Reason, also known as ReasonML, is a general-purpose, high-level, multi-paradigm, functional and object-oriented programming language and syntax extension and toolchain for OCaml created by Jordan Walke, who also created the React framework, at Facebook.[3][4] Reason uses many syntax elements from JavaScript, compiles to native code using OCaml's compiler toolchain, and can compile to JavaScript using the ReScript compiler.[5]
| Reason | |
|---|---|
 | |
| Paradigms | Multi-paradigm: functional, imperative, modular, object-oriented |
| Family | ML: Caml: OCaml |
| Designed?by | Jordan Walke |
| First?appeared | May?16, 2016[1] |
| Stable release | 3.16.0[2]? 
/ April 5, 2025 |
| License | MIT |
| Filename extensions | .re, .rei |
| Website | reasonml |
| Influenced by | |
| OCaml, JavaScript | |
The Reason community officially provides ReasonReact as a solution for React-based web applications.[6]
BuckleScript and ReScript
editReason was originally compiled to JavaScript using the BuckleScript compiler,[7] which was created at Bloomberg L.P.
In 2020, the BuckleScript and Reason toolchain was rebranded to ReScript, essentially creating a separate language that had different syntax from Reason and only compiled to JavaScript.[8]
BuckleScript was forked to Melange[9] to continue to support compiling OCaml to JavaScript.
Differences from OCaml Syntax
editWhile Reason compiles down to native code via OCaml's toolchain, it specifically differs in its syntax, error messaging, and editor tooling.[10] This allows Reason to provide an experience more similar to JavaScript or TypeScript for developers, which some may be more familiar with.
While OCaml often relies on Indentation and begin/end keywords, Reason tends to use curly braces to delimit blocks and semicolons for separation of expressions. Like JavaScript, Reason also defines functions via arrow-like syntax. Other notable differences include usage of a switch construct (instead of a match construct) and a reformatter for error messages.
Example
edittype schoolPerson = Teacher | Director | Student(string);
let greeting = person =>
switch (person) {
| Teacher => "Hey Professor!"
| Director => "Hello Director."
| Student("Richard") => "Still here Ricky?"
| Student(anyOtherName) => "Hey, " ++ anyOtherName ++ "."
};
See also
edit
- Elm – A functional language that uses an abstraction called ports to communicate with JavaScript
- PureScript – A strongly-typed, purely-functional programming language that compiles to JavaScript
References
edit- ^ "Simple, fast & type safe code that leverages the JavaScript & OCaml ecosystems: facebook/reason". GitHub. Facebook. 2025-08-14. Retrieved 2025-08-14.
- ^ "Release 3.16.0". 5 April 2025. Retrieved 27 April 2025.
- ^ "What is ReasonML?". 2ality. Retrieved 2025-08-14.
- ^ Rauschmayer, Axel (2018). Exploring ReasonML and functional programming. ReasonML Hub. Retrieved 2025-08-14.
- ^ "Overview: ReScript Documentation". rescript-lang.org. Retrieved 2025-08-14.
- ^ "ReasonReact". GitHub.
- ^ "BuckleScript". Bloomberg L.p. Retrieved 2025-08-14.
- ^ "BuckleScript & Reason Rebranding". rescript-lang. Retrieved 2025-08-14.
- ^ "ANN: Melange 1.0 – compile OCaml / ReasonML to JavaScript". OCaml. 2025-08-14. Retrieved 2025-08-14.
- ^ "What & Why · Reason". reasonml.github.io. Retrieved 2025-08-14.
