Lean

A functional programming language that makes it easy to write correct and maintainable code.

Website | Source | Docs | Theorem Proving in Lean | Functional Programming in Lean

Lean has numerous features, including:

Example:

-- Defines a function that takes a name and produces a greeting.
def getGreeting (name : String) := s!"Hello, {name}! Isn't Lean great?"

-- The `main` function is the entry point of your program.
-- Its type is `IO Unit` because it can perform `IO` operations (side effects).
def main : IO Unit :=
  -- Define a list of names
  let names := ["Sebastian", "Leo", "Daniel"]

  -- Map each name to a greeting
  let greetings := names.map getGreeting

  -- Print the list of greetings
  for greeting in greetings do
    IO.println greeting

https://github.com/leanprover/lean4
Lean is an open source theorem prover and programming language being developed at Microsoft Research. Lean aims to bridge the gap between interactive and automated theorem proving, by situating automated tools and methods in a framework that supports user interaction and the construction of fully specified axiomatic proofs.

Tags: language   functional  

Last modified 18 May 2025