This section describe usage of dune from the shell.
init subcommand provides limited support for generating dune file
stanzas and folder structures to define components.
dune init can be used to
quickly add new projects, libraries, tests, or executables without having to
manually create dune files, or it can be composed to programmatically generate
parts of a multi-component project.
Initializing a project¶
To initialize a new
dune project that uses the
libraries and supports inline tests, you can run
$ dune init proj myproj --libs base,cmdliner --inline-tests --ppx ppx_inline_test
This will create a new directory called
myproj including sub directories 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¶
To create a new directory
src, initialized as a library, you can run:
$ dune init lib mylib src --libs core --inline-tests --public
This will ensure the file
./src/dune contains the following stanza (creating
the file and directory, if needed):
(library (public_name mylib) (inline_tests) (name mylib) (libraries core) (preprocess (pps ppx_inline_tests)))
Consult the manual page
dune init --help 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 parent directories.
dune prints out the root when starting if it is not the current
$ dune runtest Entering directory '/home/jdimino/code/dune' ...
This message can be suppressed with the
command line option (as in GNU make).
Dune requires at least one
dune-workspace file or
file to operate. This file may appear in the current or a parent
directory. It is used to mark the root of the current workspace;
dune-project files are treated
slightly differently. Since there can be only a single workspace
active at a given time, Dune stops its search for the root at the
dune-workspace file it encounters. On the other hand, Dune
projects are composable, and there can be multiple projects in a
single workspace. For this reason, when Dune finds a
file it will continue searching in parent directories, in case this
project is part of a bigger workspace.
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 given 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
dune will expand it to the
corresponding target in all the build contexts where it knows how to
build it. When using
--verbose, It prints out the actual set of
targets when 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 descendant 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
So for instance:
dune build @_build/foo/runtestwill run the tests only 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:
dune build @@runtest
Note that it’s currently not 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’s a few aliases that dune automatically creates for the user
default- this alias includes 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 set to the
runtest- this is the alias to run all the tests, building them if necessary.
install- build all public artifacts - those that will be installed.
doc- build documentation for public libraries.
doc-private- build documentation for all libraries - public & private.
lint- run linting tools.
all- build all available targets in a directory and installable artifacts defined in that directory.
check- This alias will build the minimal set of targets required for tooling support. Essentially, this is
.cmtifiles and Merlin configurations.
Finding external libraries¶
When a library is not available in the workspace, dune will look it up 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
append ../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 inspect the
OCAMLPATH environment variable and use the paths defined in this
OCAMLPATH always has precedence and can have different
value 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. They will run all the tests defined in the current directory and its children 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 is 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 will search (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
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 utop instance isn’t possible.
Restricting the set of packages¶
You can 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 is not in the list
Dune provides support for building and installing your project. However it doesn’t provide helpers for distributing it. It is recommended to use dune-release for this purpose.
The common defaults are that your projects include the following files:
And that if your project contains several packages, then 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 files of your project
before creating a release tarball or when the package is pinned by the
user using opam.
This is especially interesting for the
VERSION watermark, which gets
replaced by the version obtained from the vcs. For instance if you are using
git, dune-release invokes this command to find out the version:
$ git describe --always --dirty --abbrev=7 1.0+beta9-79-g29e9b37
Projects using dune usually only need dune-release for creating and
publishing releases. However they might still want to substitute the
watermarks when the package is pinned by the user. To help with this,
dune provides the
dune subst performs the same substitution
with the default configuration. i.e. calling
dune subst at the
root of your project will rewrite in place all the files in your
More precisely, it replaces all the following watermarks in 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 name of the project 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
in particular 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 is 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 the packages available in the workspace. When several build contexts are specified via a dune-workspace file, the installation will be performed in all the build contexts.
<prefix> directory is determined as follows for a given build
- if an explicit
--prefix <path>argument is passed, use this path
opamis present in the
PATHand is configured, use the output of
opam config var prefix
- otherwise, take the parent of the directory where
As an exception to this rule, library files might be copied to a
different location. The reason for this is that they often need to be
copied to a particular location for the various build system used in
OCaml projects to find them and this location might be different from
<prefix>/lib on some systems.
Historically, the location where to store OCaml library files was
configured through findlib and the
ocamlfind command line tool was used to both install these files
and locate them. Many Linux distributions or other packaging systems
are using this mechanism to setup where OCaml library files should be
As a result, if none of
--prefix is passed to
ocamlfind is present in the
PATH, then library files will
be copied to the directory reported by
ocamlfind printconf destdir. This
dune install can be used without opam. When using opam,
ocamlfind is configured to point to the opam directory, so this rule makes
--libdir are only supported if a single build
context is in use.
dune install (and Dune’s
additional parameters to override install directories in addition to
--prefix, in particular,
--etcdir are supported
The installation can be done in specific mode (
--relocation) for creating a
directory that can be moved around. In that case the executables installed will
lookup the sites (cf How to load additional files at runtime) of the packages relatively to its location.
The –prefix directory should be used to specify the destination.
If you are using plugins that depends on installed libraries which are not dependencies of the executables – so 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 does not promote
.merlin files to the source
directories any more. Instead these configurations are stored 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 are not needed for
normal use of Dune and Merlin but can provide insightful informations when
debugging or configuring non-standard projects.
Printing the configuration¶
It is possible to manually query the generated configuration for debugging purposes:
$ dune ocaml-merlin --dump-config
This command will print 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 a s-expressions, which are used to communicate with Merlin.
Printing an approximated
It is also possible to print the configuration of the current folder in the Merlin configuration syntax by running the following command:
$ dune ocaml dump-dot-merlin > .merlin
In that case only one configuration will be printed which is the result of a
coarse merge of the configurations of the various modules present in the current
folder. 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 does not 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 filename. That means that if you have files which 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 and then the rest does not matter. Only the name before the first dot will be used by Dune 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