core: add documentation to README
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Marcel Otto
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README.md
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README.md
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# RDF.ex Core
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# RDF.ex
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An implementation of the [RDF](https://www.w3.org/TR/rdf11-primer/) data model in Elixir.
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An implementation of the RDF and the basic accompanied standards for Elixir.
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## Installation
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@ -25,46 +26,335 @@ The [Hex package](https://hex.pm/docs/publish) can be installed as usual:
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## Introduction
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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 RDF triples, consistenting of a three nodes:
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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:
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1. a subject node with an IRI or a blank node,
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2. a predicate node with the IRI of a RDF property,
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3. an object nodes with an IRI, a blank node or a RDF literal value.
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Let's start examining how the different types of nodes - the RDF standards also calls them RDF terms - are represented in Elixir.
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Let's see how the different types of nodes are represented with RDF.ex in Elixir.
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### Nodes
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### URIs
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#### Literals
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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.
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#### URIs
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Although the RDF standards speaks of IRIs, an internationalized generalization of URIs, RDF.ex currently supports only URIs. They are represented by Elixirs builtin [`URI`](http://elixir-lang.org/docs/stable/elixir/URI.html) struct.
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The `RDF` module defines a handy generator function `RDF.uri`
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The `RDF` module defines a handy constructor function `RDF.uri/1`:
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```elixir
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RDF.uri("http://www.example.com/foo")
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```
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Besides being shorter than `URI.parse`, it will provide a gentlier migration, if we decide to switch to a dedicated, more optimized URI-representation for RDF.ex.
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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.
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#### Vocabularies
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An URI can also be created with the `~I` sigil:
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But rather than having to pass a fully qualified URI string to `RDF.uri`, it allows for something similar to QNames of XML.
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```elixir
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~I<http://www.example.com/foo>
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```
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But there's an even shorter way notation for providing URI literals.
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### Vocabularies
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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.
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Furthermore, the [rdf_vocab](https://hex.pm/packages/rdf_vocab) package
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contains predefined `RDF.Vocabulary.Namespace`s for popular vocabularies.
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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.
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There are two types of terms in a `RDF.Vocabulary.Namespace` which are
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resolved differently:
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1. Capitalized terms are by standard Elixir semantics modules names, i.e.
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atoms. In all places in RDF.ex, where an URI is expected, you can use atoms
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qualified with a `RDF.Namespace` directly, but if you want to resolve it
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manually, you can pass the `RDF.Namespace` qualified atom to `RDF.uri`.
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2. Lowercased terms for RDF properties are represented as functions on a
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`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.
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```elixir
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iex> import RDF, only: [uri: 1]
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iex> alias RDF.NS.{RDFS}
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iex> RDFS.Class
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RDF.NS.RDFS.Class
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iex> uri(RDFS.Class)
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%URI{authority: "www.w3.org", fragment: "Class", host: "www.w3.org",
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path: "/2000/01/rdf-schema", port: 80, query: nil, scheme: "http",
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userinfo: nil}
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iex> RDFS.subClassOf
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%URI{authority: "www.w3.org", fragment: "subClassOf", host: "www.w3.org",
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path: "/2000/01/rdf-schema", port: 80, query: nil, scheme: "http",
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userinfo: nil}
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iex> uri(RDFS.subClassOf)
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%URI{authority: "www.w3.org", fragment: "subClassOf", host: "www.w3.org",
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path: "/2000/01/rdf-schema", port: 80, query: nil, scheme: "http",
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userinfo: nil}
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```
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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).
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```elixir
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iex> import RDF, only: [uri: 1]
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iex> RDF.type
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iex> uri(RDF.Property)
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```
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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).
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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.
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#### Description DSL
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The functions on the vocabulary namespace modules for properties, also are also available in description builder variant, which accepts subject and objects as arguments.
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```elixir
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RDF.type(EX.Foo, EX.Bar)
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```
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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:
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```elixir
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RDF.type(EX.Foo, EX.Bar, EX.Baz)
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EX.foo(EX.Bar, [1, 2, 3, 4, 5, 6])
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```
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In combination with Elixirs pipe operators this leads to a description DSL which resembles Turtle:
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```elixir
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EX.Foo
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|> RDF.type(EX.Bar)
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|> EX.baz(1, 2, 3)
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```
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The produced statements are returned by this function as a `RDF.Description` structure which will be described below.
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#### Defining vocabulary namespaces
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There are two basic ways to define a namespace for a vocabulary:
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1. You can define all terms manually.
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2. You can extract the terms from existing RDF data for URIs of resources under the specified base URI.
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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.
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A vocabulary namespace with manually defined terms can be defined in this way like that:
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```elixir
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defmodule YourApp.NS do
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use RDF.Vocabulary.Namespace
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defvocab EX,
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base_uri: "http://www.example.com/ns/",
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terms: ~w[Foo bar]
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end
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```
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The `base_uri` argument with the URI prefix of all the terms in the defined
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vocabulary is required and expects a valid URI ending with either a `"/"` or
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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`.
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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:
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```elixir
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defmodule YourApp.NS do
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use RDF.Vocabulary.Namespace
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defvocab EX,
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base_uri: "http://www.example.com/ns/",
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file: "your_vocabulary.nt"
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end
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```
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Currently only NTriple and NQuad files are supported at this place.
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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.
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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`.
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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:
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```elixir
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defmodule YourApp.NS do
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use RDF.Vocabulary.Namespace
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defvocab EX,
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base_uri: "http://www.example.com/ns/",
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terms: ~w[example-term],
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alias: [example_term: "example-term"]
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end
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```
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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:
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```elixir
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defmodule YourApp.NS do
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use RDF.Vocabulary.Namespace
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defvocab EX,
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base_uri: "http://www.example.com/ns/",
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terms: [],
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strict: false
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end
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iex> import RDF, only: [uri: 1]
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iex> alias YourApp.NS.{EX}
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iex> uri(EX.Foo)
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%URI{authority: "www.example.com", fragment: nil, host: "www.example.com",
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path: "/ns/Foo", port: 80, query: nil, scheme: "http", userinfo: nil}
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iex> EX.bar
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%URI{authority: "www.example.com", fragment: nil, host: "www.example.com",
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path: "/ns/bar", port: 80, query: nil, scheme: "http", userinfo: nil}
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iex> EX.Foo |> EX.bar(EX.Baz)
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#RDF.Description{subject: ~I<http://www.example.com/ns/Foo>
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~I<http://www.example.com/ns/bar>
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~I<http://www.example.com/ns/Baz>}
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```
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### Blank nodes
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TODO
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### Literals
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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`:
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```elixir
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RDF.Literal.new("foo")
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RDF.literal("foo")
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```
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The actual value can be accessed via the `value` struct field:
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```elixir
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RDF.literal("foo").value
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```
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An untyped literal can also be created with the `~L` sigil:
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```elixir
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import RDF.Sigils
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~L"foo"
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```
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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:
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```elixir
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RDF.literal("foo", language: "en")
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import RDF.Sigils
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~L"foo"en
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```
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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`.
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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/):
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```elixir
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RDF.literal("42", datatype: XSD.integer)
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```
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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:
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| Elixir datatype | XSD datatype |
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| :-------------- | :------------- |
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| `boolean` | `xsd:boolean` |
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| `integer` | `xsd:integer` |
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| `float` | `xsd:double` |
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| `Time` | `xsd:time` |
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| `Date` | `xsd:date` |
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| `DateTime` | `xsd:dateTime` |
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| `NaiveDateTime` | `xsd:dateTime` |
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So the former example literal can be created equivalently like this:
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```elixir
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RDF.literal(42)
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```
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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.
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```elixir
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iex> RDF.literal(42, datatype: XSD.double).value
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42.0
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```
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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:
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```elixir
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iex> RDF.Double.new("0042").value
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42.0
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iex> RDF.Double.new(42).value
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42.0
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```
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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/).
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```elixir
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iex> RDF.Literal.valid? RDF.Integer.new("42")
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true
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iex> RDF.Literal.valid? RDF.Integer.new("foo")
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false
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```
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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:
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```elixir
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iex> RDF.Literal.lexical RDF.Integer.new("0042")
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"0042"
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iex> RDF.Literal.lexical RDF.Integer.new(42)
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"42"
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```
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Although two literals might have the same value, they are not equal when they don't have the same lexical form:
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```elixir
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iex> RDF.Integer.new("0042").value == RDF.Integer.new("42").value
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true
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iex> RDF.Integer.new("0042") == RDF.Integer.new("42")
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false
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```
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The `RDF.Literal.canonical/1` function returns the given literal with its canonical lexical form according its datatype:
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```elixir
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iex> RDF.Integer.new("0042") |> RDF.Literal.canonical |> RDF.Literal.lexical
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"42"
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iex> RDF.Literal.canonical(RDF.Integer.new("0042")) ==
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RDF.Literal.canonical(RDF.Integer.new("42"))
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true
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```
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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.
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#### Blank nodes
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### RDF data structures
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#### Statements
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#### Descriptions
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#### Graphs
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#### Datasets
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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.
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### Triples
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### Graphs and Descriptions
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#### `RDF.Data` protocol
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### Serializations
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### Repositories
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## Getting help
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@ -84,4 +374,5 @@ see [CONTRIBUTING](CONTRIBUTING.md) for details.
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## License and Copyright
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(c) 2016 Marcel Otto. MIT Licensed, see [LICENSE](LICENSE.txt) for details.
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(c) 2017 Marcel Otto. MIT Licensed, see [LICENSE](LICENSE.txt) for details.
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