Acknowledgements

This book stands on work done in France over more than a decade.

Catala. Catala, from Inria, was designed by Denis Merigoux, Nicolas Chataing and Jonathan Protzenko, working with the legal scholars Sarah Lawsky and Liane Huttner; the language grew out of Lawsky’s insight into the logical structure of statutes. Their 2021 paper showed that a statute can be written as code beside its own text, with a mechanised proof of the core compilation — and that doing so finds real discrepancies: their formalisation of French family benefits surfaced an overseas-territories income-cap case (article L. 755-12 of the social security code) in OpenFisca, which was fixed after they reported it. The same research tradition produced Mlang, a modern compiler for the French income-tax code. In June 2026 the Cnaf and Inria announced a partnership to develop Catala for the computation of social benefits.

Open law. None of this would be possible without France’s open legislation and open public code: the consolidated legislation of Légifrance published as open data under the Open Licence; OpenFisca, which has modelled French taxes and benefits since 2011; the income-tax calculation code published by the tax administration; Etalab’s work on opening public data and algorithms; LexImpact, which helps Parliament simulate reforms; and the 2016 Digital Republic Act, which requires an administration to tell people when an individual decision about them relies on an algorithm, and to explain its rules on request.

This book follows an independent Arxo model built from published French law. Where it compares tools, it names the version and the date of the check (Appendix F). Where a result differs from another tool’s, the book shows the case and the text behind it.

Sources

• Denis Merigoux, Nicolas Chataing, Jonathan Protzenko, Catala: A Programming Language for the Law, ICFP 2021 — arXiv 2103.03198 (the OpenFisca discrepancy and its fix are described in the section on French family benefits; the lawyers are thanked in the acknowledgements).
• Denis Merigoux, Raphaël Monat, Jonathan Protzenko, A Modern Compiler for the French Tax Code, CC 2021 — arXiv 2011.07966.
• Cnaf, La Cnaf et Inria s’engagent ensemble pour développer Catala, 8 June 2026 — caf.fr.
• LEGI — consolidated codes, laws and regulations, Open Licence — data.gouv.fr.
• OpenFisca — history of the project — openfisca.readthedocs.io.
• DGFiP — opening of public data, including the income-tax calculation code — impots.gouv.fr.
• Etalab — practical guides on open data, source code and algorithms — etalab.gouv.fr.
• LexImpact — data.gouv.fr.
• Article L. 311-3-1 of the code on relations between the public and the administration, created by the Digital Republic Act of 7 October 2016 — Légifrance.