ε-prolog

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ε-prolog (eprolog) is a complete Prolog engine implementation written in pure Emacs Lisp. It provides a fully functional Prolog system integrated into the Emacs environment, offering traditional Prolog programming capabilities with seamless Lisp interoperability.

Features

Core Prolog Engine

• Complete unification algorithm with occurs check
• Backtracking and choice points with proper cut (!) semantics
• Clause database management for facts and rules
• Interactive query execution with solution enumeration

Built-in Predicates

• Unification: =, ==
• Type checking: atom/1, atomic/1, var/1, ground/1, number/1, string/1
• Control: fail/0, !/0, true/0, false/0, not/1 (negation as failure)
• Logical operators: and/0+, or/0+, if/2-3
• List operations: member/2, append/2-3, maplist/2-5
• Meta-call: call/1+
• Arithmetic: is/2 which is an alias of lisp/2+
• Lisp integration: lisp/2+, lisp!/1+, lispp/1+
• Dynamic parameters: store/2, fetch/2 for stateful computation

Advanced Features

• Definite Clause Grammar (DCG) support
• Spy/debugging functionality for tracing execution
• Dynamic parameters (store/2, fetch/2) with backtracking-aware state management
• Anonymous variable handling
• Variable renaming for clause application
• Explicit-stack proof engine for deep search without Lisp stack growth

Requirements

ε-prolog requires Emacs 27.2 or later. Emacs 29+ is still recommended for general performance, but deep proof search no longer depends on recursive Lisp control flow.

Installation

Place eprolog.el in your Emacs load path and add to your configuration:

(require 'eprolog)

Quick Start

For detailed examples, tutorials, and comprehensive documentation, see the docs/ directory.

Basic usage involves defining facts and rules, then querying the knowledge base:

;; Define facts
(eprolog-define-predicate (parent tom bob))
(eprolog-define-predicate (parent bob ann))

;; Define rules  
(eprolog-define-predicate (grandparent _x _z)
  (parent _x _y)
  (parent _y _z))

;; Query the database
(eprolog-query (grandparent tom _x))
;; Returns: _x = ann

;; Dynamic parameters for stateful computation
(eprolog-query 
 (store counter 0)
 (fetch counter _old)
 (is _new (+ _old 1))
 (store counter _new)
 (fetch counter _result))
;; Returns: _result = 1

ε-prolog provides convenient aliases:

• eprolog-define-predicate for eprolog-define-prolog-predicate
• eprolog-define-predicate! for eprolog-define-prolog-predicate!

For complete API documentation, advanced examples, and comprehensive tutorials, see the docs/ directory.

Testing

ε-prolog includes a comprehensive test suite in the docs/ directory with tests covering all major functionality.

Run tests with:

make test

See the docs/ directory for detailed testing instructions and interactive test examples.

Debugging

Enable spy mode to trace predicate execution:

(setq eprolog-spy-predicates '(grandparent parent))
(setq eprolog-spy-state 'prompt) ; or 'always or 'disabled

See the docs/ directory for comprehensive debugging examples and techniques.

Implementation Details

ε-prolog implements a complete Prolog engine with:

• Unification: Standard unification algorithm with optional occurs check
• Proof Search: Depth-first search with explicit goal and choice-point stacks
• Cut Implementation: Proper cut semantics by trimming choice points
• Variable Scoping: Automatic variable renaming for clause application
• Runtime Safety: Optional engine step limit that signals an explicit error

The engine evaluates queries with a while-driven state machine. Solutions are enumerated by resuming saved engine state instead of chaining continuations, which keeps deep repeat/0, member/2, append/3, and DCG searches off the Lisp call stack.

Lisp predicates defined with eprolog-define-lisp-predicate run inside that engine and must return eprolog--make-success with the next engine state or eprolog--make-failure. If eprolog-max-steps is non-nil and a query exceeds it, ε-prolog signals eprolog-step-limit-exceeded instead of reporting logical failure.

License

ε-prolog is released under the GNU General Public License v3.0. See LICENSE.org for details.

Historical Note

This implementation has an interesting lineage. It originally derives from Peter Norvig's Prolog implementation in Paradigms of Artificial Intelligence Programming (MIT licensed). The code was first reimplemented in Scheme as Athena, then ported back to Common Lisp, and finally adapted for Emacs Lisp as ε-prolog. Each iteration refined the implementation while maintaining the core algorithmic elegance of the original.

Contributing

Contributions are welcome! Please feel free to submit issues and pull requests.

Author

Masaya Taniguchi

Acknowledgments

This implementation draws inspiration from classical Prolog systems and modern functional programming techniques, adapted specifically for the Emacs Lisp environment.