  Law at large underpins modern society, codifying and governing many aspects
of citizens' daily lives. Oftentimes, law is subject to interpretation, debate
and challenges throughout various courts and jurisdictions. But in some other
areas, law leaves little room for interpretation, and essentially aims to
rigorously describe a computation, a decision procedure or, simply said, an
algorithm. Unfortunately, prose remains a woefully inadequate tool for the job.
The lack of formalism leaves room for ambiguities; the structure of legal
statutes, with many paragraphs and sub-sections spread across multiple pages,
makes it hard to compute the intended outcome of the algorithm underlying a
given text; and, as with any other piece of poorly-specified critical software,
the use of informal language leaves corner cases unaddressed. We introduce
Catala, a new programming language that we specifically designed to allow a
straightforward and systematic translation of statutory law into an executable
implementation. Catala aims to bring together lawyers and programmers through a
shared medium, which together they can understand, edit and evolve, bridging a
gap that often results in dramatically incorrect implementations of the law. We
have implemented a compiler for Catala, and have proven the correctness of its
core compilation steps using the F* proof assistant. We evaluate Catala on
several legal texts that are algorithms in disguise, notably section 121 of the
US federal income tax and the byzantine French family benefits; in doing so, we
uncover a bug in the official implementation. We observe as a consequence of
the formalization process that using Catala enables rich interactions between
lawyers and programmers, leading to a greater understanding of the original
legislative intent, while producing a correct-by-construction executable
specification reusable by the greater software ecosystem.
