How Dune Uses Dune to Build Dune

Dune’s build system is itself Dune. This works thanks to a bootstrap process. This document explains how this works.


boot/ is an OCaml script (it is interpreted, not compiled) that is a mini-build system tailored to Dune itself. It computes dependencies between the various modules by calling ocamldep, and it will generate build and link commands. It knows how to execute these commands in parallel. It does not read any dune file. However, the project structure and its system dependencies are encoded in boot/

This step produces _boot/dune.exe.

Completing the Opam Installation

_boot/dune.exe is the bootstrap Dune. Since it has been built from the Dune sources, it will act like Dune: it can read dune files, etc.

This is actually the dune executable that will get installed. But Opam does not know about this: it expects a dune.install file that explains what files to install.

The next command run by the Opam instruction is the following:

$ ./_boot/dune.exe build dune.install --release --profile dune-bootstrap

By using the dune-bootstrap build profile, it not run a full build, but only copy _boot/dune.exe to its install location, and generate dune.install.

make dev: Everything Else

The above describes how Dune itself is built through Opam, but that’s not all there is it to it: the Dune repository contains other libraries that need to be built, the bootstrap Dune did not generate files useful for editor integration, and it can not do incremental builds.

So the main Makefile has a make dev target that will run _boot/dune.exe build @install: this will rebuild the project using Dune itself.

As a special rule, this build will regenerate boot/ using the locations of the internal libraries used to build Dune.