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Lean is an open source programming language created in 2015 by Leonardo de Moura.

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Lean Theorem Prover

Tags: programming language
Lean is developed on GitHub and has 2,145 stars
There are at least 1,807 Lean repos on GitHub
Early development of Lean happened in Microsoft
Lean is written in Lean, C++, CMake, Bourne shell, Markdown, Python, C, YAML, TOML, Tex, Perl, HTML, Bash, Lua, Lex
The Google BigQuery Public Dataset GitHub snapshot shows 233 users using Lean in 265 repos on GitHub
Pygments supports syntax highlighting for Lean
GitHub supports syntax highlighting for Lean
4 PLDB concepts link to Lean: cloc, Koka, Lean, Pygments

Example from hello-world:

#print "Hello World"

Example from Linguist:

/-
Copyright (c) 2014 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.

Module: algebra.binary
Authors: Leonardo de Moura, Jeremy Avigad

General properties of binary operations.
-/

import logic.eq
open eq.ops

namespace binary
  section
    variable {A : Type}
    variables (op₁ : A → A → A) (inv : A → A) (one : A)

    local notation a * b := op₁ a b
    local notation a ⁻¹  := inv a
    local notation 1     := one

    definition commutative := ∀a b, a * b = b * a
    definition associative := ∀a b c, (a * b) * c = a * (b * c)
    definition left_identity := ∀a, 1 * a = a
    definition right_identity := ∀a, a * 1 = a
    definition left_inverse := ∀a, a⁻¹ * a = 1
    definition right_inverse := ∀a, a * a⁻¹ = 1
    definition left_cancelative := ∀a b c, a * b = a * c → b = c
    definition right_cancelative := ∀a b c, a * b = c * b → a = c

    definition inv_op_cancel_left := ∀a b, a⁻¹ * (a * b) = b
    definition op_inv_cancel_left := ∀a b, a * (a⁻¹ * b) = b
    definition inv_op_cancel_right := ∀a b, a * b⁻¹ * b =  a
    definition op_inv_cancel_right := ∀a b, a * b * b⁻¹ = a

    variable (op₂ : A → A → A)

    local notation a + b := op₂ a b

    definition left_distributive := ∀a b c, a * (b + c) = a * b + a * c
    definition right_distributive := ∀a b c, (a + b) * c = a * c + b * c
  end

  context
    variable {A : Type}
    variable {f : A → A → A}
    variable H_comm : commutative f
    variable H_assoc : associative f
    infixl `*` := f
    theorem left_comm : ∀a b c, a*(b*c) = b*(a*c) :=
    take a b c, calc
      a*(b*c) = (a*b)*c  : H_assoc
        ...   = (b*a)*c  : H_comm
        ...   = b*(a*c)  : H_assoc

    theorem right_comm : ∀a b c, (a*b)*c = (a*c)*b :=
    take a b c, calc
      (a*b)*c = a*(b*c) : H_assoc
        ...   = a*(c*b) : H_comm
        ...   = (a*c)*b : H_assoc
  end

  context
    variable {A : Type}
    variable {f : A → A → A}
    variable H_assoc : associative f
    infixl `*` := f
    theorem assoc4helper (a b c d) : (a*b)*(c*d) = a*((b*c)*d) :=
    calc
      (a*b)*(c*d) = a*(b*(c*d)) : H_assoc
              ... = a*((b*c)*d) : H_assoc
  end

end binary

Language features

Feature	Supported	Example	Token
Comments	✓
Line Comments	✓
Integers	✓
Strings	✓	"Hello world"	"
Print() Debugging	✓		#print