Overview — dune documentation

# 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 is able to build any given 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 and 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, setting up of the development tools such as merlin, etc.

In addition to the normal features one would expect from a build system for OCaml, dune provides a few additional ones that detach it from the crowd:

- you never need to tell dune where things such as libraries are. Dune will always discover them automatically. In particular, this means that when you want to re-organize your project you need to do no more than 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 always drop in the source for a dependency of your project 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 (note that dune is designed internally to make this easy but the actual support is not implemented yet)
- release directly from any revision: dune needs no setup stage. To release your project, you can simply point to a specific tag. You can of course add some release steps if you want to, but it is not necessary

The first section of this document defines some terms used in the rest of this manual. The second section specifies the dune metadata format and the third one describes how to use the `dune` command.

## Terminology

- package: a package is a set of libraries, executables, … that are built and installed as one by opam
- project: a project is a source tree, maybe containing one or more packages
- root: the root is the directory from where dune can build things. Dune 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. How the root is determined is explained in Finding the root.
- workspace: the workspace is the subtree starting from the root. It can contain any number of projects that will be built simultaneously by dune
- installed world: anything outside of the workspace, that dune takes for granted and doesn’t know how to build
- installation: this is the action of copying build artifacts or other files from the `<root>/_build` directory to the installed world
- scope: a scope determines where private items are visible. Private items include libraries or binaries that will not be installed. In dune, scopes are sub-trees rooted where at least one `<package>.opam` file is present. Moreover, scopes are exclusive. Typically, every project defines a single scope. See Scopes for more details
- build context: a build context is a subdirectory of the `<root>/_build` directory. It contains all the build artifacts of the workspace built against a specific configuration. Without specific configuration from the user, there is always a `default` build context, which corresponds to the environment in which dune is executed. Build contexts can be specified by writing a mdx (since 2.4) file
- build context root: the root of a build context named `foo` is `<root>/_build/<foo>`
- alias: an alias is a build target that doesn’t produce any file and has configurable dependencies. Aliases are per-directory. However, on the command line, asking for an alias to be built in a given directory will trigger the construction of the alias in all children directories recursively. Dune defines several Built-in Aliases.
- environment: in dune, each directory has an environment attached to it. The environment determines the default values of various parameters, such as the compilation flags. Inside a scope, each directory inherit 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 influence 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 that the `release` one

## Project layout

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

It is recommended to organize your project so that you 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`.