abstract :
In France, income tax is computed from taxpayers' individual returns, using an algorithm that is authored,
designed and maintained by the French Public Finances Directorate (DGFiP). This algorithm relies
on a legacy custom language and compiler originally designed in 1990, which unlike French wine,
did not age well with time. Owing to the shortcomings of the input language and the technical limitations
of the compiler, the algorithm is proving harder and harder to maintain, relying on ad-hoc behaviors
and workarounds to implement the most recent changes in tax law. Competence loss and aging code also
mean that the system does not benefit from any modern compiler techniques that would increase confidence
in the implementation. We overhaul this infrastructure and present Mlang, an open-source compiler toolchain
whose goal is to replace the existing infrastructure. Mlang is based on a reverse-engineered formalization
of the DGFiP’s system, and has been thoroughly validated against the private DGFiP test suite.
As such, Mlang has a formal semantics; eliminates previous hand-written workarounds in C;
compiles to modern languages (Python); and enables a variety of instrumentations, providing deep insights
about the essence of French income tax computation. The DGFiP is now officially transitioning to Mlang
for their production system.