dune/doc/manual.org

• Intro
• Terminology
• Jbuilder project layout and metadata specification
  • Metadata format
  • <package>.opam files
    • Package version
    • Odig conventions
  • jbuild
    • OCaml syntax
    • Specification
      • jbuid_version
      • library
      • executables
      • rule
      • ocamllex
      • ocamlyacc
      • alias
      • install
      • Common items
        • Ordered set language
        • Variables expansion
        • Alternative dependencies
        • Preprocessing specification
          • Per module pre-processing specification
        • Dependency specification
          • Glob
        • OCaml flags
        • User actions
• Usage
• Advanced topics
  • META file generation

This document describe the usage of Jbuilder and specifies its metadata format. It is written using the org syntax and the best way to read it is either using the Emacs org-mode or on github.

If you want quick usage example, read the quick start document instead.

Intro

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

When using Jbuilder, you give very little and high-level information to the build system, which in turns 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, Jbuilder provides a few additional ones that detach it from the crowd:

• you never need to tell Jbuilder where things such as libraries are. Jbuilder will always discover it automatically. In particular this mean that when you want to re-organize your project you need to do no more than rename your directories, Jbuilder will do the rest
• things always work the same whether your dependencies are local or installed on the system. In particular this mean that you can always drop in the source for a dependency of your project in your working copy and Jbuilder will start using immediately. This makes Jbuilder a great choice for multi-project development
• cross-platform: as long as your code is portable, Jbuilder will be able to cross-compile it (note that Jbuilder is designed internally to make this easy but the actual support is not implemented yet)
• release directly from any revision: Jbuilder 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 Jbuilder metadata format and the third one describes how to use the jbuilder 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 Jbuilder can build things. Jbuilder knows how to build target that are descendant of the root. Anything outside of the tree starting from the root is considered part of the installed world
• workspace: a workspace is the sub-tree starting from the root. It can contain any number of projects that will be built simultaneously by jbuilder
• installed world: anything outside of the workspace, that Jbuilder takes for granted and doesn't know how to build
• build context: a build context is a subdirectory of the <root>/_jbuild 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 correspond to the environment in which Jbuilder is executed. Build contexts can be specified by writing a /gitea/matthieu/dune/jbuild-workspace file
• build context root: the root of a build context named foo is <root>/_jbuild/<foo>

• alias: an alias is a build target that doesn't produce any file and has configurable dependencies. Alias are per-directory and some are recursive; asking an alias to be built in a given directory will trigger the construction of the alias in all children directories recursively. The most interesting ones are:

  • runtest which runs user defined tests
  • install which depends on everything that should be installed

Jbuilder project layout and metadata specification

A typical jbuilder project will have one or more <package>.opam file at toplevel as well as jbuild 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.

The rest of these sections describe the format of Jbuilder metadata files.

Note that the Jbuilder metadata format is versioned in order to ensure forward compatibility. Jane Street packages use a special jane_street version which correspond to a rolling and unstable version that follows the internal Jane Street development. You shouldn't use this in your project, it is only intended to make the publication of Jane Street packages easier.

Except for the special jane_street version, there is currently only one version available, but to be future proof, you should still specify it in your jbuild files. If no version is specified, the latest one will be used.

Metadata format

Most configuration files read by Jbuilder are using the S-expression syntax, which is very simple. Everything is either an atom or a list. The exact specification of S-expressions is described in the documentation of the parsexp library.

Note that the format is completely static. However you can do meta-programming on jbuilds files by writing them in /gitea/matthieu/dune/OCaml%20syntax.

<package>.opam files

When a <package>.opam file is present, Jbuilder will knows that the package named <package> exists. It will know how to construct a <package>.install file in the same directory to handle installation via opam. Jbuilder also defines the recursive install alias, which depends on all the buildable <package>.install files in the workspace. So for instance to build everything that is installable in a workspace, run at the root:

$ jbuilder build @install

Declaring a package this way will allow you to add elements such as libraries, executables, documentations, … to your package by declaring them in jbuild files.

Jbuilder will only register the existence of <package> in the subtree starting where the <package>.opam file lives, so you can only declare parts of the packages in this subtree. Typically your <package>.opam files should be at the root of your project, since this is where opam pin ... will look for them.

Package version

Note that Jbuilder will try to determine the version number of packages defined in the workspace. While Jbuilder itself makes no use of version numbers, it can be use by external tools such as ocamlfind.

Jbuilder determines the version of a package by first looking in the <package>.opam for a version variable. If not found, it will try to read the first line of a version file in the same directory as the <package>.opam file. The version file is any file whose name is, in order in which they are looked for:

• <package>.version
• version
• VERSION

The version file can be generated by a user rule.

If the version can't be determined, Jbuilder just won't assign one.

Odig conventions

Jbuilder follows the odig conventions and automatically installs any README*, CHANGE*, HISTORY* and LICENSE* files in the same directory as the <package>.opam file to a location where odig will find them.

Note that this include files present in the source tree as well as generated files. So for instance a changelog generated by a user rule will be automatically installed as well.

jbuild

jbuild files are the main part of Jbuilder, and are the origin of its name. They are used to describe libraries, executables, tests, and everything Jbuilder needs to know about.

OCaml syntax

If a jbuild file starts with (* -*- tuareg -*- *), then it is interpreted as an OCaml script that generates the jbuild file as described in the rest of this section. The code in the script will have access to a Jbuild_plugin module containing details about the build context it is executed in.

The script can use the directive #require to access libraries:

#require "base,re";;

Note that any library required by a jbuild file must be part of the installed world.

If you don't like the S-expression syntax, then this method gives you a way to use whatever else you want. For instance you could have an API to describe your project in OCaml directly:

(* -*- tuareg -*- *)
#require "my_jbuild_api"
open My_jbuild_api

let () =
  library "foo" ~modules:["plop"; "bidule"]

Currently the Jbuild_plugin module is only available inside plugins. It is however planned to make it a proper library, see the roadmap for details.

Specification

jbuild files are composed of stanzas. For instance a typical jbuild looks like:

(library
 ((name mylib)
  (libraries (base lwt))))

(rule
 ((targets (foo.ml))
  (deps    (generator/gen.exe))
  (action  (run ${<} -o ${@}))))

The following sections describe the available stanzas and their meaning.

jbuid_version

(jbuild_version 1) specifies that we are using the version 1 of the Jbuilder metadata format in this jbuild file.

library

The library stanza must be used to describe OCaml libraries. The format of library stanzas is as follow:

(library
  ((name <library-name>)
   <optional-fields>
  ))

<library-name> is the real name of the library. It determines the names of the archive files generated for the library as well as the module name under which the library will be available, unless (wrapped false) is used (see below). It must be a valid OCaml module name but doesn't need to start with a uppercase letter.

For instance, the modules of a library named foo will be available as Foo.XXX outside of foo itself. It is however allowed to write an explicit Foo module, in which case this will be the interface of the library and you are free to expose only the modules you want.

<optional-fields> are:

• (public_name <name>) this is the name under which the library can be referred as a dependency when it is not part of the current workspace, i.e. when it is installed. Without a (public_name ...) field, the library will not be installed by Jbuilder. The public name must start by the package name it is part of and optionally followed by a dot and anything else you want. The package name must be one of the packages that Jbuilder knows about, as determined by the <package>.opam files
• (synopsis <string>) should give a one-line description of the library. This is used by tools that list installed libraries
• (modules <modules>) specifies what modules are part of the library. By default Jbuilder will use all the .ml files in the same directory as the jbuild file. This include ones that are present in the file system as well as ones generated by user rules. You can restrict this list by using a (modules <modules>) field. <modules> uses the ordered set language where elements are module names and don't need to start with a uppercase letter. For instance to exclude module Foo: (modules (:standard \ foo))
• (libraries (<library-dependencies>)) is used to specify the dependencies of the library. In here you should put library names. For library that are present in the workspace, you can use either the real name or the public name. For libraries that are part of the installed world, you need to use the public name. For instance: (libraries (base re)). In addition to direct dependencies you can specify alternative dependencies. This is described in the alternative dependencies section
• (wrapped <boolean>) specifies whether the modules of the library should be available only through of the toplevel library module, or should all be exposed at toplevel. The default is true and it is highly recommended to keep it this way. Because OCaml toplevel modules must all be unique when linking an executables, polluting the toplevel namespace will make your library unusable with other libraries if there is a module name clash. This option is only intended for libraries that manually prefix all their modules by the library name and to ease porting of existing projects to Jbuilder
• (preprocess <preprocess-spec>) specifies how to pre-process files if needed. The default is no_processing. Other options are described in the preprocessing specification section
• (preprocessor_deps (<deps-conf list>)) specifies extra dependencies of the preprocessor, for instance if the preprocessor reads a generated file. The specification of dependencies is described in the dependency specification section
• (optional), if present it indicates that the library should only be built and installed if all the dependencies are available, either in the workspace or in the installed world. You can use this to provide extra features without adding hard dependencies to your project
• (c_names (<names>)), if your library has stubs, you must list the C files in this field, without the .c extension
• (cxx_names (<names>)) is the same as c_names but for C++ stubs
• (install_c_headers (<names>)), if your libraries has public C header files that must be installed, you must list them in this field, with the .h extension
• (modes (<modes>)) modes (byte and native) which should be built by default. This is only useful when writing libraries for the OCaml toplevel
• (kind <kind>) is the kind of the library. The default is normal, other available choices are ppx_rewriter and ppx_type_conv_plugin and must be set when the library is intended to be used as a ppx rewriter or a [@@deriving ...] plugin
• (ppx_runtime_libraries (<library-names>)) is for when the library is a ppx rewriter or a [@@deriving ...] plugin and has runtime dependencies. You need to specify these runtime dependencies them here
• (virtual_deps (<opam-packages>). Sometimes opam packages enable a specific feature only if another package is installed. This is for instance the case of ctypes which will only install ctypes.foreign if the dummy ctypes-forein package is installed. You can specify such virtual dependencies here. You don't need to do so unless you use Jbuilder to synthesize the depends and depopts sections of your opam file
• flags, ocamlc_flags and ocamlopt_flags. See the section about specifying OCaml flags
• (library_flags (<flags>)) is a list of flags that are passed as it to ocamlc and ocamlopt when building the library archive files. You can use this to specify -linkall for instance. <flags> is a list of strings supporting variables expansion
• (c_flags <flags>) specifies the compilation flags for C stubs, using the ordered set language. This field supports (:include ...) forms
• (cxx_flags <flags>) is the same as c_flags but for C++ stubs
• (c_library_flags <flags>) specifies the flags to pass to the C compiler when constructing the library archive file for the C stubs. <flags> uses the ordered set language and supports (:include ...) forms. When you are writing bindings for a C library named bar, you should typically write -lbar here, or whatever flags are necessary to to link against this library.

Note that when binding C libraries, Jbuilder doesn't provide special support for tools such as pkg-config, however it integrates easily with configurator by using (c_flags (:include ...)) and (c_library_flags (:include ...)).

executables

The executables stanza must be used to describe sets of executables. The format of executables stanzas is as follows:

(executables
  ((names (<entry point names>))
   <optional-fields>
  ))

<entry point names> is a list of module names that contain the main entry point of each executables. There can be additional modules in the current directory, you only need to list the entry point in (names ...). For every <name>, Jbuilder will know how to build <name>.exe and <name>.bc. <name>.exe is a native code executable and <name>.bc is a bytecode executable which requires ocamlrun to run.

Note that in case native compilation is not available, <name>.exe will in fact be a custom byte-code executable. Custom in the sense of ocamlc -custom, meaning that it is a native executable that embeds the ocamlrun virtual machine as well as the byte code. As such you can always rely on <name>.exe being available.

<optional-fields> are:

• (libraries (<library-dependencies>)) is the same as the (libraries ...) field of libraries
• (modules <modules>) specifies which modules in the current directory Jbuilder should consider when building executables. Modules not listed here will be ignored and cannot be used inside executables described by the current stanza. It is interpreted in the same way as the (modules ...) field of libraries
• (preprocess <preprocess-spec>) is the same as the (preprocess ...) field of libraries
• (preprocessor_deps (<deps-conf list>)) is the same as the (preprocessor_deps ...) field of libraries
• flags, ocamlc_flags and ocamlopt_flags. See the section about specifying OCaml flags

rule

The rule stanza is used to create custom user rules. It tells Jbuilder how to generate a specific set of files from a specific set of dependencies.

The syntax is as follow:

(rule
  ((targets (<filenames>))
   (deps    (<deps-conf list>))
   (action  <action>)))

<filenames> is a list of file names. Note that currently Jbuilder only support user rules with targets in the current directory.

<deps-conf list> specifies the dependencies of the rule. See the [[Dependency specification][dependency specification section]] for more details.

<action> is the action to run to produce the targets from the dependencies. See the actions section for more details.

ocamllex

(ocamllex (<names>)) is essentially a short-hand for:

(rule
  ((targets (<name>.ml))
   (deps    (<name>.mll))
   (action  (chdir ${ROOT} (run ${bin:ocamllex} -q -o ${<})))))

ocamlyacc

(ocamlyacc (<names>)) is essentially a short-hand for:

(rule
  ((targets (<name>.ml <name>.mli))
   (deps    (<name>.mly))
   (action  (chdir ${ROOT} (run ${bin:ocamlyacc} ${<})))))

alias

The alias stanza lets you add dependencies to an alias, or specify an action to run to construct the alias.

The syntax is as follow:

(alias
  ((name    <alias-name>)
   (deps    (<deps-conf list>))
   <optional-fields>
   ))

<name> is an alias name such as runtest.

<deps-conf list> specifies the dependencies of the rule. See the [[Dependency specification][dependency specification section]] for more details.

<optional-fields> are:

• <action>, an action to run when constructing the alias. See the actions section for more details.

The typical use of the alias stanza is to define tests:

(alias
  ((name   runtest)
   (deps   (my-test-program.exe))
   (action "./${<} blah")))

See the section about running tests for details.

install

The install stanza is what lets you describe what Jbuilder should install, either when running jbuilder install or through opam.

Libraries don't need an install stanza to be installed, just a public_name field. Everything else needs an install stanza.

The syntax is as follow:

(install
  ((section <section>)
   (files   (<filenames>))
   <optional-fields>
  ))

<section> is the installation section, as described in the opam manual. The following sections are available:

• lib
• libexec
• bin
• sbin
• toplevel
• share
• share_root
• etc
• doc
• stublibs
• man
• misc

<files> is the list of files to install.

<optional-fields> are:

• (package <name>). If there are no ambiguities, you can omit this field. Otherwise you need it to specify which package these files are part of. The package is not ambiguous when the first parent directory to contain a <package>.opam file contains exactly one <package>.opam file

Common items

Ordered set language

A few fields takes as argument am ordered set and can be specified using a small DSL.

This DSL is interpreted by jbuilder into an ordered set of strings using the following rules:

• :standard denotes to the standard value of the field when it is absent
• an atom not starting with a : is a singleton containing only this atom
• a list of sets is the concatenation of its inner sets
• (<sets1> \ <sets2>) is the set composed of elements of <sets1> that do not appear in <sets2>

In addition, some fields support the inclusion of an external file using the syntax (:include <filename>). This is useful for instance when you need to run a script to figure out some compilation flags. <filename> is expected to contain a single S-expression and cannot contain (:include ...) forms.

Most fields using the ordered set language also support [[Variables expansion][variables expansion]]. Variables are expanded after the set language is interpreted.

Variables expansion

Some fields can contains variables of the form $(var) or ${var} that are expanded by Jbuilder.

Jbuilder supports the following variables:

• ROOT is the relative path to the root of the build context
• CC is the C compiler command line being used in the current build context
• CXX is the C++ compiler command line being used in the current build context
• ocaml_bin is the path where ocamlc lives
• OCAML is the ocaml binary
• OCAMLC is the ocamlc binary
• OCAMLOPT is the ocamlopt binary
• ocaml_version is the version of the compiler used in the current build context
• ocaml_where is the output of ocamlc -where
• ARCH_SIXTYFOUR is true if using a compiler targeting a 64 bit architecture and false otherwise

In addition, (action ...) fields support the following special variables:

• @ expands to the list of target, separated by spaces
• < expands to the first dependency, or the empty string if there are no dependencies
• ^ expands to the list of dependencies, separated by spaces
• path:<path> expands to <path>
• exe:<path> is the same as <path>, except when cross-compiling, in which case it will expand to <path> from the host build context
• bin:<program> expands to a path to program. If program is installed by a package in the workspace (see install stanzas), the locally built binary will be used, otherwise it will be searched in the PATH of the current build context
• lib:<public-library-name>:<file> expands to a path to file <file> of library <public-library-name>. If <public-library-name> is available in the current workspace, the local file will be used, otherwise the one from the installed world will be used
• libexec:<public-library-name>:<file> is the same as lib:... except when cross-compiling, in which case it will expand to the file from the host build context

The ${<kind>:...} forms are what allows you to write custom rules that work transparently whether things are installed or not.

Alternative dependencies

It is sometimes the case that one wants to not depend on a specific library, but instead on whatever is already installed. For instance to use a different backend depending on the target.

Jbuilder allows this by using a (select ... from ...) form inside the list of library dependencies.

Select forms are specified as follow:

(select <target-filename> from
  ((<literals> -> <filename>)
   (<literals> -> <filename>)
   ...))

<literals> are list of literals, where each literal is one of:

• <library-name>, which will evaluate to true if <library-name> is available, either in the worksapce either in the installed world
• !<library-name>, which will evaluate to true if <library-name> is not available in the workspace or in the installed world

When evaluating a select form, Jbuilder will create <target-filename> by copying the file given by the first (<literals> -> <filename>) case where all the literals evaluate to true. It is an error if none of the clauses are selectable. You can add a fallback by adding a clause of the form (-> <file>) at the end of the list.

Preprocessing specification

Jbuilder accept three kinds of pre-processing:

• no_preprocessing, meaning that files are given as it to the compiler, this is the default
• (command <shell-command>) to pre-process files using the given shell command. The input file is given as an extra argument and the command is expected to output the result on its standard output
• (pps (<ppx-rewriters-and-flags>)) to pre-process files using the given list of ppx rewriters

Note that in any cases, files are pre-processed only once. Jbuilder doesn't use the -pp or -ppx of the various OCaml tools.

However, in the case of (command <shell-command>), the shell command is still interpreted in the same way as if it was passed to the -pp option. In particular it is executed using the system shell (sh or cmd depending on the OS). Note that you shouldn't make assumption about where the command is run from, this is an implementation detail of Jbuilder and might change in the Future.

<ppx-rewriters-and-flags> is expected to be a list where each element is either a command line flag if starting with a - or the name of a library implementing an OCaml AST rewriter. These must be libraries as Jbuilder always build a single ppx driver in order to speed up compilation.

Currently Jbuilder only knows how to buid ppx_driver based drivers, so using (pps (...)) will force a dependency on ppx_driver. You are however free to use ppx rewriters that are not based on ppx_driver in this list, since ppx_driver is able to import rewriters that where not designed for ppx_driver.

Per module pre-processing specification

By default a preprocessing specification will apply to all modules in the library/set of executables. It is possible to select the preprocessing on a module-by-module basis by using the following syntax:

(preprocess (per_file
               (<spec1> (<module-list1))
               (<spec2> (<module-list2))
               ...))

Where <spec1>, <spec2>, … are preprocessing specifications and <module-list1>, <module-list2>, … are list of module names. It is currently not possible to distinguish between .ml/.mli files, however it wouldn't be hard to support if needed.

For instance:

(preprocess (per_file
               ((command "./pp.sh X=1" (foo bar)))
               ((command "./pp.sh X=2" (baz)))))

Dependency specification

Dependencies in jbuild files can be specified using one of the following syntax:

• (file <filename>) or simply <filename>: depend on this file
• (alias <alias-name>): depend on the construction of this alias, for instance: (alias src/runtest)
• (glob_files <glob>): depend on all files matched by <glob>, see the glob section for details

In all these cases, the argument supports variables expansion.

Glob

You can use globs to declare dependencies on a set of files. Note that globs will match files that exist in the source tree as well as buildable targets, so for instance you can depend on *.cmi.

Currently jbuilder only support globbing files in a single directory. And in particular the glob is interpreted as follow:

• anything before the last / is taken as a literal path
• anything after the last /, or everything if the glob contains no /, is interpreted using the glob syntax

The glob syntax is interpreted as follow:

• \<char> matches exactly <char>, even if it is a special character (*, ?, …)
• * matches any sequence of characters, except if it comes first in which case it matches any character that is not . followed by anything
• ** matches any character that is not . followed by anything, except if it comes first in which case it matches anything
• ? matches any single character
• [<set>] matches any character that is part of <set>
• [!<set>] matches any character that is not part of <set>
• {<glob1>,<glob2>,...,<globn>} matches any string that is matched by one of <glob1>, <glob2>, …

OCaml flags

In library and executables stanzas, you can specify OCaml compilation flags using the following fields:

• (flags <flags>) to specify flags passed to both ocamlc and ocamlopt
• (ocamlc_flags <flags>) to specify flags passed to ocamlc only
• (ocamlopt_flags <flags>) to specify flags passed to ocamlopt only

For all these fields, <flags> is specified in the ordered set language.

The default value for (flags ...) includes some -w options to set warnings. The exact set depends on whether --dev is passed to Jbuilder. As a result it is recommended to write (flags ...) fields as follow:

  (flags (:standard <my options>))

User actions

(action ...) fields describe user actions. The argument can use one of these two forms:

• a simple string, in which case it is passed to bash
• using a small DSL, that is interpreted by jbuilder directly and doesn't require an external shell

In both case, all atoms in the argument of this field supports [[Variables expansion][variables expansion]]. Moreover, you don't need to specify dependencies explicitly for the special ${<kind>:...} forms, these are recognized automatically handled by Jbuilder.

The DSL is preferable in general as it will make your package more portable. It is currently quite limited, so the recommendation is to write a small OCaml program and use the DSL to invoke it. You can use shexp to write portable scripts or configurator for configuration related tasks.

The following constructions are available:

• (run <prog> <args>) to execute a program
• (chdir <dir> <DSL>) to change the current directory
• (setenv <var> <value> <DSL>) to set an environment variable
• (with-stdout-to <file> <DSL>) to redirect the output to a file
• (progn <DSL>...) to execute several commands in sequence
• (echo <string>) to output a string on stdout
• (cat <file>) to print the contents of a file to stdout
• (copy <src> <dst>) to copy a file
• (copy-and-add-line-directive <src> <dst>) to copy a file and add a line directive at the beginning
• (system <cmd>) to execute a command using the system shell: sh on Unix and cmd on Windows

Usage

TODO

Advanced topics

This section describes some details of Jbuilder for advanced users.

META file generation

Jbuilder uses META files from the findlib library manager in order to inter-operate with the rest of the world when installing libraries. It is able to generate them automatically. However, for the rare cases where you would need a specific META file, or to ease the transition of a project to Jbuilder, it is allowed to write/generate a specific one.

In order to do that, write or setup a rule to generate a META.<package> file in the same directory as the <package>.opam file. If you do that, Jbuilder will still generate a META file but it will be called META.<package>.from-jbuilder. So for instance if you want to extend the META file generated by Jbuilder you can write:

(rule
 ((targets (META.foo))
  (deps    (META.foo.from-jbuilder))
  (action  "{ cat ${<}; echo blah } > ${@}")))

Additionally, Jbuilder provides a simpler mechanism for this scheme: just write or generate a META.<package>.template file containing a line of the form # JBUILDER_GEN. Jbuilder will automatically insert its generated META contents in place of this line.