 Lean (proof assistant)

Lean is a proof assistant and a functional programming language developed primarily for formalizing mathematical theories and verifying the correctness of mathematical proofs. It was created by Leonardo de Moura and is used in both academia and industry for formal verification tasks. Key features of Lean include: 1. **Formal Language**: Lean provides a formal language in which users can write definitions, theorems, and proofs. This language is based on dependent type theory, enabling rich and expressive formulations.

Source code:

They are huge fans of Unicode characters! Check this out from a formal proof of the prime number theorem: github.com/AlexKontorovich/PrimeNumberTheoremAnd/blob/fbdbb5310d036d33b9797b35f3b04b08f2447a6e/PrimeNumberTheoremAnd/ZetaBounds.lean

Their dependency graph thingy is just beautiful however: alexkontorovich.github.io/PrimeNumberTheoremAnd/web/dep_graph_document.html

Their 2025 current installation method is bullshit, recommends VS Code extension on Ubuntu. Lol.

From CLI:Then when you run:it downloads the lean executable for you. Insane shit, could only come from a Microsoft mindset.