Table des Contenus

Opam 103: Bootstrapping a New OCaml Project with opam

Auteurs: Raja Boujbel, Dario Pinto, Artemiy Rozovyk
Date: 2025-04-19
Catégorie: Trainings
Tags: tutorial, on-boarding, opam, beginner, opam10x, dev, new project


A young camel is ready to leave on its first journey through the desert, he is well-prepared and has the perfect tools at his disposal!

A young camel is ready to leave on its first journey through the desert, he is well-prepared and has the perfect tools at his disposal!

Curious about the origins of opam?

Check out this short history on its evolution as the de facto package manager and environment manager for OCaml.

Welcome back to the opam deep-dives series!

Finally - you've asked for it since our very first opam deep-dive: it's time to explore the developer side of the opam experience.

So far, we have focused on user-facing scenarios to provide a gentle introduction. Now, we are shifting gears into project creation and development workflows.

Thank you for your patience - the wait was worth it! Today, we will guide you through starting a new OCaml project with a full opam-integrated workflow. 🚀

This guide is especially geared toward newer OCaml developers who want to master opam when setting up and managing a project. 😇

If you haven't yet, we recommend starting with Opam 101: The First steps to get comfortable with installation and usage basics and Opam 102: Pinning Packages, which already dives quite deep into package pinning, one of the first keys to tailoring your workflow and environment to your exact needs.

Also, check out the tags of each article to get an idea of the entry level required for the smoothest read possible!

Prerequisites & Context

Our goal across this post and the next one is to guide you through the full life cycle of an OCaml project - from creating a directory on your machine to publishing your package to the official opam-repository.

We will walk through each step of the journey, highlighting not just how to do things, but also why they matter in a pragmatic, real-world opam workflow. You’ll see how to:

  • Create and manage local switches
  • Select and install packages
  • Prepare your project for distribution

This post assumes you've read Opam 101: The First Step - especially the section on switches.

Nevertheless, here's a quick TL;DR for those of you who would rather get started:

What is an opam switch?

An opam switch is a development environment in the OCaml world. Opam provides you with a command-line interface for you to customise, and maintain a safe and stable environment. It's defined by all the possible combinations and valid operations between a specific version of the OCaml compiler, and any set of versioned packages.

Functionally, it is a set of environment variables that are user-updated and point to the different locations of installed versions of packages, binaries and other utilities either in a ~/.opam directory for global switches or in the current _opam directory for local ones.

👉 More in the official docs.

In this tutorial, we'll use local switches, which are especially well-suited for project-based workflows like the one we are building today. Furthermore, know that this article uses opam 2.1.5!

Ready? Let's dive in!

Setting up the environment

Before publishing an OCaml package, you have to develop it - and that means setting up your environment.

This encompasses everything from creating the working directory of your new project, to setting up a custom, local switch for it.

We will consider that you have created a new directory for your project and have since moved into it in order to progress further in the setup process.

Something like:

$ mkdir helloer
$ cd helloer

Here are the things that opam will help you accomplish at this stage of the development process:

  • Setting up a new switch (i.e, environment creation);
  • Explore packages (libraries / tooling) available in the OCaml ecosystem (i.e, technical exploration);
  • Selection and installation of OCaml software inside a switch (i.e, environment setup and tailoring);

Creating a new local switch

A switch is the isolated environment in which opam will operate and assist you in taking all the necessary steps towards an optimal workflow.

It defines a specific OCaml compiler version and a set of compatible packages, allowing you to safely build and manage your project.O

So let's first create a local switch in our helloer directory.

$ opam switch create .

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml" {>= "4.05.0"}]

We let opam select the default switch invariant when creating a new switch which is OCaml compiler version >= 4.05.0. You can define any set of switch invariants that you wish.

In the call above, the . character indicates that we are asking opam to create a switch inside the current directory, a local one. This differs from a global switch, which lives in your ~/.opam/ folder.

What's a “switch invariant”?

The idea of switch invariants is quite simple, they are the parameters of the automatic solving of package dependency trees. More specifically, this switch invariant defines the OCaml version your environment relies on. Invariants are immutable. opam will never change invariants without notifying you first and will always consider the switch invariant constraint when building the graph of available/compatible packages for your current switch, or for any other switch-altering operation for that matter.

So, back to our example:

$ opam switch create .

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml" {>= "4.05.0"}]

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
∗ installed base-bigarray.base
∗ installed base-threads.base
∗ installed base-unix.base
∗ installed ocaml-system.4.14.1
∗ installed ocaml-config.2
∗ installed ocaml.4.14.1
Done.

We can see that opam selected ocaml-system.4.14.1 as opposed to ocaml-base-compiler.4.14.1 as the OCaml compiler to install in your current local switch along with its dependencies. What's the difference?

  • ocaml-system is a system-bound compiler, typically one already installed on your system (e.g. via apt, brew, etc.), outside of your opam installation
  • ocaml-base-compiler would be a new compiler installed within your opam installation, one that opam would have permission over.

If you recall this section of Opam 101, you should know that creating a switch can be a fairly time-consuming task depending on whether or not the compiler version you have queried from opam is already installed somewhere on your machine. Therefore, every time you ask opam to install a version of the compiler, it will first scour your installation for a locally available version of that compiler to save you the time necessary for downloading, compiling and installing a brand new one. This is the reason why opam has selected an ocaml-system.4.14.1 compiler instead of installing a brand new ocaml-base-compiler.4.14.1.

A quick look at our current directory will show that an _opam directory can now be found.

$ ls
_opam

And opam switch will list the currently available switches:

$ opam switch
#  switch                        compiler             description
→  /home/ocamler/dev/helloer     ocaml.4.14.1         /home/ocamler/dev/helloer
   my-switch                     ocaml-system.4.14.1  my-switch

[NOTE] Current switch has been selected based on the current directory.
       The current global system switch is my-switch.

opam indicates that it has selected the local switch as the currently active one with the → character and then tells us that the currently active global switch outside of this directory is still a previously created one called my-switch.

Local switches were explained in detail in this section of Opam 101. We learned in it that opam automatically selects the local switch as the currently active one as soon as we move inside the directory in which it was created.

opam list will show us what packages are currently installed in our switch:

$ opam list
# Packages matching: installed
# Name        # Installed # Synopsis
base-bigarray base
base-threads  base
base-unix     base
ocaml         4.14.1      The OCaml compiler (virtual package)
ocaml-config  2           OCaml Switch Configuration
ocaml-system  4.14.1      The OCaml compiler (system version, from outside of opam)

Which is simply the list of dependencies of the OCaml compiler (since it is the only thing we have installed in our switch so far).

To confirm which OCaml binary is being used:

$ ocaml -vnum
4.14.1
$ which ocaml
/usr/bin/ocaml

This confirms you are using the system compiler (not an opam-installed one) as the path does not point to either the global ~/.opam nor /home/ocamler/dev/helloer/_opam directories.

In the case of a global switch, the following would be true:

$ ocaml -vnum
4.14.1
$ which ocaml
/home/ocamler/.config/opam/my-global-switch/bin/ocaml

To verify that everything works, and since we have a compiler, let's compile a program:

$ cat helloer.ml
let () =
  print_endline "Hello OCamlers!!"
$ ocamlc -o hello helloer.ml
$ ./hello
Hello OCamlers!!

Nice! You've just compiled your first OCaml program in a fresh local switch. 🎉

Of course, this isn't very exciting yet. Let’s spice things up by adding external libraries to our helloer program.

Getting started

helloer is a toy project - with it we can play around with the tools at hand and learn a thing or two about them.

However, if you're curious about the source code of the project, you can check it out right here.

Do keep in mind that this is rather remote from what you will encounter in the wild. Both the code base and the structure of the repository are made intentionally barebones. This will allow us to smoothly introduce what we believe are the most common features a beginner OCaml dev should get familiar with.

One of the most valuable skills in OCaml development is selecting the right libraries for the job. Over time, you will build a mental toolbox of go-to libraries for all kinds of engineering goals.

You can explore the package ecosystem in several ways.

Choose a build system

The first tool your OCaml project needs is a build system.

We are using Dune in this tutorial because it is the most widely used build system in the OCaml ecosystem today. It is well integrated with opam, supports both libraries and executables, and offers fast incremental builds with dependency tracking. Most packages in the opam-repository use dune as their build system.

That said, OCaml also has other build tools, either still used in specific contexts or maintained for compatibility - for example, ocpbuild, ocamlbuild, topkg or just make and ocamlc combined. In some cases, these alternative tools can feel more lightweight or straightforward, especially for very small projects or when fine-grained control is needed. Choosing the right tool often depends on your project’s scope and your familiarity with the ecosystem. For most new development, however, dune remains the most common and actively maintained choice.

If you happen to look for guidance or any kind of support for your OCaml developments, keep in mind that the Discuss OCaml Community Forum is the best place to engage with your peers!

We believe that introducing you to the current most common practices of the OCaml Community is a solid way to get you going.

You can expect us to cover in detail how to properly begin a project with dune in upcoming blogposts.

A call to opam install will change the state of our current switch by installing this build system:

$ opam install dune

You can now get to writing your first dune-project file. You may either refer to the documentation or get some inspiration from the dune-project file we provide at the end of this article!

Adding Command-Line Tools

Now onto finding a library to help us build a neat command-line interface for helloer.

We know that the OCaml Standard Library ships an Arg module which aims at allowing the parsing of command-line arguments. However, it's quite limited and verbose. Instead, we'll use opam search for something more ergonomic:

Using opam, a simple opam search with your keywords might help you greatly:

$ opam search "command line interface"
# Packages matching: match(*command line interface*)
# Name                  # Installed # Synopsis
bap-byteweight-frontend --          BAP Toolkit for training and controlling Byteweight algorithm
clim                    --          Command Line Interface Maker
cmdliner                --          Declarative definition of command line interfaces for OCaml
dream-cli               --          Command Line Interface for Dream applications
hg_lib                  --          A library that wraps the Mercurial command line interface
inquire                 --          Create beautiful interactive command line interface in OCaml
kappa-binaries          --          Command line interfaces of the Kappa tool suite
minicli                 --          Minimalist library for command line parsing
ocal                    --          An improved Unix `cal` utility
ocamline                --          Command line interface for user input
wcs                     --          Command line interface for Watson Conversation Service

cmdliner is one of our favourite libraries for that matter so let's use it in helloer.

Update the switch:

$ opam install cmdliner

Now is the time to implement your first command-line with cmdliner! You can check how we did it for helloer here in the helloer.ml file. You may also refer directly to the cmdliner library documentation!

$ ./helloer.exe
Hello OCamlers!!           
$ ./helloer.exe --gentle
Welcome my dear OCamlers.

Adding a Test Library

Finally, before we get to coding our little project, we should consider adding a test library to our project. This will make writing tests much easier, less time-consuming and less tedious.

Again, calling opam search with one or several keywords will yield many packages that pertain to testing OCaml binaries. Our selection for today will be alcotest, a well-known and wide-spread option for conducting tests on OCaml binaries.

$ opam search "test"
# Packages matching: match(*test*)
# Name                              # Installed # Synopsis
afl-persistent                      --          Use afl-fuzz in persistent mode
ahrocksdb                           --          A binding to RocksDB
alcotest                            --          Alcotest is a lightweight and colourful test framework
alcotest-async                      --          Async-based helpers for Alcotest
alcotest-js                         --          Virtual package containing optional JavaScript dependencies for Alcotest
alcotest-lwt                        --          Lwt-based helpers for Alcotest
alcotest-mirage                     --          Mirage implementation for Alcotest
[...]

Update the switch:

$ opam install alcotest

You can check out how we used it in helloer here, also refer to the documentation of the library for further exploration.

Run tour tests:

$ dune runtest
Testing `Tests'.                 
This run has ID `N39NJ5ZE'.

  [OK]          messages          0   normal.
  [OK]          messages          1   gentle.

Full test results in `~/ocamler/dev/helloer/_build/default/_build/_tests/Tests'.
Test Successful in 0.000s. 2 tests run.

Now that we have found and used our new tools, we need only to create a package for helloer!

This means writing an opam file. Next section will cover what information go into it.

Furthermore, the distribution of your newly developed package to the rest of the OCaml Community on the opam-repository will be covered in the next opam blog post!

Your first opam file

So how exactly does one write an opam file?

A minimal functional opam file

You will find below a minimal opam file for the helloer project.

This file is minimal in the sense that it is complete enough for you to work with your package on your local environment. However, there remains a few fields that we will explain in a moment and that are necessary for you to distribute your code.

$ cat helloer.opam
opam-version: "2.0"
depends: [
  "cmdliner"
  "ocaml"
  "alcotest" {with-test}
]
build: [
 [ "dune" "build" "-p" name ]
 [ "dune" "runtest" ] {with-test}
]
install: [ "dune" "install" ]

For now, there is enough information for opam to install and use your OCaml project locally.

What are these fields for?

opam-version: "2.0":

  • Specifies that this opam file uses syntax compatible with opam 2.0 or later.
  • Required at the top of every opam file.

depends: [...]:

  • Lists the packages that your project depends on. It lists all necessary information to build your project, and to help opam suggest what other packages have to be installed prior to it.
    • You could optionally set lower or upper bound for specific version range (e.g., cmdliner {>= "1.0.0"}), but omitting that is fine for a minimal file like this one.

build: [...]:

  • Tells opam how to build your project.
  • ["dune" "build" "-p" name]: Builds the package. -p name means "build the part of the project with the same name as the opam package".

    name here should match the actual opam package name; it’s often replaced automatically by opam internally.

  • ["dune" "runtest"] {with-test}: Runs the test suite, only if --with-test option is passed (e.g. during CI or development).
  • You can fill up with any command here, it will be launched by opam in a sandboxed environment.

install: [...]:

  • This tells opam how to install the built binaries and libraries into the opam environment.
  • Dune will install any libraries, executables, and other files you've marked as public_name.

That is it for the essential fields of an opam file. Now onto the metadata fields which are required for you to later distribute your package through the Official OCaml opam-repository.

An opam file supports about thirty-ish valid fields to specify a package, again, you can look them all up here.

A real-world opam file with opam lint

What fields are mandatory for a proper opam package?

A good question, and the answer is simple since opam features a linting command with opam lint.

This means that running this command on our small opam file will yield the following message to help us make sense of what more is required to make our newly developped package distributable:

$ opam lint .
/home/ocamler/dev/helloer/helloer.opam: Errors.
    error 23: Missing field 'maintainer'
  warning 25: Missing field 'authors'
  warning 35: Missing field 'homepage'
  warning 36: Missing field 'bug-reports'
    error 57: Synopsis must not be empty
  warning 68: Missing field 'license'

As you can see, some of these missing fields are considered errors, and others are considered mere warnings. Each also come with a designated error code. You can find all warning and error codes by either running the opam lint --help command in your terminal, or going to the corresponding opam man page on the interwebs.

Let's break each of them down and see how these linting errors can be fixed.

  • Error 23: Missing field maintainer
    What: Contact email for the package maintainer.
    Example:

    maintainer: ["Your Name <hell@er.com>"]

  • Error 57: Synopsis must not be empty
    What: One-line description of your project.
    Example:

    synopsis: "A simple and polite greeter in OCaml"

  • Warning 25: Missing field authors
    What: List of project authors.
    Example:

    authors: ["Your Name <hell@er.com>"]

  • Warning 35: Missing field homepage
    What: URL to your project's website or repository.
    Example:

    homepage: "https://github.com/OCamlPro/opam_bp_examples"

  • Warning 36: Missing field bug-reports
    What: URL for reporting issues.
    Example:

    bug-reports: "https://github.com/OCamlPro/opam_bp_examples/issues"

  • Warning 68: Missing field license
    What: License under which your project is distributed.
    Example:

    license: "ISC"

Now that you have linted an opam file manually, you will be very happy to learn that dune can actually automatically generate that file for you. Say for instance that you prefer the syntax of the dune-project file, you can let dune handle it for you!

Here's what a complete dune-project file would look like for our little helloer project.

(lang dune 3.15)

(name helloer)

(license ISC)

(authors "Your Name <hell@er.com>")

(maintainers "You Name <hell@er.com>")

(source
 (uri "https://github.com/OCamlPro/opam_bp_examples"))

(homepage "https://github.com/OCamlPro/opam_bp_examples")

(bug_reports "https://github.com/RadioPotin/ocamuse/issues")

(documentation "N/A")

(generate_opam_files true)

(package
  (name helloer)
  (synopsis "A simple and polite greeter in OCaml")
  (description
    "This is an example package for article 'opam 103' on OCamlPro's blog")
  (tags (greeter opam 103 tutorial beginner))
  (depends
    ocaml
    cmdliner
    alcotest
  )
)

Conclusion and what's next

At this point, our project is in great shape. We've seen how to setup your local switch, integrate third-party libraries, run tests, and write a minimal but functional opam file. With that foundation, you’re ready to build and manage your OCaml projects locally with confidence.

But writing code for yourself is only half the journey. The next step is sharing it with the world. In our next post, we’ll dive into how to publish your package to the official OCaml opam-repository — covering everything from the structure of an opam package submission, to working with the community through pull requests, versioning, opam-CI, and more.

Thank you for trodding along the dunes with this little OCaml caravan of ours 🚶🚶🐫🐫🐫 — and until our next Oasis, happy hacking!


Au sujet d'OCamlPro :

OCamlPro développe des applications à haute valeur ajoutée depuis plus de 10 ans, en utilisant les langages les plus avancés, tels que OCaml, Rust, et WebAssembly (Wasm) visant aussi bien rapidité de développement que robustesse, et en ciblant les domaines les plus exigeants (méthodes formelles, cybersécurité, systèmes distribués/blockchain, conception de DSLs). Fort de plus de 20 ingénieurs R&D, avec une expertise unique sur les langages de programmation, aussi bien théorique (plus de 80% de nos ingénieurs ont une thèse en informatique) que pratique (participation active au développement de plusieurs compilateurs open-source, prototypage de la blockchain Tezos, etc.), diversifiée (OCaml, Rust, Cobol, Python, Scilab, C/C++, etc.) et appliquée à de multiples domaines. Nous dispensons également des [formations sur mesure certifiées Qualiopi sur OCaml, Rust, et les méthodes formelles] (https://training.ocamlpro.com/) Pour nous contacter : contact@ocamlpro.com.