Simple, fast & type safe code that leverages the JavaScript & OCaml ecosystems
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Simple, fast & type safe code that leverages the JavaScript & OCaml ecosystems.

Build Status CircleCI

Getting Started




Go to to contribute to the Reason documentation.


See the src folder's README.

Installation for Programmatic Usage

If you're not using Reason programmatically, disregard this section and see the Getting Started guide above. This is for using Reason's refmt as a third-party library.

JavaScript API

We expose a refmt.js for you to use on the web. Again, for local development, please use the native refmt that comes with the installation here. It's an order of magnitude faster than the JS version. Don't use the JS version unless you know what you're doing. Let's keep our ecosystem fast please.

Aaaanyways, to install refmt.js: npm install reason.

Here's the API, with pseudo type annotations:

  • parseRE(code: string): astAndComments: parse Reason code
  • parseREI(code: string): astAndComments: parse Reason interface code
  • printRE(data: astAndComments): string: print Reason code
  • printREI(data: astAndComments): string: print Reason interface code
  • parseML(code), parseMLI(code), printML(data), printMLI(data): same as above, but for the OCaml syntax

The type string is self-descriptive. The type astAndComments returned by the parse* functions is an opaque data structure; you will only use it as input to the print* functions. For example:

const refmt = require('reason');

// convert the ocaml syntax to reason syntax
const ast = refmt.parseML('let f a = 1');
const result = refmt.printRE(ast);
console.log(result); // prints `let f = (a) => 1`

The parse* functions potentially throw an error of this shape:

  message: string,
  // location can be undefined
  location: {
    // all 1-indexed
    startLine: number, // inclusive
    startLineStartChar: number, // inclusive
    endLine: number, // inclusive
    endLineEndChar: number, // **exclusive**

NOTE: refmt.js requires the node module fs, which of course isn't available on the web. If using webpack, to avoid the missing module error, put node: { fs: 'empty' } into webpack.config.js. See for more information.

refmt.js is minified for you through Closure Compiler, with an accompanying The size is 2.3MB but don't get fooled; it gzips down to just 345KB. This way, you can carry it around in your own blog and use it to create interactive refmt playground, without worrying about imposing bandwidth overhead to your readers. Again, keep our ecosystem fast and lean.

Native API

We're spoiled with more APIs on the native side. To use Reason from OPAM as a native library, you have these functions. So:

  • Reason_toolchain.RE.implementation_with_comments
  • Reason_toolchain.RE.interface_with_comments
  • Reason_toolchain.RE.print_interface_with_comments
  • Reason_toolchain.ML.implementation_with_comments
  • etc.

The ML parsing functions might throw Syntaxerr.Error error. The RE parsing functions might throw:

Example usage:

let ast_and_comments =
  Lexing.from_string "let f a => 1"
  |> Reason_toolchain.RE.implementation_with_comments

(* Convert Reason back to OCaml syntax. That'll show these Reason users! *)
let ocaml_syntax =
  Format.flush_str_formatter ()


See Reason license in LICENSE.txt.

Works that are forked from other projects are under their original licenses.


The general structure of refmt repo was copied from whitequark's m17n project, including parts of the README that instruct how to use this with the OPAM toolchain. Thank you OCaml!