A Machine-Checked Proof of Birkhoff's Variety Theorem in Martin-Löf Type Theory

Abstract

The Agda Universal Algebra Library is a project aimed at formalizing the foundations of universal algebra, equational logic and model theory in dependent type theory using Agda. In this paper we draw from many components of the library to present a self-contained, formal, constructive proof of Birkhoff’s HSP theorem in Martin-Löf dependent type theory. This achieves one of the project’s initial goals: to demonstrate the expressive power of inductive and dependent types for representing and reasoning about general algebraic and relational structures by using them to formalize a significant theorem in the field. Acknowledgements This work would not have been possible without the wonderful Agda language and the Agda Standard Library , developed and maintained by The Agda Team [21]. We thank the three anonymous referees for carefully reading the manuscript and offering many excellent suggestions which resulted in a vast improvement in the overall presentation. One referee went above and beyond and provided us with a simpler formalization of free algebras which led to simplifications of the proof of the main theorem. We are extremely grateful for this.