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An Emacs major mode for ReasonML.



Note: the following setup assumes Reason and Merlin are installed. This can be achieved by installing them from OPAM (opam install reason merlin).

If you are using bucklescript, make sure you are using a compatible OCaml version (you can find the version of ocaml compatible with your bucklescript installation by running npm bsc -version). At the time of writing this documentation, install OCaml 4.06.1 (for bucklescript 7.*)

Please verify your installation:

ocamlc -version # 4.06.1 if you are using bucklescript
which ocamlmerlin # a valid path to the ocamlmerlin binary, mandatorily
which ocamlmerlin-reason # a valid path to the ocamlmerlin-reason binary, mandatorily


If your Emacs has package.el (which is automatically the case for Emacs >= 24), you can install reason-mode from the package in MELPA.


Alternatively, you can use quelpa and the following recipe:

(quelpa '(reason-mode :repo "reasonml-editor/reason-mode" :fetcher github :stable t))

Manual Installation

Download reason-indent.el, reason-interaction.el, reason-mode.el and refmt.el at the root of this repository and place it in a vendor file next to your Emacs configuration files. Then place the following somewhere in your .emacs.el:

(add-to-list 'load-path "/path/to/vendor")

Add the following to your ~/.emacs or ~/.emacs.d/init.el file:

;; Reason setup

(defun shell-cmd (cmd)
  "Returns the stdout output of a shell command or nil if the command returned
   an error"
  (car (ignore-errors (apply 'process-lines (split-string cmd)))))

(defun reason-cmd-where (cmd)
  (let ((where (shell-cmd cmd)))
    (if (not (string-equal "unknown flag ----where" where))

(let* ((refmt-bin (or (reason-cmd-where "refmt ----where")
                      (shell-cmd "which refmt")
                      (shell-cmd "which bsrefmt")))
       (merlin-bin (or (reason-cmd-where "ocamlmerlin ----where")
                       (shell-cmd "which ocamlmerlin")))
       (merlin-base-dir (when merlin-bin
                          (replace-regexp-in-string "bin/ocamlmerlin$" "" merlin-bin))))
  ;; Add merlin.el to the emacs load path and tell emacs where to find ocamlmerlin
  (when merlin-bin
    (add-to-list 'load-path (concat merlin-base-dir "share/emacs/site-lisp/"))
    (setq merlin-command merlin-bin))

  (when refmt-bin
    (setq refmt-command refmt-bin)))

(require 'reason-mode)
(require 'merlin)
(add-hook 'reason-mode-hook (lambda ()
                              (add-hook 'before-save-hook 'refmt-before-save)

(setq merlin-ac-setup t)

If you have iedit mode set up:

(require 'merlin-iedit)
(defun evil-custom-merlin-iedit ()
  (if iedit-mode (iedit-mode)
(define-key merlin-mode-map (kbd "C-c C-e") 'evil-custom-merlin-iedit)

(Thanks @sgrove:

This associates reason-mode with .re and .rei files. To enable it explicitly, do M-x reason-mode.

Project specific version of refmt

If you're using different versions of refmt between projects, you can use the project-specific installed version via the special config values:

  • 'npm (calls npx refmt ... to use the version of refmt installed in the project's node_modules)
  • 'opam (calls opam exec -- refmt ... to use the version of refmt on the current opam switch):
;; can also be set via M-x `customize-mode`
(setq refmt-command 'npm)


Reason-mode provides (opt-in) rtop support. At the moment only the native workflow is supported.

First of all you need to install the Utop Emacs integration. Make sure it is latest master because the feature is fairly new.

Then in your Emacs init file add:

(require 'utop)
(setq utop-command "opam config exec -- rtop -emacs")
(add-hook 'reason-mode-hook #'utop-minor-mode) ;; can be included in the hook above as well

After this, the function utop (C-c C-s) will start rtop in Reason buffers.


The reasonml layer is available in the develop version of spacemacs.

For the stable version of spacemacs, you can install the reason-mode package automatically.

      :location (recipe
        :repo "reasonml-editor/reason-mode"
        :fetcher github
        :files ("reason-mode.el" "refmt.el" "reason-indent.el" "reason-interaction.el")))

Afterwards add the snippet to your dotspacemacs/user-config.


Auto-format before saving

If you have refmt installed, you can add this to your .emacs file to enable auto-format:

(add-hook 'reason-mode-hook (lambda ()
          (add-hook 'before-save-hook #'refmt-before-save)))

Tests via Cask + ERT

The test folder contains tests that can be run via Cask. Once you install cask, if it is the first time run:

cask install
cask exec ./

If it is not the first time you can omit the first line and execute the tests with the second one only. The environment variable EMACS controls the program that runs emacs.


reason-mode is distributed under the terms of both the MIT license and the Apache License (Version 2.0).