Triples consist of the following three terms:

• Blank nodes are encoded by atoms that start with‘_:`.
• IRIs appear in two notations:
  • Full IRIs are encoded by atoms that do not start with‘_:`. Specifically, an IRI term is not required to follow the IRI standard grammar.
  • Abbreviated IRI notation that allows IRI prefix aliases that are registered by rdf_register_prefix/[2,3] to be used. Their notation is Alias:Local, where Alias and Local are atoms. Each abbreviated IRI is expanded by the system to a full IRI.

• Literals appear in two notations:

  String @ Lang

  A language-tagged string, where String is a Prolog string and Lang is an atom.

  Value ^^ Type

  A type qualified literal. For unknown types, Value is a Prolog string. If type is known, the Prolog representations from the table below are used.

  Datatype IRI	Prolog term
  xsd:float	float
  xsd:double	float
  xsd:decimal	float (1)
  xsd:integer	integer
  XSD integer sub-types	integer
  xsd:boolean	true or false
  xsd:date	date(Y,M,D)
  xsd:dateTime	date_time(Y,M,D,HH,MM,SS) (2,3)
  xsd:gDay	integer
  xsd:gMonth	integer
  xsd:gMonthDay	month_day(M,D)
  xsd:gYear	integer
  xsd:gYearMonth	year_month(Y,M)
  xsd:time	time(HH,MM,SS) (2)

Notes:

(1) The current implementation of xsd:decimal values as floats is formally incorrect. Future versions of SWI-Prolog may introduce decimal as a subtype of rational.

(2) SS fields denote the number of seconds. This can either be an integer or a float.

(3) The date_time structure can have a 7th field that denotes the timezone offset in seconds as an integer.

In addition, a ground object value is translated into a properly typed RDF literal using rdf_canonical_literal/2.

There is a fine distinction in how duplicate statements are handled in rdf/[3,4]: backtracking over rdf/3 will never return duplicate triples that appear in multiple graphs. rdf/4 will return such duplicate triples, because their graph term differs.

See also

- Triple pattern querying
- xsd_number_string/2 and xsd_time_string/3 are used to convert between lexical representations and Prolog terms.