RDF Terms

Reference: 4.1 RDF Term Syntax

This section discusses IRIs, variables, blank nodes, and literals in Flint and SPARQL. Many of the conventions here, including for prefixed IRIs, variables, and blank nodes, were borrowed from the Datalog grammar.

NOTE: For simplicity, many terms in Flint only allow a subset of the characters that the SPARQL spec allows. For example, the latter often accepts Unicode characters, while Flint is ASCII-only outside of IRIs or string literals.

IRIs

Internationalized Resource Identifiers (IRIs) and their subset Universal Resource Identifiers (URIs) are represented in two ways: as full IRIs or as prefixed IRIs.

Full IRIs

Examples: <http://absolute-iri-example.com/>, <relative-iri>

Full IRIs in Flint are written as strings of the form <my-iri-string>. The string inside the angle bracket pair can include any characters except for ^, <, >, ", \, |, `, or whitespace. Translating to SPARQL does not change full IRIs.

NOTE: This can mean that any string can become a IRI in Flint, though in practice they should conform to the specification for IRIs after expansion.

Prefixed IRIs

Examples: :my-prefix/foo, :bar

Prefixed IRIs in Flint are written as keywords of the form :prefix/name, where the prefix is optional. When translating to SPARQL, prefixed IRIs are transformed into the form prefix:name.

Prefix IRIs accept word characters and hyphens, with the exception of the first prefix char (which does not allow digits); periods can also be included in the middle of the prefix.

NOTE: Prefixed IRIs must have the prefix be an entry in the prefixes map; otherwise, validation will fail (unless :validate? is set to false).

a

Examples: a and :a.

There is a third IRI representation allowed in Flint for predicates: the keyword :a/symbol a. This is provided as syntactic sugar to compactly represent the IRI:

<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>

Translating to SPARQL does not affect :a/a other than stringifying it.

Therefore, in the the following query:

{:prefixes {:rdf "<http://www.w3.org/1999/02/22-rdf-syntax-ns#>"}
 :ask      []
 :where    [[?x "<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>" ?y]
            [?x :rdf/type ?y]
            [?x :a ?y]
            [?x a ?y]]}

the :where triples are all equivalent.

Variables

Examples: ?var

Variables are written as symbols prefixed with a question mark ?. The characters after the question mark can be any word character. Translating to SPARQL does not change the variable other than stringifying it.

Blank Nodes

Examples: _, _b0

Blank nodes are written as symbols prefixed with an underscore _. When translating to SPARQL, a colon is added after the underscore, e.g. _:b0. The exception is with _, which is rewritten as []. The characters after the underscore can be written as any word character; periods are also allowed in the middle.

NOTE: Blank nodes have certain restrictions - they cannot be used in any delete-related clauses, nor can the same blank node be repeated across different basic graph patterns or SPARQL updates.

Wildcard:

Examples: * and :*

The wildcard is used in certain query clauses and expressions to denote "return everything". It can be written as either a symbol or keyword.

Literals

Flint supports the following literals: simple strings, language-tagged strings, numbers, booleans, and dateTime timestamps.

NOTE: For simplicity, Flint does not allow for user-defined RDF types despite it being allowed on the SPARQL spec.

Numbers

Examples: 0, -2, 3.14

Numbers cover both integers and doubles, which are represented as integer and double literals in SPARQL. Neither are transformed during SPARQL translation beyond stringification

Booleans

Examples: true and false

Booleans are not transformed during SPARQL translation beyond stringification.

Simple strings

Examples: "Hello World!", "你好世界", "cat: \"meow\""

In Flint, string literals can contain any characters except unescaped line breaks, carriage returns, backslashes, or double quotes; this is in order to prevent SPARQL injection attacks. Strings are not transformed during SPARQL translation.

Language-tagged strings

Examples: {:en "Hello World!"}, {:zh "你好世界"}

In Flint, strings with language tags are represented by a map between one language tag keyword and the string literal (which follows the same restrictions as above).

dateTime timestamps

Examples: #inst "2022-01-01T10:10:10Z"

In Flint, dateTime timestamps include any values for which inst? is true. During SPARQL translation, an IRI denoting the type of the literal is added (since dateTime timestamps do not have a native representation in SPARQL):

"2022-01-01T10:10:10Z"^^<http://www.w3.org/2001/XMLSchema#dateTime>

However, if one includes an entry for the XMLSchema IRI prefix in their prefixes map, they can shorten the resulting string considerably; for example, if :xsd is associated with that prefix in the prefix map, then the string becomes:

"2022-01-01T10:10:10Z"^^xsd:dateTime