Introduction

Welcome to the Catala domain-specific programming language, an introductory book about Catala. The Catala domain-specific language lets you annotate a legal specification with executable and deployable code in an efficient, high-assurance fashion. Automating the enforcement of a law or regulation through the use of a computer program is not neutral for the Rule of Law as well as the rights of users. Please thread lightly when considering automating anything else than tax or social benefits computation.

Contrary to a rules engine, Catala is a fully-fledged programming language with modular abstractions inspired by functional programming. But most importantly, it enables real collaboration between lawyers and computer scientists by allowing the practice of pair programming through literate programming for writing and updating the programs and their legal specifications.

Who Catala is for

Catala is a domain-specific programming language, hence its use is targeted to a specific class of users and organizations.

Multi-disciplinary teams

Catala is designed to be used by teams mixing legal and computer science expertise. Indeed, automating the enforcement of a law or a regulation requires bridging the ambiguity gap of the legal text to a computer program that knows what to do in every possible situation. Bridging this ambiguity gap is one of missions of lawyers, trained in interpreting legal texts and performing legal research to come up with a decision that can be justified withing the framework of the Rule of Law.

If lawyers should be deciding what the program does, they are not trained in programming concepts and abstractions, nor do they know how to write thousands of lines of code in a clean and maintainable fashion. Hence, the role of the computer scientist is to ensure that the program automating the enforcement of a law or regulation is clean, concise, featuring correctly-scoped abstractions that prevent code duplication and ease maintenance and future changes. Moreover, the computer scientist is also responsible for deploying the program in a bigger IT system that might break its assumptions or stress its performance.

By reusing existing software engineering techniques and tools, while exhibiting lawyer-friendly concepts at its surface, Catala lets lawyers and computer scientists collaborate in a truly agile manner, going beyond the separation between development and quality assurance teams that communicate only via test cases and specification documents.

Government agencies and public service organizations

Automating the enforcement of a law or regulation through a computer program is no small task. To make sure all are treated fairly under the Rule of Law, your program should exhaustively take into account every situation described by the law or regulation. While automating only the most simple situation corresponding to a majority of users might acceptable in certain situations, it cannot be the basis for a production-ready public service. Indeed, creating a difference between an automated “simple path” and a non-automated handling of complex situation widens the digital divide, and increases confusion for users and case workers alike.

Catala shines when the goal is to automate exhaustively and with high assurance a given law or regulation, and to maintain this automation over time. The language and tooling will help you manage the growing complexity, maintenance over legal changes, corner cases and production-ready deployment inside a legacy IT system. If you are looking for a tool to make a first-level user-help chatbot backend that only answers basic questions, or a simplified model of a law for an economic study, then you should consider using other tools. On the other hand, if you are in an organization responsible for running a public service like taxes or social benefits, then you should have the means to properly engineer the computer program responsible for the automation, and use Catala to help you with it.

Because Catala compiles to mainstream programming languages like Python or C, it yields portable source code libraries that are embeddable in virtually any IT system, legacy or modern. More particularly, Catala programs can be deployed behind a Web API or embarked in a desktop application, enabling “write once, use anywhere”. Hence, Catala is particularly suited to historical government agencies that have been operating their IT systems for decades in the past, and will continue to do so decades in the future.

Students and academics in CS and/or Law

Formalizing law, which is a more general term for translating law into executable computer code, has been a subject of scholarly attention for a very long time. AI & Law has historically been the umbrella community for this line of work, with three sequential trends: logic programming, ontologies, and now machine learning and especially natural language processing. While these trends have uncovered important results for formalizing law, big research questions remain. How to model complex legal provisions spanning large corpuses? Can formalizing law be automated? Can legal drafting be technologically augmented? How to detect inconsistencies or loopholes by static or dynamic analysis?

With strong roots to the research community, its commitment to open-source, its formalized semantics and its extensible compiler, Catala as a programming language is an opportunity for student learning and research projects. While teachers and students can use Catala for hands-on exploring of tax and social benefits law, researchers can use Catala programs as datasets or program a new analysis that can be readily deployed to the users of Catala.

If you’re a student, a professor, or anything else actually, you’re welcome to use Catala for free and contribute back to it by filing issues, proposing pull requests, or develop plugins and tooling around it.

Who this book is for

This books is primarily geared towards programmers that want to learn Catala, set up a project using it to translate some legal text into executable code, and be guided through the process. The book assume basic knowledge of functional programming idioms, and generally software engineering experience with another mainstream programming language.

If you, a programmer, work in a multidisciplinary team with one or several lawyers, it is up to you to explain to them what Catala is and how to work with it. This guide will thus also cover various topics around the collaboration between lawyers and programmers.

If you are a lawyer and stumble across this book, you are also welcome to read it, although parts of it will not be relevant to you. You might checkout instead introductory articles that set up the context around computer code, translating law to computer code and introduce the specificities of Catala:

How to use this book

The book is organized into two parts: the user guide and the reference guide. While the user guide is meant to be read linearly and aimed at new users, the reference guide is the go-to stop for checking items about the Catala language and tooling as you’re using it, whether you are a new or experienced user.

The user guide starts with Chapter 1 and the Catala equivalent of “Hello, world“ program. Chapter 2 explains the core concepts of Catala with a hands-on tutorial centered on the automation of a basic, fictional tax system. Moving onto serious business, Chapter 3 dives into the setting up of a real-world Catala project with version control, monitoring legal changes, testing, continuous integration, automated deployment, etc. Finally, Chapter 4 speeds up your learning by answering to (almost) all the questions that you’ll normally stumble upon while coding the law with Catala.

In the reference guide, Chapter 5 details all the features available in the Catala language, while Chapter 6 and Chapter 7 focus on the command line interfaces and features of the two binaries that you will be working with: clerk and catala.

Source code

The source files from which this book is generated can be found on GitHub. While the contents of this book are expected to correspond to the latest version of Catala, some inconsistencies might appear. If you spot one, or have comments or suggestions about the book, please file an issue!