Kajabity Prolog is a lightweight Prolog interpreter written in Java, designed for learning, experimenting, and exploring the foundations of logic programming.

It is open source and available on GitHub (https://github.com/Kajabity/kajabity-prolog) under the Apache License 2.0.

I first wrote it in 2003 as a curiosity project while exploring AI and parsing algorithms, and I’ve recently published it as open source for others to learn from or extend.

It’s a work in progress, and I welcome any comments or contributions. 

There’s a lot still to do!

Usage

Kajabity Prolog is a command line Prolog interpreter written in Java (currently v25).

Build it with Maven (see instructions in the README) and run with:

kajabity-prolog

You will be promoted to enter Prolog statements and the answers printed out.

For detailed examples, see the README on GitHub.

Command Line Options

• -h, --help – Print help and exit.
• -V, --version – Print version and exit.
• --init=<inits> – Specify initialisations file(s).
• --load=<loads> – Specify Prolog source file(s) to load.
• mainFile – A Prolog source file to load at startup.

Implementation

The Prolog language is based on the unification of logical clauses collected in a database to solve a goal.

The key parts of the application are:

• Database – collects facts and clauses used in Prolog programs.
• Goal – an expression made up of terms to be tested. Variables may be instantiated as goals are solved.
• Solver – applies SLD resolution (Selective Linear Definite clause resolution): chooses the leftmost sub-goal, attempts to unify with a clause head, replaces it with the clause body, and continues recursively with backtracking.
• Unification – finds the Most General Unifier (MGU). Think of it as zipping two sides together, filling missing teeth.
• Tokenising and Parsing – reads Prolog terms into the database or as instructions/goals to solve.

Example:

C:\OSS\Prolog\kajabity-prolog>kajabity-prolog
Running: java --enable-native-access=ALL-UNNAMED -jar "C:\OSS\Prolog\kajabity-prolog\prolog-console\target\prolog-console-0.1.0-SNAPSHOT-shaded.jar"

Kajabity Prolog.  Enter "quit." to exit.
>> yes.
Prolog> ?- member(X, [apple, banana, cherry]).
        X = apple ;
        X = banana ;
        X = cherry ;
>> no.
Prolog> quit.
Bye.

Roadmap

• Automated build and deploy pipeline on GitHub.
• Extend command-line interface.
• Documentation and tests.
• Ensure a minimum suite of ISO (or similar) Prolog features are implemented.

Learn More

• GitHub Repository