.. _library_datalog:

datalog

This library provides a portable Datalog and incremental rule engine for educational purposes and non-demanding applications. This library is work-in-progress and changes, possibly breaking backwards compatibility, are to be expected.

API documentation

Open the `../../apis/library_index.html#datalog <../../apis/library_index.html#datalog>`__ link in a web browser.

Loading

To load all entities in this library, load the loader.lgt file:

::

| ?- logtalk_load(datalog(loader)).

Testing

To run the library unit tests, load the tester.lgt file:

::

| ?- logtalk_load(datalog(tester)).

Scope

• positive and stratified negation rules (neg/1 body literals)
• conservative aggregates (agg(Op, Template, Goals, Result)) with lower-strata dependencies (Op in count, sum, min, max)
• safe rules (Head and negated literal variables must appear in positive Body literals)
• fixpoint materialization
• incremental updates using support-count based propagation
• simple explanations for derived facts
• rule management (add_rule/3, remove_rule/1)
• transaction support (begin/0, commit/0, rollback/0)
• stratum introspection (predicate_stratum/3, strata/1)
• rule body normalization (positive ground literals first, then positive non-ground, then aggregates, then negation)

Public API overview

• lifecycle: clear/0, load_program/1, materialize/0
• rule management: add_rule/3, remove_rule/1, rules/1
• fact management: assert_fact/1, retract_fact/1, facts/1
• querying and explanations: query/1, query/2, explain/2
• updates: update/3
• transactions: begin/0, commit/0, rollback/0
• stratification introspection: predicate_stratum/3, strata/1 Rules and facts are represented as terms:
• rule(Id, Head, Body) where Body is a list of Literal terms:
  • positive literal: Term
  • negative literal: neg(Term)
  • aggregate literal: agg(Op, Template, Goals, Result) where Op is one of count, sum, min, max
• fact(Fact) for extensional database (EDB) facts Aggregate notes:
• Aggregate dependencies are required to be in lower strata.
• min and max require at least one matching value; otherwise no fact is derived.
• Incremental updates involving negation or aggregates currently fallback to full rematerialization for correctness.

Basic usage

::

| ?- logtalk_load(datalog(loader)). ...

| ?- Program = [ rule(path_base, path(X,Y), [edge(X,Y)]), rule(path_rec, path(X,Z), [edge(X,Y), path(Y,Z)]), fact(edge(a,b)), fact(edge(b,c)) ], datalog::load_program(Program), datalog::query(path(a,c)).

Limitations

This first version intentionally keeps the implementation simple and portable. Negation support is currently limited to stratified programs. Aggregates and cost-based optimization are planned future enhancements. Aggregate support is currently limited to count, sum, min, and max.