Haskell

Haskell is a free and open-source purely-functional programming language. The language has been around since the 1990's and has pioneered a number of advanced programming language features such as type classes, which enable type-safe operator overloading. Haskell is an open-source project of more than twenty years of scientific research and provides built-in concurrency and parallelism, debuggers, profilers and rich libraries that can be used to build robust applications while following functional programming patterns.