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The trivial OCaml package builder
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topkg — The trivial OCaml package builder

topkg is a trivial package builder for distributing OCaml software. It provides a mechanism to describe an OPAM $PKG.install file and make corresponding invocations in your build system.

topkg brings the following advantages:

  1. It frees you from implementing an install procedure in your build system: this task is delegated to OPAM or to the opam-installer tool.

  2. It doesn't reclaim control over your build system. It will only invoke it once with a list of targets determined from a package description and a build environment explicitely specified on the command line.

  3. It's a simple approach implemented by a short OCaml script.

topkg has been designed with ocamlbuild in mind but it should be usable with any other build system as long as it is able to understand the targets topkg generates.

topkg is a single BSD3 licensed OCaml script builder that you add to your repo.

Basic setup

Your repository and distribution should have the following files, you can arrange that differently but it is better to have a single set of conventions across packages.

  • pkg/META, a Findlib META file.
  • pkg/, the package builder written with See Package description.
  • pkg/, support for writing
  • opam, your package metadata, its dependencies and the instructions to build it. Having it at the root of your repository allows OPAM to use the file for build instructions and dependencies if the package is pinned.

The build instructions of your package are simply an invocation of pkg/ with a specification of the build environment as a boolean key map on the command line. The invocation is very explicit, all arguments are mandatory (this will be invoked by machines and it helps debugging builds).

Here is how you should write that invocation in the build: field of the opam file:

build: [
  "ocaml" "pkg/" "native=%{ocaml-native}%"
                         ... ]

This will execute your build system with a set of targets and generate in the root directory of your distribution an OPAM $PKG.install file.

After OPAM executed the build: commands of a package pkg, if there is a file pkg.install at the root of the distribution, it uses it to install the package. It will also use it to uninstall the package, no need to specify anything in the remove: field of the opam file. Note that in each case no ocamlfind invocation is needed, just install your META file in lib.

If you need to support another package system you can invoke pkg/ as above and then manage installation and uninstallation at a given $DESTDIR using the generated $PKG.install file and the opam-installer tool distributed with OPAM.

Package description

An OPAM $PKG.install file is a description of a standard UNIX install. It has fields for each of the standard directories (lib, bin, man, etc, etc.). Each of these fields lists the files to install in the corresponding directory (or subdirectories). See the .install file specification in the OPAM developer manual for more information.

In topkg, the package build description file pkg/ is simply a manual specification of every file you want to put in each field of the OPAM $PKG.install file by calling install field functions Pkg.{lib,bin,doc,...}. Here is an example of a pkg/ description for a single module library that also installs a command line tool called jsontrip:

#!/usr/bin/env ocaml 
#directory "pkg"
#use ""

let () =
  Pkg.describe "jsonm" ~builder:`OCamlbuild [
    Pkg.lib "pkg/META";
    Pkg.lib "src/jsonm.mli";
    Pkg.lib "src/jsonm.cmti";
    Pkg.lib "src/jsonm.cmi";
    Pkg.lib "src/jsonm.cma";
    Pkg.lib "src/jsonm.cmx";
    Pkg.lib "src/jsonm.a";
    Pkg.lib "src/jsonm.cmxa";
    Pkg.lib "src/jsonm.cmxs";
    Pkg.bin "test/jsontrip.byte";
    Pkg.bin "test/jsontrip.native";
    Pkg.doc ""; 
    Pkg.doc ""; ]

This says that we are declaring a package named jsonm which means that the generated install file will be jsonm.install. It also says that we are using ocamlbuild as a build tool and that we want all the files specified with Pkg.lib to be installed the lib directory, those with Pkg.bin in bin, those with Pkg.doc in doc etc.

Now there are two things that are unsatisfactory with the above declaration.

  1. The set of files to generate for a library is usually always the same and it's painful to write them down explicitely. This is solved by extension sets.
  2. For the binaries, we usually don't want to install both byte and native code. We want to install one tool without the byte or native suffix, and the native one if available. This is solved by auto binaries.

Using these features the above declaration can be reduced to:

#!/usr/bin/env ocaml 
#directory "pkg"
#use ""

let () = 
  Pkg.describe "jsonm" ~builder:`OCamlbuild [
    Pkg.lib "pkg/META";
    Pkg.lib ~exts:Exts.module_library "src/jsonm";
    Pkg.bin ~auto:true "test/jsontrip";
    Pkg.doc ""; 
    Pkg.doc ""; ]

Optional builds and installs are handled by declaring boolean keys specified on the command line (see Environment) and using the cond optional argument of install field functions (see Conditions). The following example compiles the vgr_pdf library only if both Uutf and Otfm are present and vgr_htmlc only if js_of_ocaml is present:

#!/usr/bin/env ocaml 
#directory "pkg";;
#use "";;

let uutf = Env.bool "uutf"
let otfm = Env.bool "otfm"
let jsoo = Env.bool "jsoo"
let vgr_pdf = uutf && otfm
let () = 
  Pkg.describe "vg" ~builder:`OCamlbuild [
    Pkg.lib "pkg/META";
    Pkg.lib ~exts:Exts.module_library "src/vg";
    Pkg.lib ~exts:Exts.module_library "src/vgr_svg";
    Pkg.lib ~cond:vgr_pdf ~exts:Exts.module_library "src/vgr_pdf";
    Pkg.bin ~cond:vgr_pdf ~auto:true "test/vecho";
    Pkg.lib ~cond:jsoo ~exts:Exts.module_library "src/vgr_htmlc";
    Pkg.doc "";
    Pkg.doc "";
    Pkg.doc "test/min_htmlc.html";
    Pkg.doc "test/";
    Pkg.doc "test/";
    Pkg.doc "test/"; ]

Extension sets

The install field functions have an optional exts argument. If present these extensions are appended to the path given to the function. The module Exts defines a few predefined extension sets. For example a module library implemented in src/ can be declared by:

Pkg.lib ~exts:Exts.library_module "src/mylib"

which is, effectively, a shortcut for:

Pkg.lib "src/mylib.mli";
Pkg.lib "src/mylib.cmti";
Pkg.lib "src/mylib.cmi";
Pkg.lib "src/mylib.cmx";
Pkg.lib "src/mylib.cma";
Pkg.lib "src/mylib.a";
Pkg.lib "src/mylib.cmxa";
Pkg.lib "src/mylib.cmxs";

Auto binaries

For generating an installing native binaries if native code compilation is available and byte code binaries if not you can use the auto optional argument of Pkg.bin and Pkg.sbin. Using it with true you can simply specify the binary name prefix. It will use the base name as the name of the tool and ask for either a .native or .byte target depending if native compilation is available or not.

Pkg.bin ~auto:true "src/mybinary" 


Conditional installation is handled through the optional argument cond of install field functions. If cond is false it's neither built nor installed. For example for a library that depends on the presence of another:

let otherlib = Env.bool "otherlib" 
Pkg.lib ~cond:otherlib ~exts:Exts.library_module "src/mylib"

Conditions related to native code and native code dynamic linking availability happen automatically:

  • In Pkg.lib paths ending with .cmxs are dropped if Env.native_dynlink is false and paths ending with .a, .cmx, .cmxa and .cmxs are dropped if Env.native is false.

  • In Pkg.{bin,sbin} path ending with .native are dropped if Env.native is false.


New boolean keys are added to the environment by calling Env.bool key. To output a sample environment you can invoke the build script with --help:

ocaml pkg/ --help

Renaming and installing in subdirectories

By default install field functions use the basename of the path given to the function as the install name. If you need to rename the build artefact or install to a subdirectory you can use the dst optional argument of install field functions. For example for a library that needs to be installed in a subdir subdirectory of lib use:

Pkg.lib ~exts:Exts.library_module ~dst:"subdir/mylib" "src/mylib"

Handling cmt and cmti files

Since the OCaml tools generate .cmt and .cmti files only as a side effect they are treated specially: they are not built. For ocamlbuild you should add this line to your _tags file:

<**/*.{ml,mli}> : bin_annot

this will build them as a side effect of other build invocations. In the $PKG.install file generated by topkg the cmt and cmti files are prefixed by a ? so that if they are not built (pre OCaml 4.01 for ocamlbuild) the install doesn't fail.

Verifying the build invocation and .install generation

For verifying the build invocation and the generated .install file you can invoke with explain as the first argument:

ocaml pkg/ explain native=true native-dynlink=true ...

This will not invoke the build system but output on stdout diagnostic information aswell as the build invocation and the generated $PKG.install file.

In contrast to the build invocation it is allowed to invoke explain with an empty or partial environment.

ocaml pkg/ explain

In that case it will use true for each unspecified environment boolean key.

Map from descriptions to targets and build artefacts

Given an invocation Pkg.{lib,bin,...} "$PATH", the system generates a target $PATH for your build system and expects to find the build artefact in $BUILD/$PATH where $BUILD is the build directory of the build tool (e.g. _build for ocamlbuild).


If you are using git bin/topkg-update is a script that automates the process of updating to the latest version. For installing the script:

opam pin add topkg

Then simply go to your repo and type:


This will make sure you have no staged changes and that is not modified. If that is the case it will update the script and commit the change on the current branch.

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