Auto-formatter for OCaml code

Disable outside project

As mentioned in options, when the option --disable-outside-detected-project is set, .ocamlformat files outside of the project (including the one in XDG_CONFIG_HOME) are not read. The project root of an input file is taken to be the nearest ancestor directory that contains a .git or .hg or dune-project file. If no configuration file is found, then the formatting is disabled.

This feature is often the behavior you can expect from OCamlFormat when it is directly run from your text editor, so it is advised to use this option.

Emacs setup

  • add $(opam config var share)/emacs/site-lisp to load-path (as done by opam user-setup install)
  • add (require 'ocamlformat) to .emacs
  • optionally add the following to .emacs to bind C-M-<tab> to the ocamlformat command and install a hook to run OCamlFormat when saving:

    (add-hook 'tuareg-mode-hook (lambda ()
      (define-key tuareg-mode-map (kbd "C-M-<tab>") #'ocamlformat)
      (add-hook 'before-save-hook #'ocamlformat-before-save)))

To pass the option --disable-outside-detected-project (or --disable) to OCamlFormat:

  • run emacs
  • run M-x customize-group⏎ then enter ocamlformat⏎
  • select the Ocamlformat Enable item
  • select the OCamlformat mode in the Value Menu: Enable (by default), Disable or Disable outside detected project
  • save the buffer (C-x C-s) then enter yes⏎ and exit

Other OCamlFormat options can be set in .ocamlformat configuration files.

With use-package

A basic configuration with use-package:

(use-package ocamlformat
  :custom (ocamlformat-enable 'enable-outside-detected-project)
  :hook (before-save . ocamlformat-before-save)

Sometimes you need to have a switch for OCamlFormat (because of version conflicts or because you don't want to install it in every switch, for example). Considering your OCamlFormat switch is named ocamlformat:

(use-package ocamlformat
  (lambda ()
         ;; Never use "/" or "\" since this is not portable (opam-user-setup does this though)
         ;; Always use file-name-as-directory since this will append the correct separator if needed
         ;; (or use a package that does it well like https://github.com/rejeep/f.el)
         ;; This is the verbose and not package depending version:
          ;; Couldn't find an option to remove the newline so a substring is needed
          (substring (shell-command-to-string "opam config var share --switch=ocamlformat --safe") 0 -1))
         (file-name-as-directory "emacs")
         (file-name-as-directory "site-lisp")))
  (ocamlformat-enable 'enable-outside-detected-project)
     (substring (shell-command-to-string "opam config var bin --switch=ocamlformat --safe") 0 -1))
  :hook (before-save . ocamlformat-before-save)

(Notice the :custom to customize the OCamlFormat binary)

This could be made simpler (by defining an elisp variable corresponding to the switch prefix when loading tuareg, for example) but it allows to have a full configuration in one place only which is often less error prone.

Vim setup

  • be sure the ocamlformat binary can be found in PATH

Optional: You can change the options passed to OCamlFormat (to use the option --disable-outside-detected-project for example), you can customize NeoFormat with:

let g:opambin = substitute(system('opam config var bin'),'\n$','','''')
let g:neoformat_ocaml_ocamlformat = {
            \ 'exe': g:opambin . '/ocamlformat',
            \ 'no_append': 1,
            \ 'stdin': 1,
            \ 'args': ['--disable-outside-detected-project', '--name', '"%:p"', '-']
            \ }

let g:neoformat_enabled_ocaml = ['ocamlformat']