Automatic Formatting

Dune can be set up to run automatic formatters for source code.

It can use OCamlformat to format OCaml source code (*.ml and *.mli files) and refmt to format Reason source code (*.re and *.rei files).

Furthermore it can be used to format code of any defined dialect (see dialect).

Configuring Automatic Formatting (Dune 2.0)

If using (lang dune 2.0), there is nothing to setup in Dune, as formatting will be set up by default. However, OCamlformat will still refuse to format sources without an .ocamlformat file present in the project root.

By default, formatting will be enabled for all languages and dialects present in the project that Dune knows about. This is not always desirable. For example, if in a mixed Reason/OCaml project, one only wants to format the Reason files to avoid pulling OCamlformat as a dependency.

It is possible to restrict the languages considered for formatting or disable it altogether by using the formatting stanza.

Formatting a Project

When this feature is active, an alias named fmt is defined. When built, it will format the source files in the corresponding project and display the differences:

$ dune build @fmt
@@ -1,3 +1 @@
-let () =
-  print_endline
-    "hello, world"
+let () = print_endline "hello, world"

Then it’s possible to accept the correction by calling dune promote to replace the source files with the corrected versions.

$ dune promote
Promoting _build/default/ to

As usual with promotion, it’s possible to combine these two steps by running dune build @fmt --auto-promote.

Starting with Dune 3.2.0, you can also run dune fmt which is equivalent to dune build @fmt --auto-promote.

Enabling and Configuring Automatic Formatting (Dune 1.x)


This section applies only to projects with (lang dune 1.x).

In (lang dune 1.x), there is no default formatting. This feature is enabled by adding the following to the dune-project file:

(using fmt 1.2)

Languages can be configured using the following syntax:

(using fmt 1.2 (enabled_for reason))

Version History

(lang dune 2.0)

  • Formatting is enabled by default.

(using fmt 1.2)

(using fmt 1.1)

  • Format Dune files.

(using fmt 1.0)