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.
The example directory contains examples of projects using jbuilder.
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,
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
bash as long as the packages themselves don't use
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 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
tool. Running simply
make will build jbuilder using the development
If you do not have
make, you can do the following:
$ ocaml bootstrap.ml $ ./boot.exe $ ./_build/default/bin/main.exe install
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:
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
you often have a few common operations that are not part of the build and
make <blah>is a good way to provide them
makeis shorter to type than
jbuilder build @install
How to add a configure step to a jbuilder project?
one way to do it which preserves composability; i.e. it doesn't require manually
./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.
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
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
.mli using this trick. For instance you can add the
following rule into your jbuild file:
(rule (with-output-to foo.ml (progn (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.
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
boot.ml file. This file is built using
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
- a few files in
srchave an alternative version. These alternatives versions are named
XXX.boot.EXT. For instance:
OCaml compatibility test
Install opam switches for all the entries in the jbuild-workspace.dev file and run:
$ make all-supported-ocaml-versions
vendor/contains dependencies of Jbuilder, that have been vendored
plugin/contains the API given to
jbuildfiles 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
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.
- src/jbuild.mli contains the internal representation
jbuildfiles and the parsing code
- src/jbuild_load.mli contains the code to scan
a source tree and build the internal database by reading
- 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