# Lean Theorem Prover

Lean 4 programming language and theorem prover

Repository: https://github.com/leanprover/lean4
Canonical: https://ross.abutalabs.com/products/lean-theorem-prover
Homepage: https://lean-lang.org
Language: Lean
License: Apache-2.0
License Family: permissive
Topics: lean4, lean
Last push: 2026-08-26T23:14:21+00:00
Link (homepage): https://lean-lang.org
Link (site_page): https://lean-lang.org/install
Link (site_page): https://lean-lang.org/fro/about
Link (site_page): https://lean-lang.org/faq

## Health v2 (maintenance only)
Score: 99/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 99, release rhythm 98, longevity 100
- inputs: {"age_days": 3062, "days_push": 7, "days_rel": 12, "gap_med": 27.5, "n_releases_24m": 31}
- flags: none
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 8913, forks 949 (observed 2026-08-28T04:10:26.256979+00:00)

## What it is
Lean 4 is an open-source dependently typed functional programming language and interactive theorem prover used for formalizing mathematics and writing formally verified software. It compiles ahead-of-time to native code, features a powerful metaprogramming framework, and is backed by the Lean FRO with a large mathematical library ecosystem (Mathlib).

## Use cases
- formalize mathematical theorems and proofs
- write formally verified software
- learn dependent type theory and proof writing
- develop high-performance functional programs with correctness guarantees
- verify cryptographic algorithms and protocols
- AI-assisted theorem proving and autoformalization

## When to choose
- you need machine-checked proofs of mathematical results
- you want formally verified code with performance competitive with C or Rust
- you need a proof assistant with strong automation and metaprogramming
- you are doing research in mathematics, logic, or verified systems

## When to avoid
- you need a general-purpose language with a large ecosystem of libraries for web or app development
- your team has no background in type theory or formal methods
- you need quick scripting rather than rigorous verification

## Facets
- artifact type: application
- maturity: active
- function: programming-language, compiler, interpreter, type-system, developer-tools
- domain: programming-languages, mathematics, education, developer-tools
- platform: windows, cli, cross-platform
- tags: theorem-prover, proof-assistant, dependent-types, formal-verification, functional-programming, mathlib, metaprogramming, interactive-proving, algorithms, linux, macos

## Member repositories
- leanprover/lean4 (main) score 99
- leanprover/lean3 (mirror) score 10

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:10:26.256979+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-29T17:24:34.105290+00:00, confidence not recorded.
  - readme: https://github.com/leanprover/lean4 (fetched 2026-08-28T04:10:26.256979+00:00, sha 0e6af638fd5c)
  - homepage: https://lean-lang.org (fetched 2026-08-29T08:24:56.043420+00:00, sha e6d2ced4de6a)
  - site_page: https://lean-lang.org/install (fetched 2026-08-29T08:24:56.046277+00:00, sha b7373849c0c1)
  - site_page: https://lean-lang.org/fro/about (fetched 2026-08-29T08:24:56.048052+00:00, sha 8d96af3de4ec)
  - site_page: https://lean-lang.org/faq (fetched 2026-08-29T08:24:56.050369+00:00, sha c9b66bc9719c)
- Data as of 2026-08-30T08:39:29.467469+00:00.