# RDF.ex An implementation of the [RDF](https://www.w3.org/TR/rdf11-primer/) data model in Elixir. ## Installation The [Hex package](https://hex.pm/docs/publish) can be installed as usual: 1. Add `rdf` to your list of dependencies in `mix.exs`: ```elixir def deps do [{:rdf, "~> 0.1.0"}] end ``` 2. Ensure `rdf` is started before your application: ```elixir def application do [applications: [:rdf]] end ``` ## Introduction The [RDF standard](http://www.w3.org/TR/rdf11-concepts/) defines a graph data model for distributed information on the web. A RDF graph is a set of statements aka RDF triples consistenting of a three nodes: 1. a subject node with an IRI or a blank node, 2. a predicate node with the IRI of a RDF property, 3. an object nodes with an IRI, a blank node or a RDF literal value. Let's see how the different types of nodes are represented with RDF.ex in Elixir. ### URIs Although the RDF standards speaks of IRIs, an internationalized generalization of URIs, RDF.ex currently supports only URIs. They are represented with Elixirs builtin [`URI`](http://elixir-lang.org/docs/stable/elixir/URI.html) struct. Its a pragmatic, temporary decision, which will be subject to changes very probably, in favour of a more dedicated representation of IRIs specialised for its usage within RDF data. See this [issue]() for progress on this matter. The `RDF` module defines a handy constructor function `RDF.uri/1`: ```elixir RDF.uri("http://www.example.com/foo") ``` Besides being a little shorter than `URI.parse` and better `import`able, it will provide a gentlier migration to the mentioned, more optimized URI-representation in RDF.ex. An URI can also be created with the `~I` sigil: ```elixir ~I ``` But there's an even shorter way notation for providing URI literals. ### Vocabularies RDF.ex supports modules which represent a RDF vocabulary as a `RDF.Vocabulary.Namespace` and comes with predefined modules for some fundamentals vocabularies in the `RDF.NS` module. Furthermore, the [rdf_vocab](https://hex.pm/packages/rdf_vocab) package contains predefined `RDF.Vocabulary.Namespace`s for popular vocabularies. These `RDF.Vocabulary.Namespace`s (a special case of a `RDF.Namespace`) allows for something similar to QNames of XML: a qualified atom with a Elixir module can be resolved to an URI. There are two types of terms in a `RDF.Vocabulary.Namespace` which are resolved differently: 1. Capitalized terms are by standard Elixir semantics modules names, i.e. atoms. In all places in RDF.ex, where an URI is expected, you can use atoms qualified with a `RDF.Namespace` directly, but if you want to resolve it manually, you can pass the `RDF.Namespace` qualified atom to `RDF.uri`. 2. Lowercased terms for RDF properties are represented as functions on a `RDF.Vocabulary.Namespace` module and return the URI directly, but since `RDF.uri` can also handle URIs directly, you can safely and consistently use it with lowercased terms too. ```elixir iex> import RDF, only: [uri: 1] iex> alias RDF.NS.{RDFS} iex> RDFS.Class RDF.NS.RDFS.Class iex> uri(RDFS.Class) %URI{authority: "www.w3.org", fragment: "Class", host: "www.w3.org", path: "/2000/01/rdf-schema", port: 80, query: nil, scheme: "http", userinfo: nil} iex> RDFS.subClassOf %URI{authority: "www.w3.org", fragment: "subClassOf", host: "www.w3.org", path: "/2000/01/rdf-schema", port: 80, query: nil, scheme: "http", userinfo: nil} iex> uri(RDFS.subClassOf) %URI{authority: "www.w3.org", fragment: "subClassOf", host: "www.w3.org", path: "/2000/01/rdf-schema", port: 80, query: nil, scheme: "http", userinfo: nil} ``` As this example shows the namespace modules can be easily `alias`ed. When required they can be also aliased to different a different module. Since the `RDF` vocabulary namespace in `RDF.NS.RDF` can't be aliased, since it would clash with top-level `RDF` module, all of its elements can be accessed directly from the `RDF` module (without an alias). ```elixir iex> import RDF, only: [uri: 1] iex> RDF.type iex> uri(RDF.Property) ``` This way of expressing URIs has the additional benefit, that the existence of the referenced URI is checked at compile time, i.e. whenever a term is used that is not part of the resp. vocabulary an error is raised by the Elixir compiler (unless the vocabulary namespace is non-strict; see below). For terms not adhering to the capitalization rules (properties lowercased, non-properties capitalized) or containing characters not allowed within atoms, these namespace define aliases accordingly. If not sure, you can look in the documentation or the vocabulary namespace definition. #### Description DSL The functions on the vocabulary namespace modules for properties, also are also available in description builder variant, which accepts subject and objects as arguments. ```elixir RDF.type(EX.Foo, EX.Bar) ``` If you want to state multiple statements with the same subject and predicate, you can either pass the objects as a list or, if there are not more not five of them, as additional arguments: ```elixir RDF.type(EX.Foo, EX.Bar, EX.Baz) EX.foo(EX.Bar, [1, 2, 3, 4, 5, 6]) ``` In combination with Elixirs pipe operators this leads to a description DSL which resembles Turtle: ```elixir EX.Foo |> RDF.type(EX.Bar) |> EX.baz(1, 2, 3) ``` The produced statements are returned by this function as a `RDF.Description` structure which will be described below. #### Defining vocabulary namespaces There are two basic ways to define a namespace for a vocabulary: 1. You can define all terms manually. 2. You can extract the terms from existing RDF data for URIs of resources under the specified base URI. It's recommended to introduce a dedicated module for the defined namespace. On this module you'll `use RDF.Vocabulary.Namespace` and define your vocabulary namespaces with the `defvocab` macro. A vocabulary namespace with manually defined terms can be defined in this way like that: ```elixir defmodule YourApp.NS do use RDF.Vocabulary.Namespace defvocab EX, base_uri: "http://www.example.com/ns/", terms: ~w[Foo bar] end ``` The `base_uri` argument with the URI prefix of all the terms in the defined vocabulary is required and expects a valid URI ending with either a `"/"` or a `"#"`. Terms will be checked for invalid character at compile-time and will raise a compile error. This handling of invalid characters can be modified with the `invalid_characters` options, which is by default set to `:fail`. By setting it explicitly to `:warn` only warnings will be raised or it can be turned off completely with `:ignore`. A vocabulary namespace with extracted terms can be by either providing RDF data directly with the `data` option or from serialized RDF data file in the `priv/vocabs` directory: ```elixir defmodule YourApp.NS do use RDF.Vocabulary.Namespace defvocab EX, base_uri: "http://www.example.com/ns/", file: "your_vocabulary.nt" end ``` Currently only NTriple and NQuad files are supported at this place. During compilation the terms will be validated, if they are properly capitalized (properties lowercased, non-properties capitalized), by analyzing the schema description of the resp. resource in the given data. This validation behaviour can be modified with the `case_violations` options, which is by default set to `:warn`. By setting it explicitly to `:fail` errors will be raised during compilation or it can be turned off with `:ignore`. When the terms contain invalid characters or violate the capitalization rules, you can fix these by defining aliases for these terms with the `alias` option and a keyword list: ```elixir defmodule YourApp.NS do use RDF.Vocabulary.Namespace defvocab EX, base_uri: "http://www.example.com/ns/", terms: ~w[example-term], alias: [example_term: "example-term"] end ``` Though strictly discouraged, a vocabulary namespace can be defined as non-strict with the `strict` option set to `false`. A non-strict vocabulary doesn't require any terms to be defined (although they can). A term is resolved dynamically at runtime by simple concatentating the term with the base uri of the resp. namespace module: ```elixir defmodule YourApp.NS do use RDF.Vocabulary.Namespace defvocab EX, base_uri: "http://www.example.com/ns/", terms: [], strict: false end iex> import RDF, only: [uri: 1] iex> alias YourApp.NS.{EX} iex> uri(EX.Foo) %URI{authority: "www.example.com", fragment: nil, host: "www.example.com", path: "/ns/Foo", port: 80, query: nil, scheme: "http", userinfo: nil} iex> EX.bar %URI{authority: "www.example.com", fragment: nil, host: "www.example.com", path: "/ns/bar", port: 80, query: nil, scheme: "http", userinfo: nil} iex> EX.Foo |> EX.bar(EX.Baz) #RDF.Description{subject: ~I ~I ~I} ``` ### Blank nodes TODO ### Literals Literals are used for values such as strings, numbers, and dates. They can be untyped, languaged-tagged or typed. In general they are created with the `RDF.Literal.new` constructor function or its alias function `RDF.literal`: ```elixir RDF.Literal.new("foo") RDF.literal("foo") ``` The actual value can be accessed via the `value` struct field: ```elixir RDF.literal("foo").value ``` An untyped literal can also be created with the `~L` sigil: ```elixir import RDF.Sigils ~L"foo" ``` A language-tagged literal can be created by providing the `language` option with a [BCP47](https://tools.ietf.org/html/bcp47)-conform language or by adding the language as a modifier to the `~L` sigil: ```elixir RDF.literal("foo", language: "en") import RDF.Sigils ~L"foo"en ``` Note: Only languages without subtags are supported as modifiers of the `~L` sigil, i.e. if you want to use `en-US` as a language tag, you would have to use `RDF.literal` or `RDF.Literal.new`. A typed literal can be created by providing the `datatype` option with an URI of a datatype. Most of the time this will be an [XML schema datatype](https://www.w3.org/TR/xmlschema11-2/): ```elixir RDF.literal("42", datatype: XSD.integer) ``` It is also possible to create a typed literal by using a native Elixir non-string value, for which the following datatype mapping will be applied: | Elixir datatype | XSD datatype | | :-------------- | :------------- | | `boolean` | `xsd:boolean` | | `integer` | `xsd:integer` | | `float` | `xsd:double` | | `Time` | `xsd:time` | | `Date` | `xsd:date` | | `DateTime` | `xsd:dateTime` | | `NaiveDateTime` | `xsd:dateTime` | So the former example literal can be created equivalently like this: ```elixir RDF.literal(42) ``` For all of these known datatypes the `value` struct field contains the native Elixir value representation according to this mapping. When a known XSD datatype is specified the given value will be converted automatically if needed and possible. ```elixir iex> RDF.literal(42, datatype: XSD.double).value 42.0 ``` For all of these supported XSD datatypes `RDF.Datatype`s are available, which are modules that allow the creation of `RDF.Literal`s with the respective datatype: ```elixir iex> RDF.Double.new("0042").value 42.0 iex> RDF.Double.new(42).value 42.0 ``` The `RDF.Literal.valid?/1` function checks if a given literal is valid according to the [XML schema datatype specification](https://www.w3.org/TR/xmlschema11-2/). ```elixir iex> RDF.Literal.valid? RDF.Integer.new("42") true iex> RDF.Literal.valid? RDF.Integer.new("foo") false ``` A RDF literal is bound to the lexical form of the initially given value. This lexical representation can be retrieved with the `RDF.Literal.lexical/1` function: ```elixir iex> RDF.Literal.lexical RDF.Integer.new("0042") "0042" iex> RDF.Literal.lexical RDF.Integer.new(42) "42" ``` Although two literals might have the same value, they are not equal when they don't have the same lexical form: ```elixir iex> RDF.Integer.new("0042").value == RDF.Integer.new("42").value true iex> RDF.Integer.new("0042") == RDF.Integer.new("42") false ``` The `RDF.Literal.canonical/1` function returns the given literal with its canonical lexical form according its datatype: ```elixir iex> RDF.Integer.new("0042") |> RDF.Literal.canonical |> RDF.Literal.lexical "42" iex> RDF.Literal.canonical(RDF.Integer.new("0042")) == RDF.Literal.canonical(RDF.Integer.new("42")) true ``` Note: Although you can create any XSD datatype by using the resp. URI with the `datatype` option of `RDF.Literal.new`, not all of them support the validation and conversion behaviour of `RDF.Literal`s and the `value` field simple contains the initially given value unvalidated and unconverted. See [this project]() for the missing XSD datatypes. ### RDF data structures #### Statements #### Descriptions #### Graphs #### Datasets Multiple graphs in an RDF document constitute an RDF dataset. An RDF dataset may have multiple named graphs and at most one unnamed ("default") graph. #### `RDF.Data` protocol ### Serializations ## Getting help - [Hex]() - [Slack]() ## Development ## Contributing see [CONTRIBUTING](CONTRIBUTING.md) for details. ## License and Copyright (c) 2017 Marcel Otto. MIT Licensed, see [LICENSE](LICENSE.txt) for details.