Coal

This is the official documentation for the Coal programming language.

About

Coal is a declarative, statically typed, purely functional programming language with simple and intuitive syntax. It provides:

• algebraic data types and pattern matching,
• extensible records,
• structural recursion,
• codata, and
• traits (type classes)

among other features. Coal’s type system, like Haskell’s and ML’s, supports type inference and parametric polymorphism, drawing on the System-F lambda calculus. The Coal compiler is implemented in Haskell and targets LLVM for code generation.

As a total language, Coal takes a different approach to recursion, following the motto that "recursion is the goto of functional programming." To guarantee that programs are provably terminating, recursion is only available in a restricted form, known as structural recursion.

  fun sum(numbers : List<int32>) : int32 =
    fold(numbers) {
      | [] => 0
      | x :: @tot => x + tot
    }

The language finds inspiration in ideas from the field of Mathematics of Program Construction, where streams and other infinite data types are described as coalgebras — hence the name Coal.

This project is licensed under the terms of the MIT license. The source code is hosted at codeberg.org/laserpants/coal.