Reason (programming language)

Reason
File:Reason-logo.svg
Paradigms	Multi-paradigm: functional, imperative, modular, object-oriented
Family	ML: Caml: OCaml
Designed by	Jordan Walke
First appeared	May 16, 2016 (2016-05-16)
License	MIT
Filename extensions	.re, .rei
Website	reasonml.github.io
Influenced by
OCaml, JavaScript

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. Reason uses many syntax elements from JavaScript, compiles to native code using OCaml's compiler toolchain, and can compile to JavaScript using the Melange compiler.

The Reason community officially provides ReasonReact as a solution for React-based web applications.

Differences from OCaml Syntax

While Reason compiles down to native code via OCaml's toolchain, it specifically differs in its syntax, error messaging, and editor tooling. This allows Reason to provide an experience more similar to JavaScript or TypeScript for developers, which some may be more familiar with.

While OCaml primarily uses parentheses or begin/end keywords to delimit blocks—with nothing separating declarations—Reason tends to use curly braces to delimit blocks and semicolons for separating declarations. Like JavaScript, Reason also defines functions via arrow-like syntax rather than OCaml's fun keyword. Other notable differences include naming the pattern matching keyword switch (instead of match), and reformatting error messages to be more familiar to JavaScript programmers.

Example

type 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 ++ "."
  };

BuckleScript and ReScript

Reason was originally compiled to JavaScript using the BuckleScript compiler, 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.

BuckleScript was forked to Melange to continue to support compiling OCaml to JavaScript.

See also

• 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

External links

• Reason Project Organisation on GitHub
• Melange documentation site
• ReasonReact