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A simple tool to indent OCaml programs

Authors: Louis Gesbert (OCamlPro), Thomas Gazagnaire (OCamlPro), Jun Furuse

License: GPL v3.0


Using OPAM

The simplest way to install ocp-indent is using OPAM:

opam install ocp-indent

From sources

You can also install ocp-indent from sources. You'll need ocaml (>= 3.12.1) and ocp-build (>= 1.99.3-beta):

make install

If you use opam and want it installed alongside ocaml, you may want to use ./configure --prefix $(opam config var prefix).


The above installation step copies elisp and vim scripts to <prefix>/share/typerex/ocp-indent/. You then need to load them in the editor of your choice to automatically use ocp-indent.


Run the following command to setup tuareg-mode to use ocp-indent:

echo '(load-file "'"$(opam config var prefix)"'/share/typerex/ocp-indent/ocp-indent.el")' >>~/.emacs


Use the following command to tell Vim to use ocp-indent:

echo 'autocmd FileType ocaml source '"$(opam config var prefix)"'/share/typerex/ocp-indent/ocp-indent.vim' >>~/.vimrc

Other editors

As ocp-indent is a command-line tool, you can easily integrate it with other editors.

ocp-indent <src-file> > <dst-file>

You can also tell it to indent only a subsets of lines, and to output only the indentation level:

ocp-indent <src-file> --lines <l1>-<l2> --numeric

How does it compare to tuareg ?

We've run some benchmarks on real code-bases and the result is quite conclusive. Keep in mind than most of existing source files are either indented manually or following tuareg standards. You can see the results here.

Moreover, as ocp-indent has a deep understanding of the OCaml syntax it shines on specific cases. See for instance the collection of unit-tests here. The currently failing tests can be seen here.


It's hard to deliver a great indenter without tests. We've built ocp-indent based on a growing collection of unit-tests. If you find an indentation bug, feel free to send us a code snippet that we will incorporate into our test suite.

The tests are organized as follows:

  • tests/passing contains tests that are properly indented and should be left unchanged by ocp-indent.
  • tests/failing contains tests for which ocp-indent currently returns the results in tests/failing-output, hence meld tests/failing{,-output} should give an overview of currently known bugs (also available online here).
  • tests/ checks the current state against the reference state (checked into git).
  • tests/ --[git-]update updates the current reference state.
  • See tests/ --help for more

Please make sure tu run make && tests/ --git-update before any commit, so that the repo always reflects the state of the program.

Configuration options

By default, ocp-indent comes with sensible default options. However, you can customize some of the indentation options using command-line options. For more details, see:

ocp-indent --config help

There is no built-in support for in-file configuration directives. Yet, some editors already provide that features, and with emacs, starting your file with a line like:

(* -*- ocp-indent-config: in=2 -*- *)

will enable you to have the indentation after in setup to 2 locally on this file.

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