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Catala

For LLMs7 sections

In short: Catala and Arxo both compute statutory outcomes from pinned legal text with auditable derivations. This page is for a reader who knows Catala — or is choosing a statute-computation tool — and wants the honest map: shared ground, real differences, and what has actually been run. For the hands-on concept mapping, see Coming from Catala.

What Catala is for

Section titled “What Catala is for”

Catala is a language for writing tax and benefit rules next to the statute they come from. One variable gets a general definition plus an exception tree; the text and the code live in the same literate file. Its strengths are statute-grade datatypes (infinite-precision decimal, money rounded to the cent, Gregorian dates with explicit ambiguity modes, and an absent-or-present marker for missing values), a testing culture of scope tests and cram tests with execution traces, and an opt-in proof plugin that checks exception trees for gaps and overlaps with a solver. The core translation from its default calculus has a mechanically checked proof; the code backends are not covered by it.

Where it meets Arxo

Section titled “Where it meets Arxo”

The overlap is direct: the same vehicle-damage assessment rules computed in both languages, Catala’s trace against Arxo’s proof graph. Both sides pin the source text, both derive the answer from rules rather than paraphrasing the act, and both keep the derivation inspectable.

Comparability has edges. Catala has no first-class forms for working-day calendars, act revisions, duties, units, or source hashes — those are done by hand-written wrappers around the program. Contradictory input (two records disagreeing about one fact) cannot be posed to a Catala variable, which holds one value; Arxo keeps both supports in a dual answer. These are recorded as limits, not defects.

Key differences

Section titled “Key differences”
  • Money rounding is a language property. Catala rounds money-by-decimal products half away from zero in its runtime; Arxo computes exact arithmetic and refuses inexact division. Fractional-cent sums are expected to diverge — the contract says so before any run.
  • Conflicting applicable definitions are an engine property. Two valid Catala definitions for one variable raise a runtime conflict error; Arxo keeps a dual status and requires a declared priority to resolve it. Detection versus status: different mechanism, neither side broken.
  • Absence is a value with a producer. Catala marks a missing value absent; Arxo answers “not established” and can name the missing premise. A Catala caller decides what to pass for the unknown; Arxo leaves it unknown.
  • Proof is opt-in. Catala’s solver-backed check of exception trees is a real strength, but it is not the default build path — results obtained with the interpreter and the standard runtime do not inherit its guarantees.

A concrete scenario

Section titled “A concrete scenario”

The prepared experiment replays fifteen vehicle-damage cases — thresholds, assessor rules, working-day deadlines, cent-level sums, coverage edges — plus an edit axis (a kilometre threshold change, a shorter reporting term, a tool upgrade), through the Catala interpreter and the Arxo package, with verdicts of match, not comparable, or mutation-killed per case.

Separately, a historical parity run on Catala 1.2.0 (September 2026, published in the project parity repository) replayed 65 act cases and 240 random inputs through both models with full agreement, including caught mutations. Those numbers belong to that run and that version — interpreter plus standard runtime on one act — not to current engines or other backends.

Choosing and combining

Section titled “Choosing and combining”

Choose Catala when the deliverable is a French-style social-law computation with literate statute text, exact money, and a mature test-and-trace workflow — and when one value per variable matches the problem. Look to Arxo when missing facts, contradictory records, declared priorities between competing grounds, or dated editions are part of the question. A cooperation pattern that fits both: Catala computes the statutory amount, Arxo frames the surrounding entitlement, its exceptions across sources, and its history across editions.

Evidence and open questions

Section titled “Evidence and open questions”
  • Sources checked: September 2026 (Catala book, tutorial, releases, and the core-translation paper; code read on pinned tags, nothing built or run).
  • Studied profile: Catala 1.2.1; historical numbers: Catala 1.2.0 interpreter with the standard runtime, on the vehicle-damage pilot subset.
  • Basis: confirmed by documentation plus a prepared protocol; comparative run not performed.
  • Open: working-day and revision handling beyond wrappers, the maturity of the explanation tooling, and solver-check semantics on new tasks.

Sources and reproducible materials

Section titled “Sources and reproducible materials”

Documentation for Arxo. Writings — blog.arxo.io.

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