebaseReader} using the standard mechanism defined
for `RDF::Reader`.

The [Freebase Dumps][] have a very normalized form, similar to N-Triples but
with prefixes. They also have a large amount of garbage. This Reader is
optimized for this format and will perform faster error recovery.

An example of reading Freebase dumps:

    require "rdf/turtle"
    fb = "../freebase/freebase-rdf-2013-03-03-00-00.ttl"
    fb_prefixes = {
      ns:  "http://rdf.freebase.com/ns/",
      key:  "http://rdf.freebase.com/key/",
      owl:  "http://www.w3.org/2002/07/owl#>",
      rdfs:  "http://www.w3.org/2000/01/rdf-schema#",
      rdf:  "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
      xsd:  "http://www.w3.org/2001/XMLSchema#"
    }
    RDF::Turtle::Reader.open(fb,
      freebase:  true,
      prefixes:  fb_prefixes) do |r|

      r.each_statement {|stmt| puts stmt.to_ntriples}
    end

## Implementation Notes
This version uses a hand-written parser using the Lexer from the [EBNF][] gem instead of a general [EBNF][] LL(1) parser for faster performance.

## Dependencies

* [Ruby](https://ruby-lang.org/) (>= 3.0)
* [RDF.rb](https://rubygems.org/gems/rdf) (~> 3.3)
* [EBNF][] (~> 2.4)

## Installation

The recommended installation method is via [RubyGems](https://rubygems.org/).
To install the latest official release of the `RDF::Turtle` gem, do:

    % [sudo] gem install rdf-turtle

## Mailing List
* <https://lists.w3.org/Archives/Public/public-rdf-ruby/>

## Author
* [Gregg Kellogg](https://github.com/gkellogg) - <https://greggkellogg.net/>

## 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 {file:UNLICENSE} file.

A copy of the [Turtle EBNF][] and derived parser files are included in the repository, which are not covered under the UNLICENSE. These files are covered via the [W3C Document License](https://www.w3.org/Consortium/Legal/2002/copyright-documents-20021231).

[Ruby]:         https://ruby-lang.org/
[RDF]:          https://www.w3.org/RDF/
[YARD]:         https://yardoc.org/
[YARD-GS]:      https://rubydoc.info/docs/yard/file/docs/GettingStarted.md
[PDD]:              https://unlicense.org/#unlicensing-contributions
[RDF.rb]:       https://ruby-rdf.github.io/rdf
[EBNF]:         https://rubygems.org/gems/ebnf
[N-Triples]:    https://www.w3.org/TR/rdf12-n-triples
[Turtle]:       https://www.w3.org/TR/rdf12-turtle
[RDF 1.1]:          https://www.w3.org/TR/rdf11-concepts/
[RDF 1.2]:          https://www.w3.org/TR/rdf12-concepts/
[Turtle doc]:   https://ruby-rdf.github.io/rdf-turtle/master/file/README.md
[Turtle EBNF]:  https://w3c.github.io/rdf-turtle/spec/turtle-bnf.html
[Freebase Dumps]: https://developers.google.com/freebase/data