Preface: Why France, and Why One Family

This book is first of all a demonstration of what modelling legislation with Arxo can do. Its model is an independent reading of published French law, built for teaching; it does not claim legal accuracy or completeness, it is not an implementation by or for the Cnaf, and its answers are not legal advice. For a real file, rely on the official texts and on the fund.

France is one of the best places in the world to learn law as code. Its legislation is published as open data. Its family benefits are defined in detail by codes, decrees and yearly instructions, and the family allowance funds — the Cafs, under the national fund, the Cnaf — pay them to millions of households. And its research and public-service teams built some of the field’s foundations: Catala, from Inria, showed that a statute can be written as code beside its own text; OpenFisca has modelled French taxes and benefits since 2011; LexImpact helps Parliament see what an amendment would do. This book stands on that work, and its acknowledgements say how.

The book follows one fictional family, the Garniers, through a benefits file: the text that applies on a given day, the amount, the duties of each side, the question only an authority can answer, and what happens next. Other fictional households join where a chapter needs them. Every answer in the book comes from a run of an executable model of the published rules, and every answer can be traced to its facts, the article behind each rule and the pinned editions of its sources.

That model is an independent Arxo model built from published French law. It is not an implementation by or for the Cnaf, and nothing in this book describes how the Cnaf or any Caf processes a file.

What the book is for

The model answers more than “how much”. A family file raises questions a calculator does not: which edition of the text applies on the date of the question, what happens when a fact is missing, who must decide when the law calls for a judgment, which reading of an ambiguous text was chosen, what the fund and the household each owe, and how a decision is explained to the person who receives it. Each chapter takes one of these questions and shows what the rules establish, what remains open, and who must act next.

The book is written for the people who build and run such systems together: lawyers and policy staff, programme analysts, and developers. You do not need to know Arxo, and basic French helps but is not required: every French term is glossed in English where it first appears, and a glossary at the end lists them all.

What the book is not

This is not legal advice, and it is not a guide to anyone’s entitlement. The families are fictional. The amounts are answers to dated questions about stated facts: a different date, a different fact, or a later edition of a text can give a different answer, and the book shows exactly how.

Dates

Every question in this book is asked at a legal date — usually a day in April 2026 — against the editions of the texts in force on that day. Where a chapter shows a recorded run, it gives the date of the recording as well. The two are different: a question about 15 April 2026 can be asked again years later and must give the same answer from the same texts.

An online edition

The same text is also an online course, with links to recorded scenes of a demonstration workbench. The book and the course are one text in two forms; corrections reach both.