Quickstart

This document gives simple usage examples of Dune. You can also look at examples for complete examples of projects using Dune.

Initializing Projects

The following subsections illustrate basic usage of the dune init proj subcommand. For more documentation, see Initializing Components and the inline help available from dune init --help.

Initializing an Executable

To initialize a project that will build an executable program, run the following (replacing project_name with the name of your project):

dune init proj project_name

This creates a project directory that includes the following contents:

project_name/
├── dune-project
├── test
│   ├── dune
│   └── project_name.ml
├── lib
│   └── dune
├── bin
│   ├── dune
│   └── main.ml
└── project_name.opam

Now, enter your project’s directory:

cd project_name

Then, you can build your project with:

dune build

You can run your tests with:

dune test

You can run your program with:

dune exec project_name

The following itemization of the generated content isn’t necessary to review at this point. But whenever you are ready, it will provide jump-off points from which you can dive deeper into Dune’s capabilities:

  • The dune-project file specifies metadata about the project, including its name, packaging data (including dependencies), and information about the authors and maintainers. You should open this in your editor to fill in the placeholder values. See dune-project for details.
  • The test directory contains a skeleton for your project’s tests. Add to the tests by editing test/project_name.ml. See Writing and Running Tests for details on testing.
  • The lib directory will hold the library you write to provide the core functionality of your executable. Add modules to your library by creating new .ml files in this directory. See library for details on specifying libraries manually.
  • The bin directory holds a skeleton for the executable program. Within the modules in this directory, you can access the modules in your lib under the namespace Project_name.Mod, where Project_name is replaced with the name of your project and Mod corresponds to the name of the file in the lib directory. You can run the executable with dune exec project_name. See Building a Hello World Program for an example of specifying an executable manually and executable for details.
  • The project_name.opam file will be freshly generated from the dune-project file whenever you build your project. You shouldn’t need to worry about this, but you can see Generating opam Files for details.
  • The dune files in each directory specify the component to be built with the files in that directory. For details on dune files, see dune.

Initializing a Library

To initialize a project for an OCaml library, run the following (replacing project_name with the name of your project):

dune init proj --kind=lib project_name

This creates a project directory that includes the following contents:

project_name/
├── dune-project
├── lib
│   └── dune
├── test
│   ├── dune
│   └── project_name.ml
└── project_name.opam

Now, enter your project’s directory:

cd project_name

Then, you can build your project with:

dune build

You can run your tests with:

dune test

All of the subcomponents generated are the same as those described in Initializing an Executable, with the following exceptions:

  • There is no bin directory generated.
  • The dune file in the lib directory specifies that the library should be public. See library for details.

Building a Hello World Program

Since OCaml is a compiled language, first create a dune file in Nano, Vim, or your preferred text editor. Declare the hello_world executable by including following stanza (shown below). Name this initial file dune and save it in a directory of your choice.

(executable
 (name hello_world))

Create a second file containing the following code and name it hello_world.ml (including the .ml extension). It will implement the executable stanza in the dune file when built.

print_endline "Hello, world!"

Next, build your new program in a shell using this command:

dune build hello_world.exe

The executable will create a directory called “build” and create the program: _build/default/hello_world.exe. Note that native code executables will have the .exe extension on all platforms (including non-Windows systems).

Finally, run it with the following command to see that it worked. In fact, the executable can both be built and run in a single step with dune exec ./hello_world.exe.

Please note: if you have Dune, opam, and OCaml installed, but you get an error that the dune command isn’t recognized, it will be necessary to run eval $(opam config env) to enable Dune in your directory. Find more information in the Dune ReadMe <https://github.com/ocaml/dune>.

Verify OCaml installation with ocaml -version Verify opam installation with opam --version

If you still get an error that the dune command isn’t recognized, try running the following in this order: opam switch create . ocaml-base-compiler opam install merlin ocp-indent dune utop Then run eval $(opam config env) again before trying to build and run your new hello_world.exe program.

Building a Hello World Program Using Lwt

Lwt is a concurrent library in OCaml.

In a directory of your choice, write this dune file:

(executable
 (name hello_world)
 (libraries lwt.unix))

This hello_world.ml file:

Lwt_main.run (Lwt_io.printf "Hello, world!\n")

And build it with:

dune build hello_world.exe

The executable will be built as _build/default/hello_world.exe

Building a Hello World Program Using Core and Jane Street PPXs

Write this dune file:

(executable
 (name hello_world)
 (libraries core)
 (preprocess (pps ppx_jane)))

This hello_world.ml file:

open Core

let () =
  Sexp.to_string_hum [%sexp ([3;4;5] : int list)]
  |> print_endline

And build it with:

dune build hello_world.exe

The executable will be built as _build/default/hello_world.exe

Defining a Library Using Lwt and ocaml-re

Write this dune file:

(library
 (name        mylib)
 (public_name mylib)
 (libraries re lwt))

The library will be composed of all the modules in the same directory. Outside of the library, module Foo will be accessible as Mylib.Foo, unless you write an explicit mylib.ml file.

You can then use this library in any other directory by adding mylib to the (libraries ...) field.

Building a Hello World Program in Bytecode

In a directory of your choice, write this dune file:

;; This declares the hello_world executable implemented by hello_world.ml
;; to be build as native (.exe) or bytecode (.bc) version.
(executable
 (name hello_world)
 (modes byte exe))

This hello_world.ml file:

print_endline "Hello, world!"

And build it with:

dune build hello_world.bc

The executable will be built as _build/default/hello_world.bc. The executable can be built and run in a single step with dune exec ./hello_world.bc. This bytecode version allows the usage of ocamldebug.

Setting the OCaml Compilation Flags Globally

Write this dune file at the root of your project:

(env
 (dev
  (flags (:standard -w +42)))
 (release
  (ocamlopt_flags (:standard -O3))))

dev and release correspond to build profiles. The build profile can be selected from the command line with --profile foo or from a dune-workspace file by writing:

(profile foo)

Using Cppo

Add this field to your library or executable stanzas:

(preprocess (action (run %{bin:cppo} -V OCAML:%{ocaml_version} %{input-file})))

Additionally, if you want to include a config.h file, you need to declare the dependency to this file via:

(preprocessor_deps config.h)

Using the .cppo.ml Style Like the ocamlbuild Plugin

Write this in your dune file:

(rule
 (targets foo.ml)
 (deps    (:first-dep foo.cppo.ml) <other files that foo.ml includes>)
 (action  (run %{bin:cppo} %{first-dep} -o %{targets})))

Defining a Library with C Stubs

Assuming you have a file called mystubs.c, that you need to pass -I/blah/include to compile it and -lblah at link time, write this dune file:

(library
 (name            mylib)
 (public_name     mylib)
 (libraries       re lwt)
 (foreign_stubs
  (language c)
  (names mystubs)
  (flags -I/blah/include))
 (c_library_flags (-lblah)))

Defining a Library with C Stubs using pkg-config

Same context as before, but using pkg-config to query the compilation and link flags. Write this dune file:

(library
 (name            mylib)
 (public_name     mylib)
 (libraries       re lwt)
 (foreign_stubs
  (language c)
  (names mystubs)
  (flags (:include c_flags.sexp)))
 (c_library_flags (:include c_library_flags.sexp)))

(rule
 (targets c_flags.sexp c_library_flags.sexp)
 (action  (run ./config/discover.exe)))

Then create a config subdirectory and write this dune file:

(executable
 (name discover)
 (libraries dune-configurator))

as well as this discover.ml file:

module C = Configurator.V1

let () =
C.main ~name:"foo" (fun c ->
let default : C.Pkg_config.package_conf =
  { libs   = ["-lgst-editing-services-1.0"]
  ; cflags = []
  }
in
let conf =
  match C.Pkg_config.get c with
  | None -> default
  | Some pc ->
     match (C.Pkg_config.query pc ~package:"gst-editing-services-1.0") with
     | None -> default
     | Some deps -> deps
in


C.Flags.write_sexp "c_flags.sexp"         conf.cflags;
C.Flags.write_sexp "c_library_flags.sexp" conf.libs)

Using a Custom Code Generator

To generate a file foo.ml using a program from another directory:

(rule
 (targets foo.ml)
 (deps    (:gen ../generator/gen.exe))
 (action  (run %{gen} -o %{targets})))

Defining Tests

Write this in your dune file:

(test (name my_test_program))

And run the tests with:

dune runtest

It will run the test program (the main module is my_test_program.ml) and error if it exits with a nonzero code.

In addition, if a my_test_program.expected file exists, it will be compared to the standard output of the test program and the differences will be displayed. It is possible to replace the .expected file with the last output using:

dune promote

Building a Custom Toplevel

A toplevel is simply an executable calling Topmain.main () and linked with the compiler libraries and -linkall. Moreover, currently toplevels can only be built in bytecode.

As a result, write this in your dune file:

(executable
 (name       mytoplevel)
 (libraries  compiler-libs.toplevel mylib)
 (link_flags (-linkall))
 (modes      byte))

And write this in mytoplevel.ml

let () = exit (Topmain.main ())