How the Lean language brings math to coding and coding to math

Uses of the functional programming language include formal mathematics, software and hardware verification, AI for math and code synthesis, and math and computer science education.

This post is an adaptation of a keynote address that Leo de Moura delivered at the International Conference on Computer Aided Verification (CAV), in July 2024.

LEAN logo.png
The Lean logo.

In 2013, I launched the Lean project with the goal of bridging the gap between automated and interactive theorem provers. Since its inception, Lean has seen unparalleled adoption in the mathematical community, surpassing previous efforts in formalized mathematics. Lean 4, the latest version, is implemented in Lean itself and is also a fully fledged, extensible programming language with robust IDE support, package management, and a thriving ecosystem.

In 2023, Sebasian Ullrich and I founded the Lean Focused Research Organization (FRO), a nonprofit dedicated to advancing Lean and supporting its community. The Lean project embraces a philosophy that promotes decentralized innovation, empowering a diverse community of researchers, developers, and enthusiasts to collaboratively push the boundaries of mathematical practice and software development. In this blog post, we will provide a brief introduction to the project and describe how it is used at AWS.

A brief introduction to Lean

Lean is an open-source, extensible, functional programming language and interactive theorem prover that makes writing correct and maintainable code easy. Lean programming primarily involves defining types and functions, allowing users to focus on the problem domain and its data rather than on coding details. Lean has four primary use cases: formal mathematics, software and hardware verification, AI for math and code synthesis, and math and computer science education.

Formal mathematics

Lean allows mathematicians to work with advanced mathematical structures using syntax that feels natural to them. The math community recognizes its usefulness: for instance, Fields medalists Peter Scholze and Terence Tao used Lean to confirm their new results; Quanta Magazine has lauded Lean as one of the biggest breakthroughs in mathematics, and it has been featured in numerous popular scientific and academic publications, including the Wired magazine article “The effort to build the mathematical library of the future”. Recently, DeepMind used Lean to build an AI engine that met the silver-medal standard at the International Math Olympiad.

As of July 2024, the Lean Mathematical Library has received contributions from over 300 mathematicians and contains 1.58 million lines of code, surpassing other formal-mathematics systems in use. This remarkable growth has come despite Lean’s concision and youth: it’s at least a decade younger than comparable libraries.

Software and hardware verification

Lean’s combination of formal verification, user interaction, and mathematical rigor makes it invaluable for both software and hardware verification. Lean is a system for programming your proofs and proving your programs. An additional benefit is that Lean produces efficient code, and its extensibility features, originally designed for mathematicians, are also highly convenient for creating abstractions when writing clean and maintainable code. Its benefits extend to any system requiring exceptional accuracy and security, including industries such as aerospace, cryptography, web services, autonomous vehicles, biomedical systems, and medical devices. Later on, we will provide several examples of Lean's applications at AWS.

AI for math and code synthesis

Lean is popular with groups developing AI for mathematics and code synthesis. One of the key reasons is that Lean formal proofs are machine checkable and can be independently audited by external proof checkers. Additionally, Lean's extensibility allows users to peer into the system internals, including data structures for representing proofs and code. This capability is also used to automatically generate animations from Lean proofs.

AI researchers are leveraging large language models (LLMs) to create Lean formal proofs and automatically translate prose into formalized mathematics. OpenAI has released lean-gym, a reinforcement learning environment based on Lean. Harmonic used Lean in the development of its Mathematical Superintelligence Platform (MSI), an AI model designed to guarantee accuracy and avoid hallucinations. Meta AI created an AI model that has solved 10 International Mathematical Olympiad problems, and DeepMind has formalized a theoretical result related to AI safety in Lean. Additionally, LeanDojo is an open-source project using LLMs to automate proof construction in Lean.

Lean's unique combination of machine-checkable proofs, system introspection, and extensibility makes it an ideal tool for advancing AI research in mathematics and code synthesis. The synergy between LLMs and Lean formal proofs is emphasized in Terence Tao's colloquium lecture at the American Mathematical Society, “Machine Assisted Proof”; in the Scientific American article “AI will become mathematicians' co-pilot”; and in the New York Times article “A.I. Is coming for mathematics, too.”

Math and CS education

Millions of people learn mathematics as students and use it throughout their careers. Since its inception, the Lean project has supported students' mathematical-reasoning needs and enabled a more diverse population to contribute to the fields of math and computer science. Numerous educational resources are available for learning Lean, including interactive computer games such as the Natural Number Game, computer science and mathematics textbooks, university courses, and on-demand tutorials. The Lean FRO is committed to expanding Lean’s educational content and envisions a future where children use Lean as a playground for learning mathematics, progressing at their own paces and receiving instantaneous feedback, similar to how many have learned to code.

A quick tour of Lean

Lean combines programming and formal verification. Let's take a quick tour through a small example to see how we write code in Lean and prove properties about that code.

Writing code in Lean

First, let's define a simple function that appends two lists:

def append (xs ys : List a) : List a :=
  match xs with
  | [] => ys
  | x :: xs => x :: append xs ys

This function is defined using pattern matching. For the base case, appending an empty list [] to ys results in ys. The notation x :: xs represents a list with head x and tail xs. For the recursive case, appending x :: xs to ys results in x :: append xs ys. Additionally, the append function is polymorphic, meaning it works with lists of any type a.

Extensible syntax

The notation x :: xs used above is not built into Lean but is defined using the infixr command:

infixr:67 " :: " => List.cons

The infixr command defines a new infix operator x :: xs, denoting List.cons x xs. This command is actually a macro implemented using Lean's hygienic macro system. Lean's extensible syntax allows users to define their own domain-specific languages. For example, Verso, the Lean documentation-authoring system, is implemented in Lean using this mechanism. Verso defines alternative concrete syntaxes that closely resemble Markdown and HTML.

Proving properties about code

Next, we'll prove a property about our append function: that the length of the appended lists is the sum of their lengths.

theorem append_length (xs ys : List a)
        : (append xs ys).length = xs.length + ys.length := by
  induction xs with
  | nil => simp [append]
  | cons x xs ih => simp [append, ih]; omega

Here, theorem introduces a new theorem named append_length. The statement (append xs ys).length = xs.length + ys.length is what we want to prove. The by ... block contains the proof. In this proof,

  • induction xs with initiates a proof by induction on xs;
  • the nil case proves the base case using simp, the Lean simplifier. The parameter append instructs the simplifier to expand append’s definition; and
  • the cons x xs ih case proves the inductive step where ih is the inductive hypothesis. It also uses simp and omega, which complete the proof using arithmetical reasoning.

In this proof, induction, simp, and omega are tactics. Tactics, which transform one state of the proof into another, are key to interactive theorem proving in Lean. Users can inspect the states of their proofs using the Lean InfoView, a panel in the IDE. The InfoView is an interactive object that can be inspected and browsed by the user. In the following picture, we see the state of our proof before the simp tactic at line 10. Note that the proof state contains all hypotheses and the goal (append (x :: xs) ys).length = (x :: xs).length + ys.length, which remains to be proved.

LEAN example.png
The state of the proof before the simp tactic at line 10, as visualized in the Lean InfoView.

How Lean is used at AWS

At AWS, Lean is used in several open-source projects to address complex verification and modeling challenges. These projects not only highlight the practical applications of Lean in different domains but also emphasize AWS's commitment to open-source development and collaboration. We cover four key projects: Cedar, LNSym, and SampCert, whose Lean source code is already available on GitHub, and AILean, which is exploring the relationship between LLMs and formal mathematics and whose code is not open source yet. 

Cedar: an open-source policy language and evaluation engine 

Cedar is an open-source policy language and evaluation engine. Cedar enables developers to express fine-grained permissions as easy-to-understand policies enforced in their applications and to decouple access control from application logic. Cedar supports common authorization models such as role-based access control and attribute-based access control. It is the first policy language built from the ground up to be verified formally using automated reasoning and tested rigorously using differential random testing.

The Cedar project uses Lean to create an executable formal model of each core component of the Cedar runtime (such as the authorization engine) and static-analysis tools (such as the type checker). This model serves as a highly readable specification, allowing the team to prove key correctness properties using Lean.

Lean was chosen for modeling Cedar due to its fast runtime, extensive libraries, IDE support, and small trusted computing base (TCB). The fast runtime enables efficient differential testing of Cedar models. The libraries provide reusable verified data structures and tactics built by the open-source community. Lean’s small TCB allows Cedar to leverage these contributions confidently, as Lean checks their correctness, requiring trust only in Lean’s minimal proof-checking kernel.

LNSym: Symbolic simulation for cryptographic verification

LNSym is a symbolic simulator for Armv8 native-code programs. It’s currently under development, with a focus on enabling automated reasoning of cryptographic machine-code programs. Many cryptographic routines are written in assembly to optimize performance and security on the underlying processor. LNSym aims to reduce the cost of verifying cryptographic routines, particularly block ciphers and secure hashes, ultimately empowering cryptography developers to formally reason about their native-code programs.

LNSym uses Lean as a specification language to model the Arm instruction semantics and cryptographic protocols and as a theorem prover for reasoning about these artifacts. Since Lean programs are executable, the specifications achieve a high degree of trust through thorough conformance testing. Lean orchestrates proofs such that the heavy and often tedious lifting is done automatically, using decision procedures like SAT solvers or custom domain-specific tactics. When proof automation fails, users can employ Lean as an interactive theorem prover. This combination of interactive and automated theorem proving ensures that progress on verification tasks is not hindered by the limitations of proof automation.

SampCert: formally verified differential-privacy primitives

SampCert is an open-source library of formally verified differential-privacy primitives used by the AWS Clean Rooms Differential Privacy service for its fast and sound sampling algorithms. Using Lean, SampCert provides the only verified implementation of the discrete Gaussian sampler and the primitives of zero concentrated differential privacy.

Although SampCert focuses on software, its verification relies heavily on Mathlib, the Lean Mathematical Library. The verification of code addressing practical problems in data privacy depends on the formalization of mathematical concepts from Fourier analysis to number theory and topology.

AILean: AI for math and math for AI

AILean is exploring the relationship between LLMs and formal mathematics in collaboration with the Technology Innovation Institute (TII). This exploration works in both directions: AI for math and math for AI. In AILean, LLMs are used to enhance proof automation and user experience in formal mathematics. LLMs can analyze theorem statements and existing proof steps, suggesting relevant lemmas, definitions, or tactics to guide users in completing proofs. They can also identify common mistakes or inconsistencies, proposing corrections or alternative approaches that avoid dead ends and thereby improving the proof development process.

Takeaways

Lean is a complex system, but its correctness relies only on a small trusted kernel. Moreover, all proofs and definitions can be exported and independently audited and checked. This is a crucial feature for both the mathematical and software verification communities because it eliminates the trust bottleneck. It doesn't matter who you are; if Lean checked your proof, the whole world can build on top of it. This enables large groups of mathematicians who have never met to collaborate and work together. Additionally, it allows users to extend Lean without fearing the introduction of soundness bugs that could compromise the logical consistency of the system.

Lean's extensibility enables customization, which was particularly important during its first ten years, when resources were limited. Lean’s extensibility allowed the community to extend the system without needing to synchronize with its developers. Self-hosting, or implementing Lean in Lean, also ensured that users can access all parts of the system without having to learn a different programming language. This makes it easy and convenient to extend Lean. Packages such as ProofWidgets and SciLean are excellent examples of user-defined extensions that leverage these features.

The FRO model introduced by Convergent Research has been instrumental in supporting Lean and helping it transition to a self-sufficient foundation. The Lean project has grown significantly, and driving it forward would have been difficult without Convergent Research’s efforts to secure philanthropic support. Just as foundations like the Rust and Linux Foundations are vital for the success and sustainability of open-source projects, the support of Convergent Research has been critical for Lean's ongoing progress.

To learn more about Lean, visit the website.

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In this role, you will: - Take your GNC algorithms from concept all the way to orbit, and stay close to them through integration, test, and on-orbit operations - Solve hard precise-pointing, payload-cueing, and fleet-autonomy problems against demanding performance requirements - Work across the full lifecycle — design, simulation, test, and flight operations — rather than a narrow slice - Join a small team of some of the top GNC engineers in the industry, building a product people will love to use This position is part of the Satellite Attitude Determination and Control team, where we own the full lifecycle of our work — from algorithm design and flight software, through lab integration, to flying our own satellites on orbit. You will design and analyze the control system and algorithms, support development of our flight hardware and software, help integrate the satellite in our labs, and operate the GNC system in flight as a constellation of satellites flows through the production line into orbit. **Export Control Requirement:** Due to applicable export control laws and regulations, candidates must be a U.S. citizen or national, U.S. permanent resident (i.e., current Green Card holder), or lawfully admitted into the U.S. as a refugee or granted asylum. Key job responsibilities - Design and analyze algorithms for state estimation, attitude determination and control, precise pointing, and payload cueing against demanding performance requirements. - Write the production C++ flight code that runs these algorithms on the satellite. Our GNC engineers own their algorithms all the way into flight software, working alongside software development engineers rather than handing prototypes off to be reimplemented by another team. - Design concepts of operation that balance pointing and payload cueing, momentum management, power, thermal, and communications across all phases of the mission, including off-nominal and contingency scenarios. - Develop momentum management strategies using reaction wheels and magnetic torquers, and help size and lay out the GNC sensor and actuator suite — star trackers, magnetometers, sun sensors, IMUs, and GPS receivers — and the algorithms that consume them. - Develop onboard autonomy, including Fault Detection, Isolation, and Recovery, so the fleet operates safely at scale with minimal intervention. - Develop and validate models and simulations of the satellite and constellation across a range of fidelity levels, including 6-DOF time-domain simulation and Monte Carlo analysis, and run hardware-in-the-loop testing to verify closed-loop performance. - Support component-level environmental testing, functional and performance checkout, and subsystem and satellite integration in our labs. - Analyze fleet telemetry in bulk and build the metrics, dashboards, and tooling that let us monitor and continuously improve pointing performance and payload operations time as the constellation grows. - Operate the GNC system on orbit — reviewing fleet data, supporting maneuvers, and resolving anomalies when on-orbit behavior diverges from the ground model. - Contribute to architecture, design, and code reviews, and document your algorithms, analyses, and results clearly. A day in the life This role spans a diverse set of problems across the spacecraft and network system. You will derive and prototype estimation and control algorithms, turn them into production flight code, and prove them out through 6-DOF and Monte Carlo simulation and hardware-in-the-loop testing in the lab. You will support satellite integration, and when something on orbit does not behave the way the ground model predicted, you will dig into fleet telemetry to find out why and drive the fix. You will own problems end to end and have the focus to go deep on each one. Because GNC sits at the center of safe flight, you will work across the whole program. The payload, power, thermal, and operations teams all depend on GNC, so you will balance their needs against your pointing and momentum budgets. To get your algorithms running reliably on the spacecraft, you will partner closely with avionics, flight software, and our embedded software development engineers. You will help the mechanical teams shape the next generation of satellite hardware, from reaction wheels to star trackers. And as satellites move from design through manufacturing into orbit, you will work alongside systems engineering and Assembly, Integration & Test. The problems are genuinely hard — demanding pointing and performance requirements, a large constellation to keep flying safely, and the chance to invent advanced capabilities that have not been built before. The work is staffed with some of the strongest engineers in the industry, and the constant collaboration across teams makes it a place to keep learning and growing. About the team Our team brings deep experience across many satellite systems and other flight vehicles, with real bench strength in both our mission and the core GNC disciplines. We design, prototype, test, iterate, and learn together, and because GNC is central to safe flight, we tend to drive Concepts of Operation and many of the system-level analyses on the program. We also have a lot of freedom. We trust our engineers to pursue the problems they face in whatever way they find best, to choose their own tools and approaches, and to own the results. If you want the autonomy to solve hard problems your way, and a team of strong engineers to do it with, you will feel at home here.