# Debian OCaml Packaging Policy

###

The Debian OCaml Task Force

`<debian-ocaml-maint@lists.debian.org>`

Revision 0.8.0

Copyright © 2002-2015 Mehdi Dogguy, Stephane Glondu, Sylvain Le Gall, Sven Luther, Samuel Mimram, Ralf Treinen, Stefano Zacchiroli

This manual is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

A copy of the GNU General Public License is available as `/usr/share/common-licenses/GPL-3` on Debian GNU/Linux systems or on the World Wide Web at The GNU Public Licence.

List of Tables

1.1. OCaml compilers

List of Examples

3.1. Dependency from a -dev package to its companion shared library stub package (if any), from the pcre-ocaml package

# Conventions used in this document

[ocaml-version] stands for the current upstream version of the OCaml system (for instance, 4.02.1).

# Chapter 1. Generalities about OCaml packages in Debian

## The name of the game

The correct name of the language is OCaml. Other spellings/capitalizations like “Objective Caml”, “ocaml”, “OCAML”, “O'Caml” should be avoided when talking about the OCaml project, the system, or the programing language.

## Bytecode and native code

The OCaml compilers can produce two kinds of executables: bytecode and native. Native executables are usual ELF executables produced for a particular CPU architecture, while pure bytecode executables are written in a language which can only be run on a OCaml bytecode interpreter. A pure bytecode executable starts with a magic string invoking the interpreter, so that bytecode executables can just be invoked like native executables. There are also “custom” bytecode executables, which embed a bytecode program along with a bytecode interpreter; these files are ELF executables. OCaml compilers to native code are not provided for every architecture. On a Debian system, the list of architectures for which the currently installed version of the OCaml system provides compilation to native code can be found in the file `/usr/lib/ocaml/native-archs`.

Native executables are in general faster to execute than bytecode executables since they are compiled specifically for an architecture. Bytecode executables are smaller than native code executables. If considering only one process the advantage in size is annihilated by the need for the OCaml runtime system for executing bytecode, however there is an advantage when running several bytecode processes in parallel since the runtime system can then be shared.

Pure bytecode executables have the advantage of being portable, which means that a pure bytecode executable can usually be run without having to be recompiled on any architecture for which an OCaml bytecode interpreter is available. For Debian, this has the advantage that packages containing only bytecode executables have `Architecture=all`, which reduces archive space and auto-builder load. It also reduces download and installation size for users in case they install several bytecode packages.

An important decision to take when building a Debian package is whether to support bytecode, native code, or both. We aim to support development both of bytecode and native code applications, hence libraries should be distributed both in bytecode and native code (see the section on library packaging). For packages containing executables, the rule is:

- If the execution of the program requires a lot of CPU power then the executables in the package should be compiled to native code on architectures where this is possible, and to bytecode elsewhere. Typical examples are coq, or edos-distcheck. See the section on native code packages.

- If the execution of the program is very fast on typical application cases then the program contained in the package should be compiled to bytecode. Typical examples are hevea, or headache. See the section on bytecode packages.

- Only in special cases, for instance when both versions provide different features, should a package contain programs both compiled to bytecode and native code. See the section on combined bytecode and native code packages.

## Overview of the OCaml compilers

### Bytecode and native code compilers

The ocaml-native-compilers package contains the OCaml compiler executables built in native mode: ocamlc.opt, which produces bytecode, and ocamlopt.opt, which produces native code. Since the OCaml compilers are themselves written in OCaml this package exists only on architectures where compilation to native code is supported.

The ocaml package contains the OCaml compiler executables built in bytecode mode: ocamlc, which produces bytecode, and on architectures where compilation to native code is supported the compiler ocamlopt, which produces native code. It is important to understand that on architectures where compilation to native code is supported both packages contain compilers from OCaml to both bytecode and native code, the difference lies in the nature (installation size and execution speed) of the compiler executables.

Table 1.1. OCaml compilers

| Compiles to bytecode | Compiles to native code
--- | --- | ---
Compiler executable in bytecode | ocamlc | ocamlopt
Compiler executable in native code | ocamlc.opt | ocamlopt.opt

See the section called “Bytecode and native code” on a discussion whether to produce native code or bytecode.

### Files used and produced by the OCaml compilers

The OCaml toolchain can produce or use the following kind of files:

- pure bytecode executables (they can be recognized since they start with the shebang line `#!/usr/bin/ocamlrun`)

- bytecode executables linked in custom mode. They are generated by ocamlc when the `-custom` flag is given at link time. Those executables are in ELF format and include both the final bytecode and the bytecode interpreter. The strip command should never be invoked on them since it will remove the bytecode part. Bytecode executables linked in custom mode are depreciated and should be avoided.

- native executables (in ELF format)

- bytecode libraries (`*.cma, *.a`)

- native libraries (`*.cmxa`)

- shared libraries (for C bindings) (`dll*.so`, `lib*.so`)

- static libraries (for C bindings) (`lib*.a`)

- bytecode object files (`*.cmo`)

- native object files (`*.cmx, *.o`)

- native plugin object files (`*.cmxs`)

- configuration files for handling libraries with ocamlfind (`META`)

## Building packages using OCaml

### Findlib and ocamlfind

The Debian OCaml Task Force has chosen to fully support the findlib library. This is a suite of tools and libraries that help to manage OCaml libraries. This software is not specific to Debian and works on all platforms where OCaml is available. Through the years, this has become the de facto standard when relying on external libraries. The main frontend is called ocamlfind and provided by the package ocaml-findlib.

Even though compiling OCaml projects in Debian is still possible without ocamlfind it is highly recommended to use it.

### The OCaml system in Debian

There are three categories of OCaml packages in Debian:

- Development packages contain OCaml objects required for the development or compilation of OCaml programs, or specifically for the creation of Debian packages containing OCaml programs.

- Runtime packages contain OCaml objects necessary to run compiled programs. A runtime package is always associated with a development package.

- Simple binary packages contain everything that does not fall into the two former categories. This includes bytecode and native executable of application programs, documentation, etc.

The ocaml package depends on all the basic packages needed to develop programs with OCaml. Here is the list of binary packages into which the OCaml system is organized:

1. The ocaml package contain the compiler and its libraries. It can be considered as a development package.

2. The ocaml-native-compilers package contains the OCaml compilers built in native mode (ocamlc.opt and ocamlopt.opt).

   ### Note

   The compilers themselves are built in native mode, nonetheless, both compilers for compiling toward bytecode and native code are contained in this package.

3. The ocaml-base package contains the interpreter and runtime libraries needed by bytecode programs compiled with OCaml (in particular, it contains the ocamlrun program). It can be considered as the runtime package associated to ocaml.

   ### Note

   ocaml-base contains also OCaml interface objects `*.cmi`, which are normally only in development packages because they are needed to run the ocaml toplevel from the package ocaml-interp.

4. The ocaml-interp package contains the toplevel system for OCaml (ocaml) which provides for an interactive interpreter of the language.

5. The ocaml-mode package contains the OCaml Emacs mode. This is the original Emacs mode provided with the OCaml upstream distribution, not to be confused with the tuareg Emacs mode which is in the package tuareg-mode.

6. The ocaml-source package contains the sources of the OCaml compiler.

7. The ocaml-compiler-libs package contains some internal libraries of the OCaml compiler (needed only in very rare cases, e.g. for developing languages which reuse OCaml internals).

Since libraries produced by OCaml are binary incompatible when compiled with different releases of OCaml, versioned virtual packages are also provided by packages at items (1), (2), (6) and (7): ocaml-[ocaml-version], etc.

### Choosing the right build dependencies

Compiling with native code versions of the compilers is generally faster than with compilers in bytecode. Unfortunately, the ocaml-native-compilers package is not available on every architecture. Packages should therefore never depend directly on this package. In order to build big programs and benefit from this natively built compiler, packages should depend on ocaml-best-compilers which itself depends on ocaml-native-compilers where available and on ocaml elsewhere. Since it is a virtual package, it cannot be a versioned dependency. The version dependency should thus be carried by the ocaml dependency. Note that dependency on ocaml-best-compilers is only necessary for really big programs for which compilation takes a lot of resources. For most small programs the compilers from the ocaml package are perfectly sufficient, and faster to install in a build environment than compiler executables in native code.

### Putting the right binary dependencies

Bytecode is specific to an upstream version of the OCaml system. Therefore, any package containing bytecode has to depend directly or indirectly on the packages providing the OCaml runtime system in precisely the version for which it was compiled (for instance, ocaml-base-[ocaml-version].) This is in particular the case for bytecode programs, but it is also the case for native code programs on the architectures on which there is no native code OCaml compiler, and on which the program will be actually compiled to bytecode (see the section on packaging native code programs). The exact dependencies of libraries packages are discussed in the chapter on library packaging.

In order for a package to be easily rebuild, or even binNMUed, in case of a transition from one OCaml version to another, these dependencies must not be hardcoded in `debian/control`. Instead, substitution variables must be used which are instantiated from the `debian/rules` file. See the discussion on build systems in the Debian OCaml Developers Reference for a discussion how this can be achieved.

### Dh_ocaml

The Debian OCaml Task Force has created a system to compute dependencies between OCaml packages and to ensure type-safe linking. It requires to use new substitution variables in `debian/control`:

- Use the `${ocaml:Depends}` substitution variable in the field `Depends`.

- Add a field `Provides: ${ocaml:Provides}` in development and runtime packages.

More detailed information about this work can be found here.

## Integrating OCaml packages in Debian

### Location of OCaml directories

The root of all installed OCaml libraries is the OCaml standard library directory, which is `/usr/lib/ocaml`. This location can be obtained from the OCaml compiler by invoking it as `ocamlc -where`.

### Debian archive section

Packages intended for the development of OCaml programs or packages, as well as packages necessary for the execution of OCaml code (like runtime systems or libraries) should have their section set to `ocaml`.

End user applications that are not specifically aiming at OCaml development or execution support but just happen to be implemented in OCaml should have their section set according to their application domain (`games`, `science`, etc.). General development tools that are not specific to OCaml development go into section `devel` (example: headache).

### Documentation

Documentation relevant to programming in OCaml should be registered with doc-base in Section `Programming/OCaml`. This is the case for API documentation generated using ocamldoc. See the section on generating documentation.

# Chapter 2. Packaging OCaml programs

## Creating packages for OCaml programs

The source package should, if possible, use the name of the upstream package without modifications.

Packages containing executables (as opposed to libraries) may be produced by compiling to bytecode, native code, or both. See this discussion of which mode to choose.

Package containing only bytecode executable, native executable or data are simple binary package, as explained previously.

## Bytecode packages

A bytecode package has all its binaries compiled into bytecode. It depends on a package containing the OCaml runtime system. As a consequence, such a package has its architecture field set to `all`.

All bytecode executables should be linked dynamically against the shared libraries for C bindings, to not bloat the archive. That said, upstream authors often favor statically linked bytecode executables (ocamlc option `-custom`), because in this case they don't need to worry about the presence of the dll stub libraries. This is not a valid reason to use statically linked bytecode in a Debian package where package dependencies can fix this problem.

A bytecode package must depend on the runtime package that is required to run it. Typically, it should depend on ocaml-base-[ocaml-version]. It can also depends on the virtual libXXX-ocaml-byte-ABCD provided by the runtime package libXXX-ocaml (see the section on binary dependencies).

A bytecode package must build-depend-indep on ocaml.

Bytecode programs which are compiled by `ocamlc -custom` must not be stripped. In particular, their package should be excluded from the dh_strip script. When compiled this way, an ELF executable is generated containing the ocaml interpreter, and the bytecode of the program in a section which is removed when the program is stripped. For more information, see the bug 256900. Custom bytecode executable is a deprecated feature of OCaml -- for now they still exist but should be avoided.

Bytecode programs should not be compiled for debugging, i.e. they should not be compiled using the `-g` option to ocamlc.

## Native versions of programs

Native OCaml compilers are not available on all architecures. For architectures missing native compiler, a bytecode version of the program should be provided. Native code packages are specific to an architecture, that is they have `Architecture=any` in the source package.

The `debian/rules` file should build the native code executable if supported on the architecture it is built on, and fall back to building the bytecode version if no working native code compiler is available. As a consequence, the rules of the section called “Bytecode packages” also apply here.

It is safe to consider that detection of native architecture means availability of ocamlopt or ocamlopt.opt when building.

## Bytecode and native versions of programs

This should be done only under exceptional circumstances. Packages providing both native and bytecode versions of a program prog usually name them respectively prog.opt and prog.byte and provide a symbolic link prog to the best available version (generally prog.opt). Please consult the rules of the section called “Bytecode packages” and the section called “Native versions of programs”.

# Chapter 3. Packaging OCaml libraries

## Creating packages for OCaml libraries

A package which provides an OCaml library called `xxx` should be split as follows:

- For libraries which are not purely programmed in OCaml (e.g. C bindings), `libxxx-ocaml` should provide the shared library stubs (`dll*.so`), and all other stuff needed to run a bytecode executable that links into this library. It should depend on ocaml-base-[ocaml-version] as well as any other library needed. The versioned dependency on ocaml-base is important since libraries are binary incompatible between releases of OCaml.

  `libxxx-ocaml` packages should be in `Section: ocaml`.

  If the library provides native plugins (`*.cmxs`) or is meant to be dynamically loaded using the `Dynlink` library, those plugins, relevant `*.cmo` or `*.cma` files, and the `META` file referencing them should also be provided by this runtime package.

- `libxxx-ocaml-dev` should provide the rest of the library package, in fact anything needed to develop programs using the library. If the library uses other libraries or C libraries then this package should depend on them.

  `libxxx-ocaml-dev` should depend on its companion `libxxx-ocaml` package (if any). The reason is that at compile time the OCaml compiler will try to load the shared library stubs, aborting the compilation in case of failure. Hence the development package is useless if the corresponding stub package is missing. To ensure compatibility the dependency among the two packages should be strictly versioned. In order for the resulting packages to be binNMU safe this requirement states that the dependency should make use of a `${binary:Version}` substitution variable.

  Example 3.1. Dependency from a -dev package to its companion shared library stub package (if any), from the pcre-ocaml package

        Package: libpcre-ocaml
        Architecture: any
        Section: ocaml
        Depends: ocaml-base-[ocaml-version], ${shlibs:Depends}, ${misc:Depends}
        ...

        Package: libpcre-ocaml-dev
        Architecture: any
        Section: ocaml
        Depends: ocaml-[ocaml-version], libpcre3-dev (>= 4.5), libpcre-ocaml (= ${binary:Version}), ${misc:Depends}
        Suggests: ocaml-findlib (>= 1.1)
        ...

  `libxxx-ocaml-dev` packages should be in `Section: ocaml`

  All OCaml bytecode libraries (`*.cma`) and bytecode object files (`*.cmo`) should be compiled for debugging, i.e. they should be compiled passing the `-g` option to ocamlc (or ocamlc.opt).

Optionally, two other packages may be created:

- `libxxx-ocaml-bin` may include binaries provided by the library source package if they are numerous. This package should conform with the same regulations as other packages providing ocaml programs. It is only needed to split off this package if there is a significant number of programs included in the library, if not, the programs should go into `libxxx-ocaml-dev`.

- `libxxx-ocaml-doc` may include any kind of documentation provided by the library source package or as separate documentation. Again, if there is only little documentation, they should go with the `-dev` package.

It is recommended that libraries use the `-pack` option to pack all the modules provided by the library into one module. We don't think upstream libraries will be moving to this scheme anytime soon (unless we actively lobby for it) so this is just a recommendation for now.

It is recommended that each library package ships a `META` file in order to make the library usable via `ocamlfind` (see the Debian package `ocaml-findlib`). See the section called “Providing `META` files” for more information on this.

## Paths for libraries

Libraries should be installed in ``/usr/lib/ocaml`/` or in a subdirectory of this directory. This includes in particular bytecode libraries (`*.cma`), native libraries (`*.cmxa, *.a`), native plugins (`*.cmxs`), bytecode object files (`*.cmo`), native object files (`*.cmx, *.o`), static libraries (`*.a`) and `META` files. The only exception to this rule is for shared libraries (`dll*.so`) which should be installed in ``/usr/lib/ocaml`/stublibs`, as can it be seen in the ``/usr/lib/ocaml`/ld.conf` file.

If upstream developers already use a subdirectory of the OCaml standard library path then this path should be preserved in the Debian package but made relative to the standard library path of OCaml. Before using the provided subdirectory, packagers should obviously check if there is no subdirectory name clash with another OCaml library.

If upstream developers do not use this scheme then packagers are encouraged not to install this library in the standard library directory. They should create at least a subdirectory per source package (in order to avoid name clashes). Packagers should also consider to do a larger separation by creating a subdirectory per binary package (in order to avoid META name clash). A suggested rule to choose the name for this subdirectory is to use either the package name provided by the META of the upstream, or the name of the library itself.

## Providing `META` files

The `ocaml-findlib` provides a tool (called `ocamlfind`) to handle OCaml libraries and store information about libraries dependencies, compiler flags, linking options, etc. Meta informations regarding a library are contained in files (usually one for each library), named `META` files, contained in the library directory. The distribution of `META` files is the best way to make more easy to use the Debian-specific organization of libraries. Packages distributing `META` files should suggest the use of ocamlfind, that is have a `Suggest: ocaml-findlib`.

By default, ocamlfind will look for `META` in this order:

- ``/usr/lib/ocaml/METAS`/`

- ``/usr/lib/ocaml`/package/`

If a library package creates its own subdirectory ``/usr/lib/ocaml`/package/` then the META file should be stored in that directory.

The naming scheme of `META` is pretty simple.

- If the `META` file is placed in the subdirectory of the package then it should be called `META`.

- If the `META` file is placed in ``/usr/lib/ocaml/METAS`/` then it should be called `META.packagename`, where `packagename` is the name of the subdirectory where the library is stored.

For example, the `META` file for the lablgtk library is named `META` and has path ``/usr/lib/ocaml`/lablgtk/META`, where ``/usr/lib/ocaml`` is the main OCaml installation directory and `lablgtk` is the lablgtk library directory.

If upstream doesn't provide a `META` then packagers are encouraged to create one. In this case, the META file should contain a comment like this, so that developers will know that they shouldn't count on the availability of a META file on non-Debian machines:

    	# This META file is delivered by the Debian distribution.

Furthermore, the `META` file should be sent to the upstream authors in order to have it included in future versions of the upstream source. For more information about `META` files see the Findlib manual, at the several META files provided by other packages (e.g. `lablgtk`, `pxp`, `pcre`, `netstring`, `lablgl`, ...) or ask on the debian-ocaml-maint mailing list for help.

## Camlp4/Camlp5

Actually, Camlp4 extensions should be processed just like standard OCaml libraries. In particular, they should provide a META file. The syntax extension should be contained in a `syntax` sub package.

The naming convention of the package is to use the same naming as with standard package, replacing `-ocaml-` by the syntax extension name, `-camlp4-`.

If a package contains at the same time syntax extension and libraries then it is up to the maintainer to choose the most relevant name for the package. Whatever the name chosen for the package, the other name should be a `Provide` of the package.

For example, consider the package sexplib310. It provides a syntax extension and a library, which is the runtime support of the additional function generated by the syntax extension. Since the most common use of sexplib310 is through its syntax extension, the package is name libsexplib-camlp4-dev and it also provide libsexplib-ocaml-dev.

Camlp5 is an alternate pretty-printer and preprocessor for OCaml (which is compatible with pre-3.10.0 version). Syntax extension are handled through exactly the same scheme as for Camlp4 except that package name use `-camlp5-` rather than `-camlp4-`.

## Documentation

The documentation is a joint effort of The Debian OCaml Task Force and upstream. There are many ways to have documentation:

- header files (`*.mli`),

- source files (`*.ml`),

- specific documentation provided by the upstream,

- OCamldoc generated documentation.

This documentation should be browsable by different means, from the most simple to the most complex one. At least, they should all be readable with a simple text editor. Specific and ocamldoc generated documentations should be provided in HTML format.

You can generate ocamldoc-specific documentation by using the `-dump` option of this command. By using this, you dump the intermediate representation of the document that will be generated by ocamldoc. This can be used to generate HTML documentation and manpages, by reloading this file (using `-load`).

# Appendix A. Appendix

## Locally installing OCaml programs and libraries

Locally installed files are files that are installed directly by the system administrator, in contrast to files installed through Debian packages. Installation and use of locally installed OCaml related programs is out of the scope of this document. However, in order to have it work with a standard Debian installation, a local system administrator should follow these guidelines:

- Executable files should be installed in `/usr/local/bin`.

- Shared libraries (for C bindings) should be installed in `/usr/local/lib/ocaml/stublibs/`

- Basically, every other file should be installed in `/usr/local/lib/ocaml/`. This includes in particular bytecode libraries (`*.cma`), native libraries (`*.cmxa`), bytecode object files (`*.cmo`), native object files (`*.cmx`), native plugin object files (`*.cmxs`), static libraries (`*.a`) and `META` files.

The default configuration of `ocamlfind` (the OCaml library manager recommended in Debian) first looks for a local installation of libraries and then for libraries installed by Debian packages.

### Warning

The `+` preceding any library in the `-I` of ocamlc or ocamlopt won't be expanded to the local standard library path. You need to specify the path entirely.