Lean is an open-source programming language and proof assistant for creating correct, maintainable, and formally verified code. Its expressive language and trusted kernel support machine-checked mathematical proofs, as well as verification of software, hardware, and security-critical systems. Lean combines interactive theorem proving with automation that can handle tasks such as pattern matching, case analysis, and systems of linear inequalities, helping users focus on the substance of a proof rather than routine steps. The language is also extensible: its metaprogramming capabilities allow developers to create domain-specific notation and new proof automation.

Lean serves a broad community of mathematicians, researchers, and engineers. Its ecosystem includes Mathlib, a large community-built library of formalized mathematics, alongside projects for cryptographic proof systems, distributed protocol verification, authorization policies, and Rust program verification. Lean is also being used to formalize advanced mathematical research, including work on Fermat’s Last Theorem. The site offers installation and learning resources, a playground, community information, use cases, and project showcases, helping newcomers and experienced users explore the language and its applications.