package jbuilder

README.org

* JBUILDER - A composable build system for OCaml

Jbuilder is a build system designed for OCaml 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 hundred of developpers, which means
that it is highly tested and productive.

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

The [[example]] directory contains examples of projects using jbuilder.

[[https://travis-ci.org/janestreet/jbuilder][https://travis-ci.org/janestreet/jbuilder.png?branch=master]]

** Overview

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 [[https://github.com/ocaml/merlin][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=
explicitely.

Especially, one should be able to install OCaml on Windows with a
binary installer and then use only the Windows Console to build
Jbuilder and packages using Jbuilder. Although this hasn't been tested
yet.

** Strengths

*** Composable

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 [[https://opam.ocaml.org/][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:

#+begin_src sh
$ jbuilder build --only-packages <package-name> @install
#+end_src

*** 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.org][roadmap]].

This feature requires [[https://opam.ocaml.org/][opam]].

*** Jenga bridge

[[https://github.com/janestreet/jenga][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 as much 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.

** Status

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

** Roadmap

See [[ROADMAP.org]] for the current plan. Help on any of these points is
welcome!

** FAQ

*** Why do many Jbuilder projects contain a Makefile?

Many Jbuilder project contain a toplevel =Makefile=. It is often only
there only 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 invokation 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
that preserve composability; i.e. it doesn't require to manually run
all =./configure= script when working on multiple projects at the same
time.

** Implementation details

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

*** Bootstrap

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.

*** OCaml compatibility test

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

#+begin_src sh
$ make all-supported-ocaml-versions
#+end_src

*** 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

*** Design

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 necesseray 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][src/build]]. In the end it makes the development
of the internal rules of Jbuilder very composable and quite pleasant.

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

**** Code flow

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