package reason

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Reason: Meta Language Toolchain


Dune Dependency






reason allows development of Meta Language syntax trees in non-text format. It allows a development model that is equivalent to collaborating on syntax trees that have been committed to a source code repository.



Published: 22 Mar 2017


Reason: Build Systems Rapidly

  • Approachable syntax.

  • Powerful, automatic source code formatting.

  • Adopt incrementally with JavaScript/C interop.

  • Ahead-of-time compilation to assembly - without a language level VM.

  • Rapidly develop and share projects.

Install Via npm

npm version > 3.0 is required - install here.

Reason Project

Installing the ReasonProject starter kit using npm is the easiest way to get started with Reason. It will install the master Reason branch and all of the dependencies for you into a local directory based sandbox. It even includes the compiler, IDE support, and REPL. Simply delete the directory when you're done and it's gone from your computer.

git clone
cd ReasonProject
npm install
npm start

While it's installing, read about how to use ReasonProject to compile your simple project and use its built-in editor support and top level.


The ReasonProject uses a very simple build system called rebuild that comes with Reason, and is enough to start a small project. For large-scale development, we are currently developing a build and namespacing workflow called rebel, which is built on jenga. It is an early work in progress, but you can also try it out via RebelExampleProject. rebel currently takes a long time to compile the first time it's used, so for getting started quickly, use ReasonProject instead.

rebel features:

  • Easy to use bucklescript integration.

  • Easy to use js_of_ocaml integration.

  • Automatic namespacing based on your package.json package name.

  • Automatic generation of .merlin files.

  • Powered by jenga, a fast, parallel, recoverable build system.

  • Uses modern compiler features such as "module aliases" for faster compilation of projects.

Installing via OPAM

The OPAM installation doesn't use the isolated directory based sandbox model that ExampleProject does, but if you are very familiar with opam, you should be able to work out conflicts in the global switch, or create a new switch for the purpose of using Reason.

# On OSX, install opam via Homebrew:
brew update
brew install opam
# On Linux, see here (you will need opam >= 1.2.2):

opam init
# Add this to your ~/.bashrc (or ~/.zshrc):
#   eval $(opam config env)

opam update
opam switch 4.02.3
eval $(opam config env)
opam install reason
Testing OPAM installation.

Test the installation by compiling the following program:

echo 'print_string "Hello world"' >

rebuild Hello.native # Automatically generates Hello.native from


Get Started Now

Check out the docs which guide you through the basic syntax and toolchain features.

Contribute back to that documentation in the docs folder.


Get in touch! We're on IRC freenode #reasonml, and Discord.

Contributing To Development

# On OSX, install opam via Homebrew:
brew update
brew install opam
# On Linux, see here (you will need opam >= 1.2.2):

opam init
# Add this to your ~/.bashrc (or ~/.zshrc):
#   eval $(opam config env)

opam update
opam switch 4.03.0
eval $(opam config env)
git clone
cd reason
opam pin add -y reason .

Note: during the last opam pin step, make sure your local repo is clean. In particular, remove artifacts and node_modules. Otherwise the pinning might go stale or stall due to the big node_modules.


See Reason license in LICENSE.txt.

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

Editor plugins (which have also been forked) in the editorSupport/ directory include their own licenses.

Dependencies (9)

  1. ppx_tools_versioned = "5.0alpha"
  2. ocaml-migrate-parsetree < "2.0.0"
  3. topkg = "0.8.1"
  4. result = "1.2"
  5. merlin-extend >= "0.3" & < "0.4"
  6. menhir >= "20160303" & <= "20170101"
  7. utop >= "1.17" & < "2.0"
  8. ocamlfind build
  9. ocaml >= "4.02" & < "4.05"

Dev Dependencies


Conflicts (1)

  1. utop < "1.17"

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