Libraries With Dune


Dune provides several means to arrange modules into libraries. We look at Dune's mechanisms for structuring projects with libraries that contain modules.

This tutorial uses the Dune build tool. Make sure you have version 3.7 or later installed.

Requirements: Modules and Functors.

Minimum Project Setup

This section details the structure of an almost-minimum Dune project setup. Check Your First OCaml Program for automatic setup using the dune init proj command.

$ mkdir mixtli; cd mixtli

In this directory, create four more files: dune-project, dune,, and


(lang dune 3.7)
(package (name wmo-clouds))

This file contains the global project configuration. It's kept almost to the minimum, including the lang dune stanza that specifies the required Dune version and the package stanza that makes this tutorial simpler.


  (name cloud)
  (public_name nube))

Each folder that requires some sort of build must contain a dune file. The executable stanza means an executable program is built.

  • The name cloud stanza means the file contains the executable.
  • The public_name nube stanza means the executable is made available using the name nube.

module Stratus = struct
  let nimbus = "Nimbostratus (Ns)"

module Cumulus = struct
  let nimbus = "Cumulonimbus (Cb)"

let () =
  Wmo.Stratus.nimbus |> print_endline;
  Wmo.Cumulus.nimbus |> print_endline

Here is the resulting output:

$ opam exec -- dune exec nube
Nimbostratus (Ns)
Cumulonimbus (Cb)

Here is the folder contents:

$ tree
├── dune
├── dune-project

Dune stores the files it creates in a folder named _build. In a project managed using Git, the _build folder should be ignored

$ echo _build >> .gitignore

In OCaml, each .ml file defines a module. In the mixtli project, the file defines the Cloud module, the file defines the Wmo module that contains two submodules: Stratus and Cumulus.

Here are the different names:

  • mixtli is the project's name (it means cloud in Nahuatl).
  • is the OCaml source file's name, referred as cloud in the dune file.
  • nube is the executable command's name (it means cloud in Spanish).
  • Cloud is the name of the module associated with the file
  • Wmo is the name of the module associated with the file
  • wmo-clouds is the name of the package built by this project.

The dune describe command allows having a look at the project's module structure. Here is its output:

((root /home/cuihtlauac/caml/mixtli-dune)
 (build_context _build/default)
  ((names (cloud))
   (requires ())
    (((name Wmo)
      (impl (_build/default/
      (intf ())
      (cmt (_build/default/.cloud.eobjs/byte/wmo.cmt))
      (cmti ()))
     ((name Cloud)
      (impl (_build/default/
      (intf ())
      (cmt (_build/default/.cloud.eobjs/byte/cloud.cmt))
      (cmti ()))))
   (include_dirs (_build/default/.cloud.eobjs/byte)))))


In OCaml, a library is a collection of modules. By default, when Dune builds a library, it wraps the bundled modules into a module. This allows having several modules with the same name, inside different libraries, in the same project. That feature is known as namespaces for module names. This is similar to what module do for definitions; they avoid name clashes.

Dune creates libraries from folders. Let's look at an example. Here the folder is lib:

$ mkdir lib

The lib folder is populated with the following files.


(library (name wmo))


val nimbus : string


let nimbus = "Cumulonimbus (Cb)"


val nimbus : string


let nimbus = "Nimbostratus (Ns)"

All the modules found in the lib folder are bundled into the Wmo module. This module is the same as what we had in the file. To avoid redundancy, we delete it:

$ rm

We update the dune file building the executable to use the library as a dependency.


  (name cloud)
  (public_name nube)
  (libraries wmo))


  • Dune creates a module Wmo from the contents of folder lib.
  • The folder's name (here lib) is irrelevant.
  • The library name appears uncapitalised (wmo) in dune files:
    • In its definition, in lib/dune
    • When used as a dependency in dune

Library Wrapper Modules

By default, when Dune bundles modules into a library, they are automatically wrapped into a module. It is possible to manually write the wrapper file. The wrapper file must have the same name as the library.

Here, we are creating a wrapper file for the wmo library from the previous section.


module Cumulus = Cumulus
module Stratus = Stratus

Here is how to make sense of these module definitions:

  • On the left-hand side, module Cumulus means module Wmo contains a submodule named Cumulus.
  • On the right-hand side, Cumulus refers to the module defined in the file lib/

Run dune exec nube to see that the behaviour of the program is the same as in the previous section.

When a library folder contains a wrapper module (here, it is the only one exposed. All other file-based modules from that folder that do not appear in the wrapper module are private.

Using a wrapper file makes several things possible:

  • Have different public and internal names, module CumulusCloud = Cumulus
  • Define values in the wrapper module, let ... =
  • Expose module resulting from functor application, module StringSet = Set.Make(String)
  • Apply the same interface type to several modules without duplicating files
  • Hide modules by not listing them

Include Subdirectories

By default, Dune builds a library from the modules found in the same folder as the dune file, but it doesn't look into subfolders. It is possible to change this behaviour.

In this example, we create subdirectories and move files there.

$ mkdir lib/cumulus lib/stratus
$ mv lib/ lib/cumulus/
$ mv lib/cumulus.mli lib/cumulus/m.mli
$ mv lib/ lib/stratus/
$ mv lib/stratus.mli lib/stratus/m.mli

Change from the default behaviour with the include_subdirs stanza.


(include_subdirs qualified)
(library (name wmo))

Update the library wrapper to expose the modules created from the subdirectories.

module Cumulus = Cumulus.M
module Stratus = Stratus.M

Run dune exec nube to see that the behaviour of the program is the same as in the two previous sections.

The include_subdirs qualified stanza works recursively, except on subfolders containing a dune file. See the Dune documentation for more on this topic.


The OCaml module system allows organising a project in many ways. Dune provides several means to arrange modules into libraries.

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