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How to Flatpak OCaml applications

Posted Updated
By Jose Castillo Lema
10 min read
How to Flatpak OCaml applications

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

The SDK is only needed during the building phase.

Table of contents

Creating a hello world CLI application

Create a hello world example.ml:

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let () = print_endline "hello world"

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

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

Flatpak manifest

Some things to note in the following manifest:

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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:

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$ 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:

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$ flatpak run flatpak.ocaml.example
hello world

The application has an installed size of 369.2 kB:

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$ 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:

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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):

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$ 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:

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$ 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:

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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 ↵
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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:

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$ 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:

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$ 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:

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$ 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:

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$ 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:

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[
  {
    "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:

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$ ./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 ↵
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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:

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$ 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:

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$ 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:

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$ 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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