Installing OCaml#

If all you need is a way to follow along with the code examples in this book, you don’t actually have to install OCaml! The code on each page is executable in your browser, as described earlier in this Preface.

If you want to take it a step further but aren’t ready to spend time installing OCaml yourself, we provide a virtual machine with OCaml pre-installed inside a Linux OS.

But if you want to do OCaml development on your own, you’ll need to install it on your machine. There’s no universally “right” way to do that. The instructions below are for Cornell’s CS 3110 course, which has goals and needs beyond just OCaml. Nonetheless, you might find them to be useful even if you’re not a student in the course.

Here’s what we’re going to install:

  • A Unix development environment

  • OPAM, the OCaml Package Manager

  • An OPAM switch with the OCaml compiler and some packages

  • The Visual Studio Code editor, with OCaml support

The installation process will rely heavily on the terminal, or text interface to your computer. If you’re not too familiar with it, you might want to brush up with a terminal tutorial.


If this is your first time installing development software, it’s worth pointing out that “close doesn’t count”: trying to proceed past an error usually just leads to worse errors, and sadness. That’s because we’re installing a kind of tower of software, with each level of the tower building on the previous. If you’re not building on a solid foundation, the whole thing might collapse. The good news is that if you do get an error, you’re probably not alone. A quick google search will often turn up solutions that others have discovered. Of course, do think critically about suggestions made by random strangers on the internet.

Let’s get started!

Unix Development Environment#


First, upgrade your OS. If you’ve been intending to make any major OS upgrades, do them now. Otherwise when you do get around to upgrading, you might have to repeat some or all of this installation process. Better to get it out of the way beforehand.


If you’re already running Linux, you’re done with this step. Proceed to Install OPAM, below.


Beneath the surface, macOS is already a Unix-based OS. But you’re going to need some developer tools and a Unix package manager. There are two to pick from: Homebrew and MacPorts. From the perspective of this textbook and CS 3110, it doesn’t matter which you choose:

  • If you’re already accustomed to one, feel free to keep using it. Make sure to run its update command before continuing with these instructions.

  • Otherwise, pick one and follow the installation instructions on its website. The installation process for Homebrew is typically easier and faster, which might nudge you in that direction. If you do choose MacPorts, make sure to follow all the detailed instructions on its page, including XCode and an X11 server. Do not install both Homebrew and MacPorts; they aren’t meant to co-exist. If you change your mind later, make sure to uninstall one before installing the other.

After you’ve finished installing/updating either Homebrew or MacPorts, proceed to Install OPAM, below.


Unix development in Windows is made possible by the Windows Subsystem for Linux (WSL). If you have a recent version of Windows (build 20262, released November 2020, or newer), WSL is easy to install. If you don’t have that recent of a version, try running Windows Update to get it.


If you get an error about the “virtual machine” while installing WSL, you might need to enable virtualization in your machine’s BIOS. The instructions for that are dependent on the manufacturer of your machine. Try googling “enable virtualization [manufacturer] [model]”, substituting for the manufacturer and model of your machine. This Red Hat Linux page might also help.

With a recent version of Windows, and assuming you’ve never installed WSL before, here’s all you have to do:

  • Open Windows PowerShell as Administrator. To do that, click Start, type PowerShell, and it should come up as the best match. Click “Run as Administrator”, and click Yes to allow changes.

  • Run wsl --install. (Or, if you have already installed WSL but not Ubuntu before, then instead run wsl --install -d Ubuntu.) When the Ubuntu download is completed, it will likely ask you to reboot. Do so. The installation will automatically resume after the reboot.

  • You will be prompted to create a Unix username and password. You can use any username and password you wish. It has no bearing on your Windows username and password (though you are free to re-use those). Do not put a space in your username. Do not forget your password. You will need it in the future.


Do not proceed with these instructions if you were not prompted to create a Unix username and password. Something has gone wrong. Perhaps your Ubuntu installation did not complete correctly. Try uninstalling Ubuntu and reinstalling it through the Windows Start menu.

Now skip to the “Ubuntu setup” paragraph below.

Without a recent version of Windows, you will need to follow Microsoft’s manual installation instructions. WSL2 is preferred over WSL1 by OCaml (and WSL2 offers performance and functionality improvements), so install WSL2 if you can.

Ubuntu setup. These rest of these instructions assume that you installed Ubuntu (22.04) as the Linux distribution. That is the default distribution in WSL. In principle other distributions should work, but might require different commands from this point forward.

Open the Ubuntu app. (It might already be open if you just finished installing WSL.) You will be at the Bash prompt, which looks something like this:



If that prompt instead looks like root@...#, something is wrong. Did you create a Unix username and password for Ubuntu in the earlier step above? If so, the username in this prompt should be the username you chose back then, not root. Do not proceed with these instructions if your prompt looks like root@...#. Perhaps you could uninstall Ubuntu and reinstall it.

In the current version of the Windows terminal, Ctrl+Shift+C will copy and Ctrl+Shift+V will paste into the terminal. Note that you have to include Shift as part of that keystroke. In older versions of the terminal, you might need to find an option in the terminal settings to enable those keyboard shortcuts.

Run the following command to update the APT package manager, which is what helps to install Unix packages:

sudo apt update

You will be prompted for the Unix password you chose. The prefix sudo means to run the command as the administrator, aka “super user”. In other words, do this command as super user, hence, “sudo”.


Running commands with sudo is potentially dangerous and should not be done lightly. Do not get into the habit of putting sudo in front of commands, and do not randomly try it without reason.

Now run this command to upgrade all the APT software packages:

sudo apt upgrade -y

Then install some useful packages that we will need:

sudo apt install -y zip unzip build-essential

File Systems. WSL has its own filesystem that is distinct from the Windows file system, though there are ways to access each from the other.

  • When you launch Ubuntu and get the $ prompt, you are in the WSL file system. Your home directory there is named ~, which is a built-in alias for /home/your_ubuntu_user_name. You can run explorer.exe . (note the dot at the end of that) to open your Ubuntu home directory in Windows explorer.

  • From Ubuntu, you can access your Windows home directory at the path /mnt/c/Users/your_windows_user_name/.

  • From Windows Explorer, you can access your Ubuntu home directory under the Linux icon in the left-hand list (near “This PC” and “Network”), then navigating to Ubuntu → homeyour_ubuntu_user_name. Or you can go there directly by typing into the Windows Explorer path bar: \\wsl$\Ubuntu\home\your_ubuntu_user_name.

Practice accessing your Ubuntu and Windows home directories now, and make sure you can recognize which you are in. For advanced information, see Microsoft’s guide to Windows and Linux file systems.

We recommend storing your OCaml development work in your Ubuntu home directory, not your Windows home directory. By implication, Microsoft also recommends that in the guide just linked.

Install OPAM#

Linux. Follow the instructions for your distribution.

Mac. If you’re using Homebrew, run this command:

brew install opam

If you’re using MacPorts, run this command:

sudo port install opam

Windows. Run this command from Ubuntu:

sudo apt install opam

Initialize OPAM#


Do not put sudo in front of any opam commands. That would break your OCaml installation.

Linux, Mac, and WSL2. Run:

opam init --bare -a -y

(Don’t worry if you get a note about making sure .profile is “well-sourced” in .bashrc. You don’t need to do anything about that.)

WSL1. Hopefully you are running WSL2, not WSL1. But on WSL1, run:

opam init --bare -a -y --disable-sandboxing

It is necessary to disable sandboxing because of an issue involving OPAM and WSL1.

Create an OPAM Switch#

A switch is a named installation of OCaml with a particular compiler version and set of packages. You can have many switches and, well, switch between them —whence the name. Create a switch for this semester’s CS 3110 by running this command:

opam switch create cs3110-2024sp ocaml-base-compiler.5.1.1


If that command fails saying that the 5.1.1 compiler can’t be found, you probably installed OPAM sometime back in the past and now need to update it. Do so with opam update.

You might be prompted to run the next command. It won’t matter whether you do or not, because of the very next step we’re going to do (i.e., logging out).

eval $(opam env)

Now we need to make sure your OCaml environment was configured correctly. Logout from your OS (or just reboot). Then re-open your terminal and run this command:

opam switch list

You should get output like this:

#  switch         compiler                    description
→  cs3110-2024sp  ocaml-base-compiler.5.1.1   cs3110-2024sp

There might be other lines if you happen to have done OCaml development before. Here’s what to check for:

  • You must not get a warning that “The environment is not in sync with the current switch. You should run eval $(opam env)”. If either of the two issues below also occur, you need to resolve this issue first.

  • There must be a right arrow in the first column next to the cs3110-2024sp switch.

  • That switch must have the right name and the right compiler version, 5.1.1.


If you do get that warning about opam env, something is wrong. Your shell is probably not running the OPAM configuration commands that opam init was meant to install. You could try opam init --reinit to see whether that fixes it. Also, make sure you really did log out of your OS (or reboot).

Continue by installing the OPAM packages we need:

opam install -y utop odoc ounit2 qcheck bisect_ppx menhir ocaml-lsp-server ocamlformat

Make sure to grab that whole line above when you copy it. You will get some output about editor configuration. Unless you intend to use Emacs or Vim for OCaml development, you can safely ignore that output. We’re going to use VS Code as the editor in these instructions, so let’s ignore it.

You should now be able to launch utop, the OCaml Universal Toplevel.



You should see a message “Welcome to utop version … (using OCaml version 5.1.1)!” If the OCaml version is incorrect, then you probably have an environment issue. See the tip above about the opam env command.

Enter 3110 followed by two semicolons. Press return. The # is the utop prompt; you do not type it yourself.

# 3110;;
- : int = 3110

Stop to appreciate how lovely 3110 is. Then quit utop. Note that this time you must enter the extra # before the quit directive.

# #quit;;

A faster way to quit is to type Control+D.

Double-Check OCaml#

If you’re having any trouble with your installation, follow these double-check instructions. Some of them repeat the tips we provided above, but we’ve put them all here in one place to help diagnose any issues.

First, reboot your computer. We need a clean slate for this double-check procedure.

Second, run utop, and make sure it works. If it does not, here are some common issues:

  • Are you in the right Unix prompt? On Mac, make sure you are in whatever Unix shell is the default for your Terminal: don’t run bash or zsh or anything else manually to change the shell. On Windows, make sure you are in the Ubuntu app, not PowerShell or Cmd.

  • Is the OPAM environment set? If utop isn’t a recognized command, run eval $(opam env) then try running utop again. If utop now works, your login shell is somehow not running the right commands to automatically activate the OPAM environment; you shouldn’t have to manually activate the environment with the eval command. Probably something went wrong earlier when you ran the opam init command. To fix it, follow the “redo” instructions below.

  • Is your switch listed? Run opam switch list and make sure a switch named cs3110-2024sp is listed, that it has the 5.1.1 compiler, and that it is the active switch (which is indicated with an arrow beside it). If that switch is present but not active, run opam switch cs3110-2024sp then see whether utop works. If that switch is not present, follow the “redo” instructions below.

Redo Instructions: Remove the OPAM directory by running rm -r ~/.opam. Then go back to the OPAM initialization step in the instructions way above, and proceed forward. Be extra careful to use the exact OPAM commands given above; sometimes mistakes occur when parts of them are omitted. Finally, double-check again: reboot and see whether utop still works.


You want to get to the point where utop immediately works after a reboot, without having to type any additional commands.

Visual Studio Code#

Visual Studio Code is a great choice as a code editor for OCaml. (Though if you are already a power user of Emacs or Vim those are great, too.)

First, download and install Visual Studio Code (henceforth, VS Code). Launch VS Code. Open the extensions pane, either by going to View → Extensions, or by clicking on the icon for it in the column of icons on the left — it looks like four little squares, the top-right of which is separated from the other three.

At various points in the following instructions you will be asked to “open the Command Palette.” To do that, go to View → Command Palette. There is also an operating system specific keyboard shortcut, which you will see to the right of the words “Command Palette” in that View menu.

Second, follow one of these steps if you are on Windows or Mac:

  • Windows only: Install the “WSL” extension.

  • Mac only: Open the Command Palette and type “shell command” to find the “Shell Command: Install ‘code’ command in PATH” command. Run it.

Third, regardless of your OS, close any open terminals — or just logout or reboot — to let the new path settings take effect, so that you will later be able to launch VS Code from the terminal.

Fourth, on Windows only, open the Command Palette and run the command “WSL: Connect to WSL”. (If you on Mac, skip ahead to the next step.) The first time you do this, it will install some additional software. After that completes, you will see a “WSL: Ubuntu” indicator in the bottom-left of the VS Code window. Make sure that you see “WSL: Ubuntu” there before proceeding with the next step below. If you see just an icon that looks like >< then click it, and choose “Connect to WSL” from the Command Palette that opens.

Fifth, again open the VS Code extensions pane. Search for and install the “OCaml Platform” extension from OCaml Labs. Be careful to install the extension with exactly that name.


The extensions named simply “OCaml” or “OCaml and Reason IDE” are not the right ones. They are both old and no longer maintained by their developers.

Double-Check VS Code#

Let’s make sure VS Code’s OCaml support is working.

  • Reboot your computer again. (Yeah, that really shouldn’t be necessary. But it will detect so many potential mistakes now that it’s worth the effort.)

  • Open a fresh new Unix shell. Windows: remember that’s the Ubuntu, not PowerShell or Cmd. Mac: remember that you shouldn’t be manually switching to a different shell by typing zsh or bash.

  • Navigate to a directory of your choice, preferably a subdirectory of your home directory. For example, you might create a directory for your 3110 work inside your home directory:

    mkdir ~/3110
    cd ~/3110

    In that directory open VS Code by running:

    code .

    Go to File → New File. Save the file with the name VS Code should give it an orange camel icon.

  • Type the following OCaml code then press Return/Enter:

    let x : int = 3110

    As you type, VS Code should colorize the syntax, suggest some completions, and add a little annotation above the line of code. Try changing the int you typed to string. A squiggle should appear under 3110. Hover over it to see the error message. Go to View → Problems to see it there, too. Add double quotes around the integer to make it a string, and the problem will go away.

If you don’t observe those behaviors, something is wrong with your installation. Here’s how to proceed:

  • Make sure that, from the same Unix prompt as which you launched VS Code, you can successfully complete the double-check instructions for your OPAM switch: Can you run utop? Is the right switch active? If not, that’s the problem you need to solve first. Then return to the VS Code issue. It might be fixed now.

  • If you’re on WSL and VS Code does add syntax highlighting but does not add squiggles as described above, and/or you get an error about “Sandbox initialization failed”, then double-check that you see a “WSL” indicator in the bottom left of the VS Code window. If you do not, make sure you installed the “WSL” extension as described above, and that you are launching VS Code from Ubuntu rather than PowerShell or from the Windows GUI. If you do, make sure that the “OCaml Platform” extension is installed.

If you’re still stuck with an issue, try uninstalling VS Code, rebooting, and re-doing all the installation instructions above from scratch. Pay close attention to any warnings or errors.


While troubleshooting any VS Code issues, do not hardcode any paths in the VS Code settings file, despite any advice you might find online. That is a band-aid, not a cure of whatever the underlying problem really is. More than likely, the real problem is an OCaml environment issue that you can investigate with the OCaml double-check instructions above.

VS Code Settings#

We recommend tweaking a few editor settings. Open the user settings JSON file by (i) going to View → Command Palette, (ii) typing “user settings json”, and (iii) selecting Open User Settings (JSON). Copy and paste these settings into the window:

    "editor.tabSize": 2,
    "editor.rulers": [ 80 ],
    "editor.formatOnSave": true

Save the file and close the tab.

Using VS Code Collaboratively#

VS Code’s Live Share extension makes it easy and fun to collaborate on code with other humans. You can edit code together like collaborating inside a Google Doc. It even supports a shared voice channel, so there’s no need to spin up a separate Zoom call. To install and use Live Share, follow Microsoft’s tutorial.

If you are a Cornell student, log in with your Microsoft account, not GitHub. Enter your Cornell NetID email, e.g., That will take you to Cornell’s login site. Use the password associated with your NetID.