OCaml is a high-performance, functional-first programming language with a powerful static type system and type inference. It is pragmatic, blending functional, imperative, and object-oriented features.

Core Pillars

  1. Strong, Static Type System: Types are checked at compile time, eliminating a huge class of runtime errors. Its standout feature is type inference, where the compiler deduces types for you, so you rarely need to write them. This provides the safety of static types with the feel of a dynamic language.

  2. Functional-First, but Multi-Paradigm: It encourages a functional style with immutable data structures and first-class functions. However, it is not purely functional; you can use mutable state, loops, and objects when they are the right tool for the job.

  3. Excellent Performance: It compiles to fast native machine code. Its generational garbage collector is highly optimized, making it suitable for performance-critical systems.

  4. Powerful Module System: It has a sophisticated system for structuring large programs. A key feature is functors, which are “functions” that take modules as arguments and produce new modules, enabling highly generic and reusable code.

Key Language Features

  • Pattern Matching: A powerful and safe way to deconstruct data structures, like a switch statement on steroids. The compiler ensures you handle all possible cases.
  • Algebraic Data Types (ADTs): Also known as variants, they allow you to define complex, custom data types concisely and safely.
  • Immutability by Default: Variables and data structures are immutable unless you explicitly mark them as mutable, which helps prevent bugs.

Use Cases

OCaml is used for complex, mission-critical systems where correctness and performance are paramount. Notable users include:

  • Jane Street: A quantitative trading firm that uses OCaml for nearly all of its software.
  • MirageOS: A library operating system that builds secure, specialized unikernels.
  • Tezos: A blockchain platform.
  • Coq & F/: Proof assistants and verification tools written in OCaml.

In short, OCaml is a pragmatic language for building robust, performant, and maintainable software.