Overview¶

Introduction¶

Dune is a build system for OCaml (with support for Reason and Coq). It is not intended as a completely generic build system that’s able to build any project in any language. On the contrary, it makes lots of choices in order to encourage a consistent development style.

This scheme is inspired from the one used inside Jane Street and adapted to the opam world. It has matured over a long time and is used daily by hundreds of developers, which means that it is highly tested and productive.

When using Dune, you give very little, high-level information to the build system, which in turn takes care of all the low-level details from the compilation of your libraries, executables, and documentation to the installation, setting up of tests, and setting up development tools such as Merlin, etc.

In addition to the normal features expected from an OCaml build system, Dune provides a few additional ones that separate it from the crowd:

You never need to tell Dune the location of things such as libraries. Dune will discover them automatically. In particular, this means that when you want to reorganise your project, you need nothing other than to rename your directories, Dune will do the rest.
Things always work the same whether your dependencies are local or installed on the system. In particular, this means that you can insert the source for a project dependency in your working copy, and Dune will start using it immediately. This makes Dune a great choice for multi-project development.
Cross-platform: as long as your code is portable, Dune will be able to cross-compile it. Read more in the Cross-Compilation section.
Release directly from any revision: Dune needs no setup stage. To release your project, simply point to a specific Git tag (named revision). Of course, you can add some release steps if you’d like, but it isn’t necessary. For more information, please refer to dune-release.

The first section below defines some terms used in this manual. The second section specifies the Dune metadata format, and the third one describes how to use the dune command.

Terminology¶

root¶

The top-most directory in a GitHub repo, workspace, and project, differentiated by variables such as %{workspace_root} and %{project_root}. Dune builds things from this directory. It knows how to build targets that are descendants of the root. Anything outside of the tree starting from the root is considered part of the installed world. Refer to Finding the Root to learn how the workspace root is determined.

workspace¶

The subtree starting from each root. It can contain any number of projects that will be built simultaneously by Dune, and it must contain a dune-workspace file.

project¶

A collection of source files that must include a dune-project file. It may also contain one or more packages. A project consists in a hierarchy of directories. Every directory (at the root, or a subdirectory) can contain a dune file that contains instructions to build files in that directory. Projects can be shared between different applications.

package¶

A set of libraries and executables that opam builds and installs as one.

installed world¶

Anything outside of the workspace. Dune doesn’t know how to build things in the installed world.

installation¶

The action of copying build artifacts or other files from the <root>/_build directory to the installed world.

scope¶

Defined by any directory that contains at least one <package>.opam file. Typically, every project defines a single scope that is a subtree starting from this directory. Moreover, scopes are separate from your project’s dependencies. The scope also determines where private items are visible. Private items include libraries or binaries that will not be installed. See Scopes for more details.

build context¶

A specific configuration written in a dune-workspace file, which has a corresponding subdirectory in the <root>/_build directory. It contains all the workspace’s build artifacts. Without this specific configuration from the user, there is always a default build context that corresponds to the executed Dune environment.

build context root¶

The root of a build context named foo is <root>/_build/<foo>.

build target¶

Specified on the command line, e.g., dune build <target_path.exe>. All targets that Dune knows how to build live in the _build directory.

alias¶

A build target that doesn’t produce any file and has configurable dependencies. Targets starting with @ on the command line are interpreted as aliases (e.g., dune build @src/runtest). Aliases are per-directory. However, asking to build an alias in a given directory will also trigger alias construction in all children directories recursively. If no target is specified, Dune builds the default alias. Dune defines several Built-in Aliases.

environment¶

Determines the default values of various parameters, such as the compilation flags. In Dune, each directory has an environment attached to it. Inside a scope, each directory inherits the environment from its parent. At the root of every scope, a default environment is used. At any point, the environment can be altered using an env stanza.

build profile¶

A global setting that influences various defaults. It can be set from the command line using --profile <profile> or from dune-workspace files. The following profiles are standard:

release which is the profile used for opam releases
dev which is the default profile when none is set explicitly, it has stricter warnings than the release one

dialect¶

An alternative frontend to OCaml (such as ReasonML). It is described by a pair of file extensions, one corresponding to interfaces and one to implementations. It can use the standard OCaml syntax, or it can specify an action to convert from a custom syntax to a binary OCaml abstract syntax tree. It can also specify a custom formatter.

placeholder substitution¶

A build step in which placeholders such as %%VERSION%% in source files are replaced by concrete values such as 1.2.3. It is performed by dune subst for development versions and dune-release for releases.

Project Layout¶

A typical Dune project will have a dune-project and one or more <package>.opam files at the root as well as dune files wherever interesting things are: libraries, executables, tests, documents to install, etc.

We recommended organising your project to have exactly one library per directory. You can have several executables in the same directory, as long as they share the same build configuration. If you’d like to have multiple executables with different configurations in the same directory, you will have to make an explicit module list for every executable using modules.

History¶

Dune started as jbuilder in late 2016. When its 1.0.0 version was released in 2018, the name has been changed to dune. It used to be configured with jbuild and jbuild-workspace files with a slightly different syntax. After a transition period, this syntax is not supported anymore.