Lean 4

Lean 4 is both a programming language and an interactive theorem prover, designed to support formal reasoning while also functioning as an efficient and extensible general-purpose language. The project serves researchers, mathematicians, programmers, and formal methods users who need a system for writing machine-checked proofs as well as executable programs in the same environment. One of its defining characteristics is its emphasis on extensibility, since Lean 4 is built to allow users to develop custom automation, metaprogramming tools, and domain-specific extensions instead of being limited to a fixed proving workflow. The broader Lean ecosystem also includes official tutorials, language references, examples, and installation and build tools, which reflects that the project is not just a core compiler repository but the center of a mature development platform.