Foreword

There has long been a gap between the languages we use to think about a domain and the languages we use to run code over it. On one side, ontology engineers reach for OWL, OntoUML, or hand-rolled formalisms in the description-logic tradition: precise, mechanically reasonable, but awkward to compose, package, or evolve like real software. On the other side, programmers reach for Python, TypeScript, or Rust: ergonomic, well-tooled, but almost defenseless against a domain whose constraints really do live in the type system.

Argon is an attempt to close that gap from the programming-language side.

It is a programming language whose primary subjects are concepts, relations, and rules. It compiles. It has a package manager. It has an LSP, an editor extension, and a registry. It also has refinement types, a decidability tier ladder, bitemporal axes, standpoint-aware queries, and a mechanically-verified core. The same source can model a residential lease, a regulatory regime, and the events that move money between them — and the compiler can tell you, before you run anything, whether your rules are decidable, whether your refinement is satisfiable, and whether your invariants follow from your axioms.

This book is the path into that language. It moves atoms-to-molecules, the way The Rust Programming Language does for Rust, and it does so against a single running example — a residential-lease ontology that we build up piece by piece across the chapters. By the end of Part 3, the reader has a working, queryable, validated model of a non-trivial legal domain. Part 5 covers the formal foundations: the meta-property calculus, the seven-tier decidability ladder, defeasibility semantics, and the bitemporal + standpoint runtime. Where the chapters say “mechanically verified,” they state the result and the math behind it — termination, uniqueness, soundness, decidability — and let the reader inspect the engine the result is about.

Argon is not finished. The team is in the middle of a measured redesign: cleaning up vocabulary, unifying body shapes, collapsing a few item kinds into more general primitives. The book teaches today’s syntax and flags every place where the language is moving. Migration notes point forward, and Appendix D is the lever that will keep the book honest as the redesign rolls out.

The audience the book imagines first is the foundational-ontology research community: people who already know UFO, BFO, DOLCE, OntoUML, OWL — what an ontology is, what reasoning is, why decidability matters. The book skips that preamble and goes straight to what is distinctive: how Argon turns an ontology into a program you can ship.

If you arrive from the other side — Rust, TypeScript, Haskell, a working programmer who has never touched description logic — the book still works. The early chapters teach the formal vocabulary as it goes. You will not be asked to read a paper before the first compile.

— the Argon team, 2026