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 Dialects).

Enabling automatic formatting

This feature is enabled by adding the following to the dune-project file:

(using fmt 1.2)

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"

It is then possible to accept the correction by calling dune promote to replace the source files by the corrected versions.

$ dune promote
Promoting _build/default/ to

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

Only enabling it for certain languages

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.

In these cases, it is possible to use the enabled_for argument to restrict the languages that are considered for formatting.

(using fmt 1.2 (enabled_for reason))

Version history



  • Format Dune files.