A composable build system for OCaml
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Signed-off-by: David Allsopp <david.allsopp@metastack.com>
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Jbuilder - A composable build system

Jbuilder is a build system designed for OCaml/Reason projects only. It focuses on providing the user with a consistent experience and takes care of most of the low-level details of OCaml compilation. All you have to do is provide a description of your project and Jbuilder will do the rest.

The scheme it implements is inspired from the one used inside Jane Street and adapted to the open source 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.

Jbuilder comes with a manual. If you want to get started without reading too much, you can look at the quick start guide.

The example directory contains examples of projects using jbuilder.

Travis status AppVeyor status


Jbuilder reads project metadata from jbuild files, which are either static files in a simple S-expression syntax or OCaml scripts. It uses this information to setup build rules, generate configuration files for development tools such as merlin, handle installation, etc...

Jbuilder itself is fast, has very low overhead and supports parallel builds on all platforms. It has no system dependencies: all you need to build jbuilder and packages using jbuilder is OCaml. You don't need make or bash as long as the packages themselves don't use bash explicitly.

Especially, one can install OCaml on Windows with a binary installer and then use only the Windows Console to build Jbuilder and packages using Jbuilder.



Take n repositories that use Jbuilder, arrange them in any way on the file system and the result is still a single repository that Jbuilder knows how to build at once.

This make simultaneous development on multiple packages trivial.

Gracefully handles multi-package repositories

Jbuilder knows how to handle repositories containing several packages. When building via opam, it is able to correctly use libraries that were previously installed even if they are already present in the source tree.

The magic invocation is:

$ jbuilder build --only-packages <package-name> @install

Building against several configurations at once

Jbuilder is able to build a given source code repository against several configurations simultaneously. This helps maintaining packages across several versions of OCaml as you can tests them all at once without hassle.

This feature should make cross-compilation easy, see details in the roadmap.

This feature requires opam.

Jenga bridge

Jenga is another build system for OCaml that has more advanced features such as polling or much better editor integration. Jenga is more powerful and more complex and as a result has many more dependencies. It is planned to implement a small bridge between the two so that a Jbuilder project can build with Jenga using this bridge.


Jbuilder requires OCaml version 4.02.3 or greater.


The recommended way to install jbuilder is via the opam package manager:

$ opam install jbuilder

You can also build it manually with:

$ make release
$ make install

Note however that make install requires the opam-installer tool. Running simply make will build jbuilder using the development settings.

If you do not have make, you can do the following:

$ ocaml bootstrap.ml
$ ./boot.exe
$ ./_build/default/bin/main.exe install


If you have questions about jbuilder, you can send an email to ocaml-core@googlegroups.com or open a ticket on github.


Jbuilder is now in beta testing stage. Once a bit more testing has been done, it will be released in 1.0.


See the roadmap for the current plan. Help on any of these points is welcome!


Why do many Jbuilder projects contain a Makefile?

Many Jbuilder project contain a toplevel Makefile. It is often only there for convenience, for the following reasons:

  1. there are many different build systems out there, all with a different CLI. If you have been hacking for a long time, the one true invocation you know is make && make install, possibly preceded by ./configure

  2. you often have a few common operations that are not part of the build and make <blah> is a good way to provide them

  3. make is shorter to type than jbuilder build @install

How to add a configure step to a jbuilder project?

example/sample-projects/with-configure-step shows one way to do it which preserves composability; i.e. it doesn't require manually running ./configure script when working on multiple projects at the same time.

Can I use topkg with jbuilder?

Yes, have a look at the topkg-jbuilder project for more details.

Known issues

Optional libraries inside a multilib directory


If a directory contains several libraries and some are marked as optional (by adding (optional) in the (library ...) stanza), then the dependencies will still be required to perform the build.

This could be sorted out with some refactoring, but there is a simple workaround, so it is low-priority.


Put each optional library in a separate directory.

mli only modules


Due to the low-level details of OCaml compilation, it is currently possible to write a module that has only a .mli and no .ml file. This works as long as the mli contains only type declarations.

This is not a properly supported feature of the compiler, and in particular it is not possible to alias such modules or use them as the argument of a functor. Moreover, if you do write a value declaration, or even just define an exception in the .mli, then you won't get an error until the point where you link an executable using this module.

For these reason, mli only modules are not recommended by Jbuilder until the compiler support them properly.


As long as a module type contains no value declaration, it is possible to turn in to an implementation by using a recursive module:

module rec M : sig
  type t = A | B
end = M
include M

So if you have a module without a .ml file, simply generate a .ml from the .mli using this trick. For instance you can add the following rule into your jbuild file:

(rule (with-output-to foo.ml
        (echo "module rec HACK : sig\n")
        (cat foo.mli)
        (echo "\nend = HACK\ninclue HACK\n"))))

In fact, jbuilder will automatically add this rule if you have a module without imlpementation. However it will print a warning.

Implementation details

This section is for people who want to work on Jbuilder itself.


In order to build itself, Jbuilder uses an OCaml script (bootstrap.ml) that dumps most of the sources of Jbuilder into a single boot.ml file. This file is built using ocamlopt or ocamlc and used to build everything else.

Note that we don't include all of the sources in boot.ml. We skip a few parts to speed up the build. In particular:

  • vendored libraries are replaced by simpler implementations taken from vendor/boot
  • a few files in src have an alternative version. These alternatives versions are named XXX.boot.EXT. For instance: glob_lexer.boot.ml

OCaml compatibility test

Install opam switches for all the entries in the jbuild-workspace.dev file and run:

$ make all-supported-ocaml-versions

Repository organization

  • vendor/ contains dependencies of Jbuilder, that have been vendored
  • plugin/ contains the API given to jbuild files that are OCaml scripts
  • src/ contains the core of Jbuilder, as a library so that it can be used to implement the Jenga bridge later
  • bin/ contains the command line interface
  • doc/ contains the manual and rules to generate the manual pages


Jbuilder was initially designed to sort out the public release of Jane Street packages which became incredibly complicated over time. It is still successfully used for this purpose.

One necessary feature to achieve this is the ability to precisely report the external dependencies necessary to build a given set of targets without running any command, just by looking at the source tree. This is used to automatically generate the <package>.opam files for all Jane Street packages.

To implement this, the build rules are described using a build arrow, which is defined in src/build.mli. In the end it makes the development of the internal rules of Jbuilder very composable and quite pleasant.

To deal with process multiplexing, Jbuilder uses a simplified Lwt/Async-like monad, implemented in src/future.mli.

Code flow

  • src/jbuild.mli contains the internal representation of jbuild files and the parsing code
  • src/jbuild_load.mli contains the code to scan a source tree and build the internal database by reading the jbuild files
  • src/gen_rules.mli contains all the build rules of Jbuilder
  • src/build_system.mli contains a trivial implementation of a Build system. This is what Jenga will provide when implementing the bridge