Your First OCaml Program

To complete this tutorial, you need to have installed OCaml. Optionally, we recommend configuring your editor.

We will work with files containing OCaml source code and compile them to produce executable binaries. However, this is not a detailed tutorial on OCaml compilation, project modularisation, or dependencies management; it only gives a glimpse at those topics. The goal is to sketch the bigger picture in order to avoid getting lost in the details. In other words, we do breadth-first learning instead of depth-first learning.

In the previous tutorial most commands were entered in UTop. In this tutorial, the majority of commands should be entered into a terminal. Code examples starting with a dollar sign $ are intended to be entered in the terminal, while lines starting with a hash sign # are intended to be entered in UTop.

Once you've completed this tutorial, you should be able to create, compile, and execute an OCaml project using Dune, OCaml's build system. You will be able to work with files, make private definitions within modules, and know how to install and use opam packages.

Working Within an opam Switch

When you installed OCaml, a global opam switch was created automatically. This tutorial can be completed while working inside this global opam switch.

When you work on several OCaml projects simultaneously, you should create more opam switches. For instructions on how to do that, see "Introduction to opam Switches".

Compiling OCaml Programs

By default, OCaml comes with two compilers: one translating sources into native binaries and another turning sources into a bytecode format. OCaml also comes with an interpreter for that bytecode format. This tutorial demonstrates how to use the native compiler to write OCaml programs.

We start by setting up a traditional “Hello World!” project using Dune. Make sure to have installed version 3.7 or later. The following creates a project named hello:

$ dune init proj hello
Entering directory '/home/'
Success: initialized project component named hello

Note: Throughout this tutorial, outputs generated Dune might vary slightly because of the Dune version installed. This tutorial shows the output for Dune 3.7. If you'd like to get the most recent version of Dune, run opam update; opam upgrade dune in a terminal.

The project is stored in a directory named hello. The tree command lists the files and directories created. It might be necessary to install tree if you don't see the following. Through Homebrew, for example, run brew install tree.

Note: If you get an error in Homebrew from this in an Apple silicon macOS, it's likely an issue with the architecture switch from Intel to ARM. Please refer to the ARM64 Fix to remedy the ARM64 error.

$ cd hello
$ tree
├── bin
│   ├── dune
│   └──
├── _build
│   └── log
├── dune-project
├── hello.opam
├── lib
│   └── dune
└── test
    ├── dune

4 directories, 8 files

OCaml source files have the .ml extension, which stands for “Meta Language.” Meta Language (ML) is an ancestor of OCaml. This is also what the “ml” stands for in “OCaml.” Here is the content of the bin/ file:

let () = print_endline "Hello, World!"

The project-wide metadata is available in the dune-project file. It contains information about the project name, dependencies, and global setup.

Each folder containing source files that need to be built must contain a dune file describing how.

This builds the project:

$ dune build
Entering directory '/home/'

This launches the executable it creates:

$ dune exec hello
Entering directory '/home/'
Hello, World!

Let's see what happens when we edit the bin/ file directly. Open it in your editor and replace the word World with your first name. Recompile the project with dune build as before, and then launch it again with dune exec hello.

Voilà! You've just written your first OCaml program.

In the rest of this tutorial, we will make more changes to this project in order to illustrate OCaml's tooling.

Why Isn't There a Main Function?

Although bin/'s name suggests it contains the application entry point into the project, it does not contain a dedicated main function, and there is no requirement that a project must contain a file with that name in order to produce an executable. A compiled OCaml file behaves as though that file were entered line by line into the toplevel. In other words, an executable OCaml file's entry point is its first line.

Double semicolons aren't needed in source files like they are in the toplevel. Statements are just processed in order from top to bottom, each triggering the side effects it may have. Definitions are added to the environment. Values resulting from nameless expressions are ignored. Side effects from all those will take place in the same order. That's OCaml main.

However, it is common practice to single out a value that triggers all the side effects and mark it as the intended main entry point. In OCaml, that's the role of let () =, which evaluates the expression on the right, including all the side effects, without creating a name.

Modules and the Standard Library, Cont'd

Let's summarise what was said about modules in the "Tour of OCaml":

  • A module is a collection of named values.
  • Identical names from distinct modules don't clash.
  • The standard library is collection of several modules.

Modules aid in organising projects; concerns can be separated into isolated modules. This is outlined in the next section. Before creating a module ourselves, we'll demonstrate using a definition from a module of the standard library. Change the content of the file bin/ to this:

let () = Printf.printf "%s!\\n" "Hello, World!"

This replaces the function print_endline with the function printf from the Printf module in the standard library. Building and executing this modified version should produce almost the same output as before. Use dune exec hello to try it for yourself.

Every File Defines a Module

Each OCaml file defines a module, once compiled. This is how separate compilation works in OCaml. Each sufficiently standalone concern should be isolated into a module. References to external modules create dependencies. Circular dependencies between modules are not allowed.

To create a module, let's create a new file named lib/ containing this:

let world = "Hello from a module"

Here is a new version of the bin/ file:

 let () = Printf.printf "%s\n"

Now execute the resulting project:

$ dune exec hello
Entering directory '/home/'
Hello from a module

The file lib/ creates the module named Hello, which in turn defines a string value named world. This definition is referred to as from the bin/ file.

Dune can launch UTop to access the modules exposed by a project interactively. Here's how:

$ dune utop

Then, inside the utop toplevel, it is possible to inspect our Hello module:

# #show Hello;;
module Hello : sig val world : string end

Now exit utop with Ctrl-D or enter #quit;; before going to the next section.

Defining Module Interfaces

UTop's #show command displays an API (in the software library sense): the list of definitions provided by a module. In OCaml, this is called a module interface. An .ml file defines a module. In a similar way, an .mli file defines a module interface. The module interface file corresponding to a module file must have the same base name, e.g., hello.mli is the module interface for module Create a lib/hello.mli file with this content:

val world : string

Observe that only the list of declarations of the module signature (which is between sig and end) has been written in the interface file lib/hello.mli. This is explained in more detail in the tutorial dedicated to modules.

Module interfaces are also used to create private definitions. A module definition is private if it is not listed in its corresponding module interface. If no module interface file exists, everything is public.

In your preferred editor, amend the lib/ file to add the mundo definition. Replace what's there with the following:

let mundo = "Hola Mundo!"
let world = mundo

Also edit the bin/ file like this:

let () = Printf.printf "%s\n" Hello.mundo

Trying to compile this fails.

$ dune build
Entering directory '/home/'
File "hello/bin/", line 1, characters 30-41:
1 | let () = Printf.printf "%s\n" Hello.mundo
Error: Unbound value Hello.mundo

This is because we haven't changed lib/hello.mli. Since it does not list mundo, it is therefore private.

Installing and Using Modules From a Package

OCaml has an active community of open-source contributors. Most projects are available using the opam package manager, which you installed in the Install OCaml tutorial. The following section shows you how to install and use a package from opam's open-source repository.

To illustrate this, let's turn our hello project into a web server using Anton Bachin's Dream web framework. First install the dream package with this command:

$ opam install dream

Next, run the Dream web server in the bin/ file by changing the code to read:

let () = Dream.(run (router [ get "/" (fun (_ : request) -> html ]))

This gives us a web server that responds with the content of to HTTP requests to the '/' path. Refer to the Dream documentation for more information.

You need to tell Dune it needs Dream to compile the project. Do this by adding the last line below. This puts dream in the library stanza of the bin/dune file.

 (public_name hello)
 (name main)
 (libraries hello dream))

Launch the server from a new terminal.

$ dune exec hello
20.07.23 13:14:07.801                       0
20.07.23 13:14:07.801                       Type Ctrl+C to stop

Note: If on macOS a key icon is displayed, like when asking for a password, you can ignore it and type Ctrl+C to get back to the command prompt.

Then test from the first terminal:

$ curl http://localhost:8080/
¡Hola Mundo!

Using the Preprocessor to Generate Code

Let's assume we'd like hello to display its output as if it was a list of strings in UTop: ["hello"; "using"; "an"; "opam"; "library"]. To do that, we need a function turning a string list into a string, adding brackets, spaces, and commas. Instead of defining it ourselves, let's generate it automatically with a package. We'll use ppx_show, which was written by Thierry Martinez. Here is how to install it:

$ opam install ppx_show

Dune needs to be explained how to use it, so edit the lib/dune file to look like this:

 (name hello)
 (preprocess (pps ppx_show))
 (libraries ppx_show.runtime))

Here is the meaning of the two new lines:

  • (libraries ppx_show.runtime) means our project is using definitions found in the ppx_show.runtime library, provided by the package ppx_show;
  • (preprocess (pps ppx_show)) means that before compilation the source needs to be transformed using the preprocessor provided by the package ppx_show.

The files lib/ and lib/hello.mli need to be edited, too:

val string_of_string_list : string list -> string
val world : string list

let string_list_pp = [%show: string list]

let string_of_string_list = Format.asprintf "@[%a@]" string_list_pp

let world = String.split_on_char ' ' "Hello using an opam library"

Let's read this from the bottom up:

  • world has the type string list. We're using String.split_on_char to turn a string into a string list by splitting the string on space characters.
  • string_of_string_list has type string list -> string. This converts a list of strings into a string, applying the expected formatting.
  • string_list_pp has type Format.formatter -> string list -> unit, which means it is a custom formatter that turns a string list into a string (this type does not appear in the signature).

Finally, you'll also need to edit bin/

let () = print_endline Hello.(string_of_string_list world)

Here is the result:

dune exec hello
Entering directory '/home/'
Done: 90% (19/21, 2 left) (jobs: 1)["Hello"; "using"; "an"; "opam"; "library"]

A Sneak-Peek at Dune as a One-Stop Shop

This section explains the purpose of the files and folders created by dune proj init which haven't been mentioned earlier.

Along the history of OCaml, several build systems have been used. As of writing this tutorial (Summer 2023), Dune is the mainstream one, which is why it is used in the tutorial. Dune automatically extracts the dependencies between the modules from the files and compiles them in a compatible order. It only needs one dune file per folder where there is something to build. The three folders created by dune proj init have the following purposes:

  • bin: executable programs
  • lib: libraries
  • test: tests

There will be a tutorial dedicated to Dune. This tutorial will present the many features of Dune, a few of which are listed here:

  • Running tests
  • Generating documentation
  • Producing packaging metadata (here in hello.opam)
  • Creating arbitrary files using all-purpose rules

The _build folder is where Dune stores all the files it generates. It can be deleted at any time, but subsequent builds will recreate it.

Minimum Setup

In this last section, let's create a bare minimum project, highlighting what's really needed for Dune to work. We begin by creating a fresh project folder:

$ cd ..
$ mkdir minimo
$ cd minimo

At the very least, Dune only needs two files: dune-project and one dune file. Here is how to write them with as little text as possible:


(lang dune 3.6)


(executable (name minimo))

let () = print_endline "My name is Minimo"

That's all! This is sufficient for Dune to build and execute the file.

$ dune exec ./minimo.exe
Entering directory '/home/'
My name is Minimo

Note: minimo.exe is not a file name. This is how Dune is told to compile the file using OCaml's native compiler instead of the bytecode compiler. As a fun fact, note that an empty file is valid OCaml syntax. You can use that to reduce minimo even more; of course, it will not display anything, but it will be a valid project!


This tutorial is the last of the "Getting Started" series. Moving forward, you have enough to pick and choose among the other tutorials to follow your own learning path.

Recommended next tutorials:

  1. Values and Functions
  2. Basic Data Types and Pattern Matching
  3. If Statements, Loops, and Recursions
  4. Lists

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