Catala

Rules as Code language from Inria

Introductory paper

straightforward and systematic translation of statutory law into an executable implementation

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natural and easy to express the general case / exceptions logic

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bring together lawyers and programmers through a shared medium

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We now know that since at least 2000 B.C.E. and the Code of Ur-Nammu [Wikipedia contributors 2021], various societies have attempted to edict, codify and record their governing principles, customs and rules in a set of legal texts - the law.

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he practice of law thus oftentimes requires substantial human input

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relevant to a given case

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classify and categorize

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computational law or sometimes rules as code, concerns itself with situations where entities are well-defined, and where human appreciation, judgement or interpretation are not generally expected

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the law is the result of a centuries-long process: its convoluted structure demands tremendous expertise and training to successfully navigate and understand, something that a computer programmer may not have.

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A natural thought is perhaps to try to simplify the law itself. Unfortunately, this recurrent theme of the political discourse often conflicts with the political reality

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the authors do not anticipate a drastic improvement around the world concerning legal complexity

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computation of the French income tax, only to discover that the tax returns of an entire nation were processed using an antiquated system designed in 1990, relying on 80,000 lines of code written in a custom, in-house language

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inability to evolve

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reproducing the computation extremely challenging

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lack of accountability

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tailored specifically for the purpose of faithfully, crisply translating computational law into executable specifications

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While a program’s control-flow (as a first approximation) makes forward progress, statutes frequently back-patch previous definitions and re-interpret earlier paragraphs within different contexts.

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even though (b)(1) is a key piece of information for the application of Section 121, the reader will find it only if they keep going after (a). This is a general feature of legal texts: relevant information is scattered throughout, and (a) alone is nowhere near enough information to make a determination

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paragraph (A) modifies (b)(1) in place

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law is re-entrant and can call itself recursively under different conditions

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key takeaway is that, for a piece of law, one must process the entire document; barring that, the reader might be missing a crucial limitation that only surfaces much later in the text.

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pattern of having a default case followed by exceptions. This nonmonotonic logic is known as default logic

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closest to the purposes of the law is known as prioritized default logic

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Prioritized logic adds a strict partial order over defaults, to resolve conflicts when multiple defaults may be admissible at the same time.

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Legal prose is very dense, and uses a number of concepts without explicitly defining them in the text.

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dates are triples of a year, month and a day

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Distinct from date is duration, the type of a time interval, always expressed as a number of days

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up to the user to specify how "two years" should be interpreted

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non-experts are for the most part puzzled by compact notations

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generous keywords improve readability

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context keyword indicates that the value of the field might be determined later, depending on the context.

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intuition is that scopes are functions and contexts are their parameters and local variables.

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the user relates context variables together, via the definition keyword, or the rule keyword for conditions

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for legal minds, conditions and data are different objects

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Catala desugars rules to definitions equipped with a default value of false

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Catala at first glance resembles a functional language with heavy syntactic sugar, diving into the subtleties of the law exhibits the need for two features

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define context variables through a combination of an (optional) default case, along with an arbitrary number of special cases

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out-of-order nature of definitions means that Catala is entirely declarative

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law does not have general recursion

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our language is not Turing-complete

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seem to be universal features found in all statutes, regardless of the country or even the language

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posit that Catala captures the fundamentals of legal reasoning

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guiding design principle was to always guarantee that the Catala source code matches the structure of the law

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general case / exceptions drafting style

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law encodes a static priority structure

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either one exception applies, and the computation stops. Or if no exception applies, the parent definition is evaluated instead

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internal default calculus representation was drastically simplified

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made the semantics nearly impossible to explain; made the rules overly complicated and the proof of soundness very challenging; and more importantly, was not necessary to capture the essence of Western statutory law

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When the Catala code is compiled to OCaml, execution time drops to 0.5ms. Therefore, we conclude that performance problems are, at this stage of the project, nonexistent.

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the final intermediate representation of the Catala compiler is a pure and generic lambda calculus operating over simply-typed values

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possible to compile Catala to any programming language that has functions, arrays, structures, unions, and support for exceptions

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in France, administrative agencies always use a V-shaped development cycle for their legal expert systems

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this theoretical division of labor is artificial. Indeed, the natural language specification often proves insufficient or ambiguous

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validation of the implementation depends on lawyer-written test cases

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software engineering expertise is needed to produce maintainable, performant, high-quality code

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We deliberately introduced a bug in the code shown to the lawyers in this user study. The bug involved a ⩽ operator replaced by ⩾. Of the 7 who answered "Yes" to (5) in the second round, 2 were able to spot it, which we interpret to be a very encouraging indication

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major limitation concerning the expression of negation in the legal text

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line of work around logic programming never took hold

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so far unsuccessful attempts were made to automate the translation using natural language processing techniques

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Others

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so that lawyers can code their own legal expert systems

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optimistic proponents of computational law

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we focus on the challenges related to maintaining existing legal expert systems

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