ReasonML

Definition

ReasonML is a syntax and toolchain, created by Facebook, that provides a more JavaScript-familiar surface syntax for the OCaml language, allowing developers to write statically typed, functional-first code that compiles to native binaries or, via BuckleScript/ReScript tooling, to JavaScript.

Overview

ReasonML was released by Facebook in 2016 as an attempt to make OCaml — a mature, statically typed functional language with a fast, sound type system — more approachable to JavaScript and web developers. Rather than inventing a new language from scratch, ReasonML is fundamentally a new syntax layered on top of OCaml's existing compiler and semantics, resembling JSX-flavored JavaScript far more closely than OCaml's traditional ML-style syntax. Code written in Reason syntax is functionally OCaml underneath, meaning it can leverage OCaml's mature ecosystem, its type inference, pattern matching, and its compiler's speed. ReasonML gained particular traction for React development, since Facebook paired it with ReasonReact, offering type-safe bindings to the React library and a natural fit given JSX's origins at the same company. Compilation to JavaScript was originally handled by BuckleScript, a separate OCaml-to-JS compiler; that project was later renamed and repositioned as ReScript, which became a full language and toolchain in its own right, eventually somewhat eclipsing ReasonML as the primary path for OCaml-flavored functional programming on the web. Because it compiles to native code as well as JavaScript, ReasonML occupies a distinctive niche: teams can, in principle, share a single codebase across a web front end and native or server-side components compiled natively. In practice, adoption has been concentrated among functional-programming-oriented teams already comfortable with OCaml's ecosystem and tooling, and much of the community's energy and new development has since shifted toward ReScript, which offers a more polished, JavaScript-first developer experience with its own package manager and simplified interop story.

Key Features

• JavaScript-familiar syntax layered over the OCaml language and compiler
• Sound, fast static type inference inherited from OCaml
• Compiles to both native binaries and JavaScript (originally via BuckleScript)
• First-class pattern matching and algebraic data types
• Strong historical integration with React via ReasonReact
• Access to OCaml's mature package ecosystem and tooling
• Immutable-by-default data structures with opt-in mutability
• Precursor to and closely related with the ReScript language

Use Cases

Alternatives

History

Reason (often "ReasonML") is a programming language and toolchain that provides a JavaScript-like syntax over OCaml, aiming to make OCaml's powerful type system approachable for JavaScript developers. It was created by Jordan Walke — the creator of the React framework — in 2016 while he was at Facebook; notably, React's earliest prototype had been written in an ML-family language, so Reason brought that lineage full circle. Reason code can compile to native via OCaml's toolchain or to JavaScript, and the community provided ReasonReact for building React applications. Reason is closely intertwined with BuckleScript (introduced the same year), the OCaml-to-JavaScript compiler that later evolved, together with Reason's syntax, into ReScript.

Frequently Asked Questions

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