How to Flatpak OCaml applications

16 minute read

How to Flatpak an OCaml application

Let’s take a couple OCaml applications to illustrate Flatpak distributing and packaging leveraging the Flatpak SDK Extension for OCaml:

a hello world CLI application
a simple GTK program from the Introduction to Gtk OCaml tutorial

The SDK is only needed during the building phase.

Table of contents

How to Flatpak an OCaml application

Creating a hello world CLI application

Create a hello world example.ml:

let () = print_endline "hello world"

You can compile your code with the OCaml compiler ocamlopt (no need to do so at this point):

$ ocamlopt -o example example.ml
$ ./example
hello world

Flatpak manifest

Some things to note in the following manifest:

The SDK extension pointing to the Flatpak SDK Extension for OCaml ↵
The build options setting the PATH ↵
Build some pre-requisistes using dune ↵
Installing the application to /app/bin ↵

app-id: flatpak.ocaml.example
runtime: org.freedesktop.Sdk
runtime-version: '22.08'
sdk: org.freedesktop.Sdk
sdk-extensions:
  - org.freedesktop.Sdk.Extension.ocaml
command: example

build-options:
  append-path: /usr/lib/sdk/ocaml/bin

modules:
  - name: camlp-streams
    buildsystem: simple
    sources:
      - type: git
        url: https://github.com/ocaml/camlp-streams
        branch: trunk
    build-commands:
      - dune build

  - name: example
    buildsystem: simple
    sources:
      - type: file
        path: example.ml
    build-commands:
      - ocamlopt -o example example.ml
      - install -Dm755 example -t /app/bin/

Install and run the application

Build and install the Flatpak package locally using flatpak-builder:

$ flatpak-builder --force-clean build-dir flatpak.ocaml.example.yaml
$ flatpak-builder --user --install --force-clean build-dir flatpak.ocaml.example.yaml

Run the application:

$ flatpak run flatpak.ocaml.example
hello world

The application has an installed size of 369.2 kB:

$ flatpak info flatpak.ocaml.example
          ID: flatpak.ocaml.example
         Ref: app/flatpak.ocaml.example/x86_64/master
        Arch: x86_64
      Branch: master
      Origin: example-origin
  Collection:
Installation: user
   Installed: 368.3 kB
     Runtime: org.freedesktop.Sdk/x86_64/22.08
         Sdk: org.freedesktop.Sdk/x86_64/22.08
      Commit: 02386f983e23b6d991651e86c1b1a55531fd4fa6504c730c938bc06033c19f40
     Subject: Export flatpak.ocaml.example
        Date: 2023-10-09 12:34:32 +0000

Creating a simple GTK program

Let’s use the simple lablgtk program from the Introduction to Gtk OCaml tutorial simple.ml:

open GMain
open GdkKeysyms

let locale = GtkMain.Main.init ()

let main () =
  let window = GWindow.window ~width:320 ~height:240
                              ~title:"Simple lablgtk program" () in
  let vbox = GPack.vbox ~packing:window#add () in
  window#connect#destroy ~callback:Main.quit;

  (* Menu bar *)
  let menubar = GMenu.menu_bar ~packing:vbox#pack () in
  let factory = new GMenu.factory menubar in
  let accel_group = factory#accel_group in
  let file_menu = factory#add_submenu "File" in

  (* File menu *)
  let factory = new GMenu.factory file_menu ~accel_group in
  factory#add_item "Quit" ~key:_Q ~callback: Main.quit;

  (* Button *)
  let button = GButton.button ~label:"Push me!"
                              ~packing:vbox#add () in
  button#connect#clicked ~callback: (fun () -> prerr_endline "Ouch!");

  (* Display the windows and enter Gtk+ main loop *)
  window#add_accel_group accel_group;
  window#show ();
  Main.main ()

let () = main ()

You can install the dependencies and compile your code (no need to do so at this point):

$ opam install lablgtk
$ ocamlfind ocamlopt -package lablgtk2 -linkpkg simple.ml -o simple
$ ./simple
Ouch!

This is what you should see when you run it:

Preparations

Import GTK2 library

We need the GTK2 library, let’s grab it from Flathub shared-modules repo:

$ git submodule add https://github.com/flathub/shared-modules.git

Wrapper script

LD_LIBRARY_PATH is empty by default when running Flatpak applications (flatpak/flatpak-builder#474) so we will need a wrapper script simple.sh around the application:

export LD_LIBRARY_PATH=/app/lib
exec /app/bin/simple

Building from sources

lablgtk can be built from its sources.

Flatpak manifest

Some things to note in the following manifest:

The SDK extension pointing to the Flatpak SDK Extension for OCaml ↵
Permissions for the Flatpak app to access X11 (--share=ipc and --socket=fallback-x11) ↵
The build options setting the PATH and some OCaml related environment variables ↵
For some reason debuginfo was breaking ocamlfind, so it has been disabled ↵
We are importing GTK2 from the shared modules repository ↵
Installing some lablgtk pre-requisites: ocamlfind and camlp-streams (required only with OCaml>=5) ↵
lablgtk is build from its sources instead of using opam ↵
Installing the application to /app/bin and lablgtk library to /app/lib ↵
Removal of the build files at the post-install phase to limit the size of the resulting Flatpak application ↵
Wrapper setup ↵

app-id: flatpak.ocaml.lablgtk.build
runtime: org.freedesktop.Sdk
runtime-version: '22.08'
sdk: org.freedesktop.Sdk
sdk-extensions:
  - org.freedesktop.Sdk.Extension.ocaml
command: simple.sh
finish-args:
  - --share=ipc
  - --socket=fallback-x11
build-options:
  append-path: /usr/lib/sdk/ocaml/bin:/app/share/runtime/ocaml
  env:
    CAML_LD_LIBRARY_PATH: /app/share/runtime/ocaml/lib:/usr/lib/sdk/ocaml/lib/ocaml:/usr/lib/sdk/ocaml/lib/ocaml/stublibs
    OCAMLFIND_CONF: /app/share/runtime/ocaml/findlib.conf
  no-debuginfo: true

modules:
  - shared-modules/gtk2/gtk2.json

  - name: ocamlfind
    buildsystem: simple
    sources:
      - type: git
        url: https://github.com/ocaml/ocamlfind
        branch: master
    build-commands:
      - mkdir -p ${FLATPAK_DEST}/share/runtime/ocaml
      - ./configure -bindir ${FLATPAK_DEST}/share/runtime/ocaml -config ${FLATPAK_DEST}/share/runtime/ocaml -no-topfind -sitelib ${FLATPAK_DEST}/share/runtime/ocaml/lib
      - make all
      - make install
    post-install:
      - ocamlfind printconf

  - name: camlp-streams
    buildsystem: simple
    sources:
      - type: git
        url: https://github.com/ocaml/camlp-streams
        branch: trunk
    build-commands:
      - dune build
      - dune install --prefix /app/share/runtime/ocaml
    post-install:
      - ocamlfind query camlp-streams

  - name: lablgtk
    buildsystem: simple
    sources:
      - type: archive
        url: https://github.com/garrigue/lablgtk/archive/refs/tags/2.18.13.tar.gz
        sha256: 7b9e680452458fd351cf8622230d62c3078db528446384268cd0dc37be82143c
    build-commands:
      - ./configure
      - make world
      - make old-install DESTDIR=/app/share/runtime/ocaml

  - name: simple
    buildsystem: simple
    sources:
      - type: file
        path: simple.ml
    build-commands:
      - mv /app/share/runtime/ocaml/usr/lib/sdk/ocaml/lib/ocaml/lablgtk2 /app/share/runtime/ocaml/lib
      - ocamlfind ocamlopt -package lablgtk2 -linkpkg simple.ml -o simple
      - install -Dm755 simple -t /app/bin/
      - mv /app/share/runtime/ocaml/usr/lib/sdk/ocaml/lib/ocaml/stublibs/dlllablgtk2.so /app/lib
    post-install:
      - rm -rf /app/share/runtime/ocaml

  - name: simple-wrapper
    buildsystem: simple
    sources:
      - type: file
        path: simple.sh
    build-commands:
      - cp simple.sh /app/bin

Install and run the application

Build and install the Flatpak package locally using flatpak-builder:

$ flatpak-builder --force-clean build-dir flatpak.ocaml.lablgtk.build.yaml
$ flatpak-builder --user --install --force-clean build-dir flatpak.ocaml.lablgtk.build.yaml

Run the application:

$ flatpak run flatpak.ocaml.lablgtk.build
Ouch!

This is what you should see when you run it:

The application has an installed size of 34.8 MB:

$ flatpak info flatpak.ocaml.lablgtk.build
          ID: flatpak.ocaml.lablgtk.build
         Ref: app/flatpak.ocaml.lablgtk.build/x86_64/master
        Arch: x86_64
      Branch: master
      Origin: build-origin
  Collection:
Installation: user
   Installed: 34.8 MB
     Runtime: org.freedesktop.Sdk/x86_64/22.08
         Sdk: org.freedesktop.Sdk/x86_64/22.08
      Commit: 6acda68f932e9728d942e976b6d6b94cab2929eb4c91b087f58792028f18d536
     Subject: Export flatpak.ocaml.lablgtk.build
        Date: 2023-12-19 09:29:13 +0000

Building with opam

Source files

We can leverage the OCaml Package Manager (opam) to install lablgtk into the Flatpak. For that, let’s leverage flatpak-builder-tools, a collection of various scripts to aid in using flatpak-builder that among other things contains a helper tool to automatically generate flatpak-builder source files from a .json generated by opam tree:

$ git clone https://github.com/flatpak/flatpak-builder-tools
$ cd flatpak-builder-tools/opam
$ opam tree --json=lablgtk.json lablgtk
$ ./flatpak-opam-generator.py lablgtk.json > sources/lablgtk.json

The output sources file sources/lablgtk.json should look something like this:

[
  {
    "type": "file",
    "url": "https://github.com/garrigue/lablgtk/archive/2.18.13.tar.gz",
    "name": "lablgtk.2.18.13",
    "md5": "d0a326b99475216cc22232e72c89415f",
    "dest": "cache/md5/d0",
    "dest-filename": "d0a326b99475216cc22232e72c89415f"
  },
  {
    "type": "file",
    "url": "https://github.com/ocaml/camlp-streams/archive/v5.0.1.tar.gz",
    "name": "camlp-streams.5.0.1",
    "md5": "afc874b25f7a1f13e8f5cfc1182b51a7",
    "dest": "cache/md5/af",
    "dest-filename": "afc874b25f7a1f13e8f5cfc1182b51a7"
  },
  {
    "type": "file",
    "url": "https://github.com/ocaml/dune/releases/download/3.10.0/dune-3.10.0.tbz",
    "name": "dune.3.10.0",
    "sha256": "9ff03384a98a8df79852cc674f0b4738ba8aec17029b6e2eeb514f895e710355",
    "dest": "cache/sha256/9f",
    "dest-filename": "9ff03384a98a8df79852cc674f0b4738ba8aec17029b6e2eeb514f895e710355"
  },
  {
    "type": "file",
    "url": "http://download.camlcity.org/download/findlib-1.9.6.tar.gz",
    "name": "ocamlfind.1.9.6",
    "md5": "96c6ee50a32cca9ca277321262dbec57",
    "dest": "cache/md5/96",
    "dest-filename": "96c6ee50a32cca9ca277321262dbec57"
  }
]

Generate Flatpak manifest code

Let’s use the helper tool to generate the corresponding Flatpak manifest code:

$ ./flatpak-opam-generator.py --generate lablgtk lablgtk.json
...
# Manifest code generated by flatpak-opam-generator
- name: lablgtk
  buildsystem: simple
  sources:
    - sources/lablgtk.json
    - type: git
      branch: master
      url: https://github.com/ocaml/opam-repository
  build-commands:
    - ls -A --color=never | grep -Ev "cache|packages|repo" | xargs rm -rf
    - opam admin filter -y lablgtk.2.18.13 camlp-streams.5.0.1 dune.3.10.0 ocamlfind.1.9.6 
    - opam admin cache
    - opam repo add lablgtk .
    - opam install -y lablgtk.2.18.13 camlp-streams.5.0.1 dune.3.10.0 ocamlfind.1.9.6 
    - opam repo remove --all lablgtk
  post-install:
    - opam info --field name,all-installed-versions lablgtk

Flatpak manifest

Some things to note in the following manifest:

The SDK extension pointing to the Flatpak SDK Extension for OCaml ↵
Permissions for the Flatpak app to access X11 (--share=ipc and --socket=fallback-x11) ↵
The build options setting the PATH and some OCaml related environment variables ↵
We are importing GTK2 from the shared modules repository ↵
Creation of a new switch for installing lablgtk ↵
The manifest code generated by flatpak-opam-generator for lablgtk ↵
Installing the application to /app/bin and lablgtk library to /app/lib ↵
Removal of the ocaml switch at the post-install phase to limit the size of the resulting Flatpak application ↵
Wrapper setup ↵

app-id: flatpak.ocaml.lablgtk.switch
runtime: org.freedesktop.Sdk
runtime-version: '22.08'
sdk: org.freedesktop.Sdk
sdk-extensions:
  - org.freedesktop.Sdk.Extension.ocaml
command: simple.sh
finish-args:
  - --share=ipc
  - --socket=fallback-x11
build-options:
  append-path: /app/share/runtime/ocaml/bin:/app/share/runtime/ocaml/switch/_opam/bin:/usr/lib/sdk/ocaml/bin
  env:
    CAML_LD_LIBRARY_PATH: /app/share/runtime/ocaml/switch/_opam/lib/stublibs:/app/share/runtime/ocaml/switch/_opam/lib/ocaml/stublibs:/app/share/runtime/ocaml/switch/_opam/lib/ocaml
    OCAML_TOPLEVEL_PATH: /app/share/runtime/ocaml/switch/_opam/lib/toplevel
    OPAMROOT: /app/share/runtime/ocaml
    OPAMSWITCH: /app/share/runtime/ocaml/switch
    OPAM_SWITCH_PREFIX: /app/share/runtime/ocaml/switch/_opam

modules:
  - shared-modules/gtk2/gtk2.json

  - name: switch
    buildsystem: simple
    sources:
      - type: git
        branch: master
        url: https://github.com/ocaml/opam-repository
      - type: archive
        url: https://github.com/ocaml/ocaml/archive/refs/tags/5.1.0.tar.gz
        dest: switch
        sha256: 43a3ac7aab7f8880f2bb6221317be55319b356e517622fdc28359fe943e6a450
    build-commands:
      - mkdir -p ${FLATPAK_DEST}/share/runtime/ocaml
      - mv switch ${FLATPAK_DEST}/share/runtime/ocaml
      - opam init -y --bare --disable-sandboxing .
      - opam switch create -y --deps-only ${FLATPAK_DEST}/share/runtime/ocaml/switch
      - opam pin -y ${FLATPAK_DEST}/share/runtime/ocaml/switch
    post-install:
      - opam switch list

  # Manifest code generated by flatpak-opam-generator
  - name: lablgtk
    buildsystem: simple
    sources:
      - sources/lablgtk.json
      - type: git
        branch: master
        url: https://github.com/ocaml/opam-repository
    build-commands:
      - ls -A --color=never | grep -Ev "cache|packages|repo" | xargs rm -rf
      - opam admin filter -y lablgtk.2.18.13 camlp-streams.5.0.1 dune.3.10.0 ocamlfind.1.9.6
      - opam admin cache
      - opam repo add lablgtk .
      - opam install -y lablgtk.2.18.13 camlp-streams.5.0.1 dune.3.10.0 ocamlfind.1.9.6
      - opam repo remove --all lablgtk
    post-install:
      - opam info --field name,all-installed-versions lablgtk

  - name: simple
    buildsystem: simple
    sources:
      - type: file
        path: simple.ml
    build-commands:
      - ocamlfind ocamlopt -package lablgtk2 -linkpkg simple.ml -o simple
      - install -Dm755 simple -t /app/bin/
      - cp /app/share/runtime/ocaml/switch/_opam/lib/stublibs/dlllablgtk2.so /app/lib
    post-install:
      - rm -rf /app/share/runtime/ocaml

  - name: simple-wrapper
    buildsystem: simple
    sources:
      - type: file
        path: simple.sh
    build-commands:
      - cp simple.sh /app/bin

Install and run the application

Build and install the Flatpak package locally using flatpak-builder:

$ flatpak-builder --force-clean build-dir flatpak.ocaml.lablgtk.switch.yaml
$ flatpak-builder --user --install --force-clean build-dir flatpak.ocaml.lablgtk.switch.yaml

Run the application:

$ flatpak run flatpak.ocaml.lablgtk.switch
Ouch!

This is what you should see when you run it:

The application has an installed size of 11.3 MB:

$ flatpak info flatpak.ocaml.lablgtk.switch
          ID: flatpak.ocaml.lablgtk.switch
         Ref: app/flatpak.ocaml.lablgtk.switch/x86_64/master
        Arch: x86_64
      Branch: master
      Origin: lablgtk-origin
  Collection:
Installation: user
   Installed: 11.3 MB
     Runtime: org.freedesktop.Sdk/x86_64/22.08
         Sdk: org.freedesktop.Sdk/x86_64/22.08
      Commit: b3159cf05f2d89d7caaae3bc7bc6fd6a2ab70d5195374b4222a3cbedf8f635b0
      Parent: 499a0892394a54c93976d422aa533563b6a1b1212fd9036fbc4fcbdd023f87dd
     Subject: Export flatpak.ocaml.lablgtk.switch
        Date: 2023-10-16 12:01:01 +0000

Publish the application

Info on how to later publish the application on Flathub can be found here.

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Jose Castillo Lema

How to Flatpak OCaml applications

How to Flatpak an OCaml application

Creating a hello world CLI application

Flatpak manifest

Install and run the application

Creating a simple GTK program

Preparations

Import GTK2 library

Wrapper script

Building from sources

Flatpak manifest

Install and run the application

Building with opam

Source files

Generate Flatpak manifest code

Flatpak manifest

Install and run the application

Publish the application

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