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This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2026 Paulo Moura <pmoura@logtalk.org> SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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.