This section describes using
dune from the shell.
dune init command is still under development and subject to
init subcommand provides limited support for generating Dune file
stanzas and folder structures to define components. The
dune init command can be used to
quickly add new projects, libraries, tests, and executables without having to
manually create Dune files in a text editor, or it can be composed to programmatically generate
parts of a multi-component project.
Initializing a Project¶
You can run the following command to initialize a new Dune project that uses the
libraries and supports inline tests:
$ dune init proj myproj --libs base,cmdliner --inline-tests --ppx ppx_inline_test
This creates a new directory called
myproj, including subdirectories and
dune files for library, executable, and test components. Each component’s
dune file will also include the declarations required for the given
This is the quickest way to get a basic
dune project up and building.
Initializing an Executable¶
To add a new executable to a
dune file in the current directory
(creating the file if necessary), run
$ dune init exe myexe --libs base,containers,notty --ppx ppx_deriving
This will add the following stanza to the
(executable (name main) (libraries base containers notty) (preprocess (pps ppx_deriving)))
Initializing a Library¶
Run the following command to create a new directory
src, initialized as a library:
$ dune init lib mylib src --libs core --inline-tests --public
This will ensure the file
./src/dune contains the below stanza (creating
the file and directory, if necessary):
(library (public_name mylib) (inline_tests) (name mylib) (libraries core) (preprocess (pps ppx_inline_tests)))
Consult the manual page using the
dune init --help command for more details.
Finding the Root¶
The root of the current workspace is determined by looking up a
dune-project file in the current directory and its
parent directories. Dune requires at least one of these two files to operate.
If it isn’t in the current directory, Dune prints out the root when starting:
$ dune runtest Entering directory '/home/jdimino/code/dune' ...
This message can be suppressed with the
command line option (as in GNU make).
More precisely, Dune will choose the outermost ancestor directory containing a
dune-workspace file, which is used to mark the root of the current workspace.
dune-workspace file is present, the same strategy applies with
In case of a mix of dune-workspace and dune-project files, workspace files
take precedence over project files in the sense that if a
file is found, only parent
dune-workspace files will be considered when
looking for the root; however, if a dune-project file is found both parent
dune-project files will be considered.
dune-workspace file is also a configuration file. Dune will read
it unless the
--workspace command line option is used. See the
section dune-workspace for the syntax of this file. The scope
dune-project files is wider than the scope
files. For instance, a
dune-project file may specify the name of
the project which is a universal property of the project, while a
dune-workspace file may specify an opam switch name which is valid
only on a given machine. For this reason, it is common and recommended
dune-project files in repositories, while it is less
common to commit
If the previous rule doesn’t apply, i.e., no ancestor directory has a
dune-workspace, then the current directory will be used
Forcing the Root (for Scripts)¶
You can pass the
--root option to
dune to select the root
explicitly. This option is intended for scripts to disable the automatic lookup.
Note that when using the
--root option, targets given on the command line
will be interpreted relative to the given root, not relative to the current
directory, as this is normally the case.
Interpretation of Targets¶
This section describes how Dune interprets the targets provided on
the command line. When no targets are specified, Dune builds the
default alias, see Default Alias for more details.
All targets that Dune knows how to build live in the
Although, some are sometimes copied to the source tree for the need of external
tools. These includes
<package>.install files when either
--promote-install-files is passed on the command line.
As a result, if you want to ask Dune to produce a particular
file you would have to type:
$ dune build _build/default/bin/prog.exe
However, for convenience, when a target on the command line doesn’t
_build, Dune expands it to the
corresponding target in all the build contexts that Dune knows how to
build. When using
--verbose, it prints out the actual set of
targets upon starting:
$ dune build bin/prog.exe --verbose ... Actual targets: - _build/default/bin/prog.exe - _build/4.03.0/bin/prog.exe - _build/4.04.0/bin/prog.exe
Targets starting with a
@ are interpreted as aliases. For instance
@src/runtest means the alias
runtest in all descendants of
src in all build contexts where it is defined. If you want to
refer to a target starting with a
@, simply write:
To build and run the tests for a particular build context, use
dune build @_build/foo/runtestonly runs the tests for the
dune build @runtestwill run the tests for all build contexts
You can also build an alias non-recursively by using
@@ instead of
@. For instance, to run tests only from the current directory, use:
dune build @@runtest
Please note: it’s not currently possible to build a target directly if that target
lives in a directory that starts with the
@ character. In the rare cases
where you need to do that, you can declare an alias like so:
(alias (name foo) (deps @foo/some.exe))
@foo/some.exe can then be built with:
dune build @foo
When no targets are given to
dune build, it builds the special
default alias. Effectively
dune build is equivalent to:
dune build @@default
When a directory doesn’t explicitly define what the
means via an alias stanza, the following implicit
definition is assumed:
(alias (name default) (deps (alias_rec all)))
Which means that by default
dune build will build everything that
When using a directory as a target, it will be interpreted as building the default target in the directory. The directory must exist in the source tree.
dune build dir
Is equivalent to:
dune build @@dir/default
There are a few aliases that Dune automatically creates for the user:
defaultincludes all the targets that Dune will build if a target isn’t specified, i.e.,
$ dune build. By default, this is set to the
allalias. Note that for Dune 1.x, this was initially set to the
runtestruns all the tests, building them if necessary.
installbuilds all public artifacts that will be installed.
docbuilds documentation for public libraries.
doc-privatebuilds documentation for all libraries, both public & private.
lintruns linting tools.
allbuilds all available targets in a directory and also builds installable artifacts defined in that directory.
checkbuilds the minimal set of targets required for tooling support. Essentially, this is
.cmtifiles and Merlin configurations.
Finding External Libraries¶
When a library isn’t available in the workspace, Dune will search for it in the installed world and expect it to be already compiled.
It looks up external libraries using a specific list of search paths, and each build context has a specific list of search paths.
When running inside an opam environment, Dune will look for installed
$OPAM_SWITCH_PREFIX/lib. This includes both opam
build context configured via the
dune-workspace file and the
default build context when the variable
Otherwise, Dune takes the directory where
ocamlc was found and
appends ../lib` to it. For instance, if
ocamlc is found in
/usr/bin, Dune looks for installed libraries in
In addition to the two above rules, Dune always inspects the
OCAMLPATH environment variable and uses the paths defined in this
OCAMLPATH always has precedence and can have different
values in different build contexts. For instance, you can set it
manually in a specific build context via the
There are two ways to run tests:
dune build @runtest
dune test(or the more explicit
The two commands are equivalent, and they will run all the tests defined in the current directory and its children directories recursively. You can also run the tests in a specific sub-directory and its children by using:
dune build @foo/bar/runtest
dune test foo/bar(or
dune runtest foo/bar)
dune build and
dune runtest commands support a
--watch) flag. When it’s passed, Dune will perform the action as usual and
then wait for file changes and rebuild (or rerun the tests). This feature
fswatch to be installed.
Launching the Toplevel (REPL)¶
Dune supports launching a utop instance with locally defined libraries loaded.
$ dune utop <dir> -- <args>
<dir> is a directory under which Dune searches (recursively) for
all libraries that will be loaded.
<args> will be passed as arguments to the
utop command itself. For example,
dune utop lib -- -implicit-bindings will
utop, with the libraries defined in
lib and implicit bindings for
Dune also supports loading individual modules unsealed by their signatures into the toplevel. This is accomplished by launching a toplevel and then asking dune to return the toplevel directives needed to evaluate the module:
$ utop # use_output "dune top-module path/to/module.ml";;
Requirements & Limitations¶
- Utop version >= 2.0 is required for this to work.
- This subcommand only supports loading libraries. Executables aren’t supported.
- Libraries that are dependencies of utop itself cannot be loaded. For example Camomile.
- Loading libraries that are defined in different directories into one
utopinstance isn’t possible.
Restricting the Set of Packages¶
Restrict the set of packages from your workspace that Dune can see with
$ dune build --only-packages pkg1,pkg2,... @install
This option acts as if you went through all the Dune files and
commented out the stanzas referring to a package that isn’t in the list
Dune provides support for building and installing your project; however, it doesn’t provide helpers for distributing it. It’s recommended to use dune-release for this purpose.
The common defaults are that your projects include the following files:
If your project contains several packages, all the package names must be prefixed by the shortest one.
One of the features
dune-release provides is watermarking; it replaces
various strings of the form
%%ID%% in all your project files
before creating a release tarball or when the opam user pins the package.
This is especially interesting for the
VERSION watermark, which gets
replaced by the version obtained from the Version-Control System (VCS). For instance, if you’re using
dune-release invokes this command to find out the version:
$ git describe --always --dirty --abbrev=7 1.0+beta9-79-g29e9b37
If no VCS is detected,
dune subst will do nothing.
Projects using Dune usually only need
dune-release for creating and
publishing releases. However, they may still substitute the
watermarks when the user pins the package. To help with this,
Dune provides the
dune subst performs the same substitution that
with the default configuration, i.e., calling
dune subst at the
root of your project will rewrite all your project files.
More precisely, it replaces the following watermarks in the source files:
NAME, the name of the project
VERSION, output of
git describe --always --dirty --abbrev=7
VERSION_NUM, same as
VERSIONbut with a potential leading
VCS_COMMIT_ID, commit hash from the vcs
PKG_MAINTAINER, contents of the
maintainerfield from the opam file
PKG_AUTHORS, contents of the
authorsfield from the opam file
PKG_HOMEPAGE, contents of the
homepagefield from the opam file
PKG_ISSUES, contents of the
issuesfield from the opam file
PKG_DOC, contents of the
docfield from the opam file
PKG_LICENSE, contents of the
licensefield from the opam file
PKG_REPO, contents of the
repofield from the opam file
The project name is obtained by reading the
file in the directory where
dune subst is called. The
dune-project file must exist and contain a valid
dune subst is meant to be called from the opam file and
behaves a bit different to other Dune commands. In
particular it doesn’t try to detect the root of the workspace and must
be called from the root of the project.
Custom Build Directory¶
By default Dune places all build artifacts in the
_build directory relative
to the user’s workspace. However, one can customize this directory by using the
--build-dir flag or the
DUNE_BUILD_DIR environment variable.
$ dune build --build-dir _build-foo # this is equivalent to: $ DUNE_BUILD_DIR=_build-foo dune build # Absolute paths are also allowed $ dune build --build-dir /tmp/build foo.exe
Installing a Package¶
When releasing a package using Dune in opam, there’s nothing special
to do. Dune generates a file called
<package-name>.install at the
root of the project. This contains a list of files to install, and
opam reads it in order to perform the installation.
When not using opam, or when you want to manually install a package,
you can ask Dune to perform the installation via the
$ dune install [PACKAGE]...
This command takes a list of package names to install. If no packages are specified, Dune will install all available packages in the workspace. When several build contexts are specified via a dune-workspace file, Dune performs the installation in all the build contexts.
For a given build context, the installation directories are determined with a
single scheme for all installation sections. Taking the
section as an example, the priorities of this scheme are as follows:
- if an explicit
--lib <path>argument is passed, use this path
- if an explicit
--prefix <path>argument is passed, use
--lib <path>argument is passed before during dune compilation to
./configure, use this paths
OPAM_SWITCH_PREFIXis present in the environment use
- otherwise, fail
The installation can be done in specific mode (
--relocation) for creating a
directory that can be moved. In that case, the installed executables will
look up the package sites (cf How to Load Additional Files at Runtime) relative to its location.
The –prefix directory should be used to specify the destination.
If you’re using plugins that depend on installed libraries and aren’t executable dependencies, like libraries that need to be loaded at runtime, you must copy the libraries manually to the destination directory.
Querying Merlin Configuration¶
Since Version 2.8, Dune no longer promotes
.merlin files to the source
directories. Instead, Dune stores these configurations in the _build
folder, and Merlin communicates directly with Dune to obtain its configuration
via the ocaml-merlin subcommand. The Merlin configuration is now stanza-specific,
allowing finer control. The following commands aren’t needed for
normal Dune and Merlin use, but they can provide insightful information when
debugging or configuring non-standard projects.
Printing the Configuration¶
It’s possible to manually query the generated configuration for debugging purposes:
$ dune ocaml-merlin --dump-config
This command prints the distinct configuration of each module present in the current directory. This directory must be in a Dune workspace and the project must be already built. The configuration will be encoded as s-expressions, which are used to communicate with Merlin.
Printing an Approximated
It’s also possible to print the current folder’s configuration in the Merlin configuration syntax by running the following command:
$ dune ocaml dump-dot-merlin > .merlin
In that case, Dune prints only one configuration: the result of the configuration’s
coarse merge in the current folder’s various modules.
This folder must be in a Dune workspace, and the project must be already
built. Preprocessing directives and other flags will be commented out and must
be un-commented afterward. This feature doesn’t aim at writing exact or correct
.merlin files. Its sole purpose is to lessen the burden of writing the
configuration from scratch.
Merlin configuration loading is based on filenames, so if you have files that are preprocessed by custom rules before they are built, they should respect the following naming convention: the unprocessed file should start with the name of the resulting processed file followed by a dot. The rest does not matter. Dune uses only the name before the first dot to match with available configurations.
For example, if you use the
cppo preprocessor to generate the file
real_module_name.ml, then the source file could be named