package ocamlformat-rpc

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Auto-formatter for OCaml code (RPC mode)







OCamlFormat is a tool to automatically format OCaml code in a uniform style. This package defines a RPC interface to OCamlFormat

Published: 19 Jul 2021



OCamlFormat is a tool to format OCaml code.

OCamlFormat works by parsing source code using the OCaml compiler's standard parser, deciding where to place comments in the parse tree, and writing the parse tree and comments in a consistent style.

See the source code of OCamlFormat itself and Infer for examples of the styles of code it produces.

Table of Contents

FAQ for new users

Hello, new user! Welcome! :wave:

If you are here, you are probably interested in using a formatting tool for your code base, so that you do not have to worry about formatting it by hand, and to speed up code review by focusing on the important parts.

OCamlFormat is probably what you are after! But there are some things that you should know before formatting all the things.

Should I use OCamlFormat?

OCamlFormat is already being used by several projects, but it comes with some important caveats. This FAQ should help you decide if it can work for you.

OCamlFormat is beta software. While we do not follow SemVer, we expect the program to change considerably before we reach version 1.0.0. In particular, upgrading the ocamlformat package will cause your program to get reformatted. Sometimes it is relatively pain-free, but sometimes it will make a diff in almost every file. This can be a hard price to pay, since this means losing the corresponding git history.

If you use a custom configuration, options you rely on might also get removed in a later release.

Moreover if you adopt OCamlFormat in one project it will not break your workflow in your other projects. Indeed OCamlFormat modifies a file only if it can find an .ocamlformat file, so adding a save hook in your editor will only simplify your workflow in projects using OCamlFormat.

What configuration should I use?

The recommended way is to use a versioned default profile, such as:


(or replace with the output of ocamlformat --version)

This ensures two things:

  • you are using the default formatting configuration.

  • the version that you use to format is recorded somewhere. If somebody else working on the project tries to use a different version, they will see an error message instead of reformatting the whole project in a different way.

Can OCamlFormat support my style?

No. It is better to see OCamlFormat as a tool to apply a style, rather than a tweakable tool to enforce your existing style. There are some knobs that you can turn, such as overriding margin to determine the maximum line width. But it is better not to set individual options to override what the default profile is doing.

To quote (and sed) prettier's page on option philosophy:

How to locally disable OCamlFormat?

To disable the formatting of a specific toplevel item you must attach an [@@ocamlformat "option=VAL"] attribute to this item in the processed file, such as:

let do_not_touch
    (x : t)
      (y : t)
        (z : t) = [
  x; y; z
] [@@ocamlformat "disable"]

To disable the formatting of a specific expression you must attach an [@ocamlformat "option=VAL"] attribute to this expression in the processed file, such as:

let do_not_touch (x : t) (y : t) (z : t) = [
  x; y; z
] [@ocamlformat "disable"]

To disable a whole file, the preferred way is to add the name of the file to a local .ocamlformat-ignore file. An .ocamlformat-ignore file specifies files that OCamlFormat should ignore. Each line in an .ocamlformat-ignore file specifies a filename relative to the directory containing the .ocamlformat-ignore file. Shell-style regular expressions are supported. Lines starting with # are ignored and can be used as comments.



OCamlFormat requires source code that meets the following conditions:

  • Does not trigger warning 50 (“Unexpected documentation comment.”). For code that triggers warning 50, it is unlikely that OCamlFormat will happen to preserve the documentation string attachment.

  • Parses without any preprocessing, using the version of the standard OCaml (not camlp4) parser used to build OCamlFormat. Attributes and extension points should be correctly preserved, but other mechanisms such as camlp4, cppo, etc. will not work.

  • Is either a module implementation (.ml), an interface (.mli) or a sequence of toplevel phrases (.mlt). dune files in OCaml syntax also work.

Under those conditions, OCamlFormat is expected to produce output equivalent to the input. As a safety check in case of bugs, prior to terminating or modifying any input file, OCamlFormat enforces the following checks:

  • The parse trees obtained by parsing the original and formatted files are equal up to some minor normalization (see Normalize.equal).

  • The documentation strings, and their attachment, has been preserved (implicit in the parse tree check).

  • The set of comments in the original and formatted files is the same up to their location.

Code style

There are a number of preset code style profiles, selected using the --profile option by passing --profile=<name> on the command line or adding profile = <name> to an .ocamlformat configuration file. Each profile is a collection of settings for all options, overriding lower priority configuration of individual options. So a profile can be selected and then individual options can be overridden if desired.

The conventional or default profile aims to be as familiar and "conventional" appearing as the available options allow.

The ocamlformat profile aims to take advantage of the strengths of a parsetree-based auto-formatter, and to limit the consequences of the weaknesses imposed by the current implementation. This is a style which optimizes for what the formatter can do best, rather than to match the style of any existing code. Instead of familiarity, the focus is on legibility, keeping the common cases reasonably compact while attempting to avoid confusing formatting in corner cases. General guidelines that have directed the design include:

  • Legibility, in the sense of making it as hard as possible for quick visual parsing to give the wrong interpretation, is of highest priority;

  • Whenever possible the high-level structure of the code should be obvious by looking only at the left margin, in particular, it should not be necessary to visually jump from left to right hunting for critical keywords, tokens, etc;

  • All else equal compact code is preferred as reading without scrolling is easier, so indentation or white space is avoided unless it helps legibility;

  • Attention has been given to making some syntactic gotchas visually obvious.

The compact profile is similar to ocamlformat but opts for a generally more compact code style.

The sparse profile is similar to ocamlformat but opts for a generally more sparse code style.

If no profile is selected, the conventional one is used.


The full options' documentation is available in [ocamlformat-help.txt] and through ocamlformat --help. Options can be modified by the means of:

  • an .ocamlformat configuration file with an option = VAL line

  • using the OCAMLFORMAT environment variable: OCAMLFORMAT=option=VAL,...,option=VAL

  • an optional parameter on the command line

  • a global [@@@ocamlformat "option=VAL"] attribute in the processed file

  • an [@@ocamlformat "option=VAL"] attribute on an expression in the processed file

.ocamlformat files in the containing and all ancestor directories for each input file are used, as well as the global .ocamlformat file defined in $XDG_CONFIG_HOME/ocamlformat. The global .ocamlformat file has the lowest priority, then the closer the directory is to the processed file, the higher the priority.

When the option --enable-outside-detected-project is not 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 config file is found, formatting is disabled.

An .ocamlformat-ignore file specifies files that OCamlFormat should ignore. Each line in an .ocamlformat-ignore file specifies a filename relative to the directory containing the .ocamlformat-ignore file. Lines starting with # are ignored and can be used as comments.


OCamlFormat can be installed with opam:

opam install ocamlformat

Alternately, see ocamlformat.opam for manual build instructions.

Editor setup

Disable outside project

As mentioned in the Options section, 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 config 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
         ;; 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

  • install the Neoformat plugin

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']


OCamlFormat is documented in its man page and through its internal help:

  • ocamlformat --help

  • man ocamlformat

You can also view it online.


OCamlFormat is influenced by and follows the same basic design as refmt for Reason, but outputs OCaml instead of Reason.

This tool is not able to deal directly with Reason code (*.re/*.rei files), but it is possible to first convert these files to OCaml syntax using refmt -p ml and then running ocamlformat on this output.


See CONTRIBUTING for how to help out.


OCamlFormat is MIT-licensed.

Dependencies (18)

  1. uutf >= "1.0.1"
  2. uuseg >= "10.0.0"
  3. stdio
  4. re >= "1.7.2"
  5. odoc-parser >= "0.9.0" & < "1.0.0"
  6. ocp-indent
  7. menhirSdk >= "20201216"
  8. menhirLib >= "20201216"
  9. menhir >= "20201216"
  10. fpath
  11. fix
  12. dune-build-info
  13. cmdliner < "1.1.0"
  14. base-unix
  15. base >= "v0.12.0"
  16. ocamlformat-rpc-lib = version
  17. ocaml >= "4.08" & < "4.14"
  18. dune >= "2.8"

Dev Dependencies (3)

  1. odoc with-doc
  2. bisect_ppx dev & >= "2.5.0"
  3. alcotest with-test

Used by