lean4-htt/doc/whatIsLean.md

What is Lean

Lean is a functional programming language that makes it easy to write correct and maintainable code.

Lean programming primarily involves defining types and functions. This allows your focus to remain on the problem domain and manipulating its data, rather than the details of programming.

-- Defines a function that takes a name and produces a greeting.
def getGreeting (name : String) := "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.
def main : IO Unit :=
  -- Defines a list of names
  let names := ["Sebastian", "Leo", "Daniel"]

  -- Print the list of greetings `names.map getGreeting`
  for greeting in names.map getGreeting do
    IO.println greeting

Lean has numerous features, including:

• Type inference
• First-class functions
• Powerful data types
• Pattern matching
• Type classes
• Extensible syntax
• Hygienic macros
• Dependent types
• Metaprogramming framework
• Async programming
• Verification, you can prove properties of your functions using Lean itself