Beyond Basics


These are early days for Diskuv OCaml. We frequently update the software for bug fixes. Stay informed about new features, bug fixes and security updates on Twitter:

If you are a student, talk with your instructor before applying a major update. They will likely want you to stay on your existing version until the course is complete.

Learn OCaml - A first project


This section is almost verbatim from A first project - Learn OCaml. Since you already installed Diskuv OCaml, almost everything else on that page is already done for you!

Let’s begin the simplest project with Dune and OCaml. We create a new directory and ask dune to initialise a new project:

  1. Open the Command Prompt (press the Windows key ⊞ and R, and then type “cmd” and ENTER).

  2. Type:

    C:\Users\you>if not exist "%USERPROFILE%\DiskuvOCamlProjects" mkdir %USERPROFILE%\DiskuvOCamlProjects
    C:\Users\you>cd %USERPROFILE%\DiskuvOCamlProjects
    C:\Users\you\DiskuvOCamlProjects>mkdir helloworld
    C:\Users\you\DiskuvOCamlProjects>cd helloworld/
    C:\Users\you\DiskuvOCamlProjects\helloworld>dune init exe helloworld
    Success: initialized executable component named helloworld

Building our program is as simple as typing dune build:

C:\Users\you\DiskuvOCamlProjects\helloworld>dune build
Info: Creating file dune-project with this contents:
| (lang dune 2.9)

When we change our program, we type dune build again to make a new executable. We can run the executable with dune exec (it’s called helloworld.exe even when we’re not using Windows):

C:\Users\you\DiskuvOCamlProjects\helloworld>dune exec ./helloworld.exe
Hello, World!

Let’s look at the contents of our new directory. Dune has added the file, which is our OCaml program. It has also added our dune file, which tells dune how to build the program, and a _build subdirectory, which is Dune’s working space.

Volume in drive C has no label.
Volume Serial Number is A00E-4711

Directory of C:\Users\you\DiskuvOCamlProjects\helloworld

10/14/2021  02:47 PM    <DIR>          .
10/14/2021  02:46 PM    <DIR>          ..
10/14/2021  02:46 PM                32 dune
10/14/2021  02:47 PM                17 dune-project
10/14/2021  02:46 PM                40
10/14/2021  02:47 PM    <DIR>          _build
            3 File(s)             89 bytes
            3 Dir(s)  116,767,272,960 bytes free

The helloworld.exe executable is stored inside the _build/default subdirectory, so it’s easier to run with dune exec. To ship the executable, we can just copy it from inside _build/default to somewhere else.

Here is the contents of the automatically-generated dune file. When we want to add components to your project, such as third-party libraries, we can edit this file:

  (name helloworld))


Editing files

Now is a good time to talk about editing a file. Editing is how you change the contents of a file. You probably already know how to use Microsoft Word to edit Word documents: just start up Microsoft Word and then use the Word menu to “Open” a Word document. But Microsoft Word only works with Word documents that end with .doc or .docx! On Windows you can use the program Notepad (press the Windows key ⊞, and then type “notepad”) to edit “text” documents.

All programming languages, including OCaml, use text documents. These are also called text files and source files. (We’ll use the term “source file” from now on.) Source files are not Word documents. In fact, you will mess up your source file if you use Microsoft Word to edit it. You have to use a text editor. Other than that difference, editing should still be familiar to you:

  • Open your editor (example: open Notepad)

  • Use the editor menu to “Open” a source file, or make a “New” source file

  • Type in your code

  • Save the source file with an appropriate name and ending.

Click on the animated image below (use your mouse!) to see how to open a file:

Opening a source file with Notepad on Windows

Click on the picture below to see how you change the Save As type box while you are saving a file:

Editing a source file with Notepad on Windows

We should always save with All file types (*.*), not “Text documents (*.txt)”, because Notepad and other simple editors will add “.txt” to the ending of the filename (also known as the file extension) without telling you!

It bears repeating:

The name, extension and location of the source file is critical! As you go through this documentation make sure you Save the text file exactly where it tells you with the exact name and extension it tells you!

Continuous building

Eventually you may get tired of running dune build all the time.

Try running the following:

C:\Users\you\DiskuvOCamlProjects>cd %USERPROFILE%\DiskuvOCamlProjects\helloworld
C:\Users\you\DiskuvOCamlProjects\helloworld>with-dkml sh -c 'while true; do dune build --watch; sleep 1; done'

and then edit your to say “This is so fast!” instead of “Hello, World!”.

Then open a new Command Prompt (press the Windows key ⊞ and R, and then type “cmd” and ENTER) to run:

C:\Users\you>cd %USERPROFILE%\DiskuvOCamlProjects\helloworld
This is so fast!

Anytime you edit your source code, it will recompile what has changed.

Installing packages

Opam is the OCaml package manager. It gives you access to thousands of third-party packages that you can use in your own projects.

Each project is a local directory with source code and its own set of OCaml packages. Opam will manage the OCaml packages in a local subdirectory named _opam. The technical term for _opam is a local switch. In this section we will create a project called my-first-switch.

Let’s start by finding which switches are available:

  1. Open the Command Prompt (press the Windows key ⊞ and R, and then type “cmd” and ENTER).

  2. Type:

    C:\Users\you>opam switch
    #  switch                                                      compiler
    →  playground
    [WARNING] The environment is not in sync with the current switch.
              You should run: for /f "tokens=*" %i in ('opam env') do @%i

You just found that you have at least two (2) switches: the directory ...\0\dkml and the playground. We will avoid the dkml reserved switch, and for now we’ll ignore the playground switch.

Let’s create our own my-first-switch switch. All we need to do is create a directory and run opam dkml init inside our new (or existing) directory:


Press y (yes) whenever you are prompted!

C:\Users\you>if not exist "%USERPROFILE%\DiskuvOCamlProjects" mkdir %USERPROFILE%\DiskuvOCamlProjects
C:\Users\you>cd %USERPROFILE%\DiskuvOCamlProjects

C:\Users\you\DiskuvOCamlProjects>mkdir my-first-switch
C:\Users\you\DiskuvOCamlProjects>cd my-first-switch
C:\Users\you\DiskuvOCamlProjects\my-first-switch>opam dkml init

C:\Users\you\DiskuvOCamlProjects\my-first-switch>opam switch
#  switch                                                                           compiler
→  C:\Users\you\DiskuvOCamlProjects\my-first-switch                                    ocaml-system.4.12.1

[NOTE] Current switch has been selected based on the current directory.
      The current global system switch is C:\Users\you\AppData\Local\Programs\DiskuvOCaml\0\dkml.
[WARNING] The environment is not in sync with the current switch.
         You should run: for /f "tokens=*" %i in ('opam env') do @%i

Notice how the switch was created with opam dkml init, and also notice how opam switch tells you in its [NOTE] that it knows which switch should be used based on the current directory.

If we want our my-first-switch to be remembered regardless what the directory currently is, we can follow the [WARNING] and add the option --set-switch.

Let’s do that now so we learn how to do it:

C:\Users\you\DiskuvOCamlProjects\my-first-switch>for /f "tokens=*" %i in ('opam env --set-switch') do @%i

C:\Users\you\DiskuvOCamlProjects\my-first-switch>opam switch
#  switch                                                                           compiler
→  C:\Users\you\DiskuvOCamlProjects\my-first-switch                                    ocaml-system.4.12.1

[NOTE] Current switch is set locally through the OPAMSWITCH variable.
      The current global system switch is C:\Users\you\AppData\Local\Programs\DiskuvOCaml\0\dkml.

Great! You are now ready to install some packages for the my-first-switch project. Let’s see what packages are installed with opam list and available with opam list -a:

C:\Users\you\DiskuvOCamlProjects\my-first-switch>opam list
# Packages matching: installed
# Name        # Installed # Synopsis
base-bigarray base        pinned to version base
base-threads  base        pinned to version base
base-unix     base        pinned to version base
conf-withdkml 1           Virtual package relying on with-dkml
ocaml         4.12.1      pinned to version 4.12.1
ocaml-config  3           pinned to version 3
ocaml-system  4.12.1      The OCaml compiler (system version, from outside of opam)

C:\Users\you\DiskuvOCamlProjects\my-first-switch>opam list -a
# Packages matching: available
# Name                                          # Installed                # Synopsis
0install                                        --                         pinned to version 2.17
0install-gtk                                    --                         pinned to version 2.17
0install-solver                                 --                         pinned to version 2.17
ANSITerminal                                    --                         pinned to version 0.8.2
zstandard                                       --                         pinned to version v0.14.0
zstd                                            --                         pinned to version 0.2
zxcvbn                                          --                         pinned to version 2.4+1

There are a lot! You will probably find it easier to use the OCaml Packages browser in your web browser.

Since this section is following the Learn OCaml tutorials, let’s install the Graphics library which gives you the Graphics module. In Opam the package names are always lowercase, so the module Graphics will be available in the graphics Opam package:

C:\Users\you\DiskuvOCamlProjects\my-first-switch>opam install graphics


Press y when asked if you want to continue, then sit back while it compiles and installs the graphics package.

Learn OCaml - A First Hour with OCaml

You are almost ready to follow the tutorial A First Hour with OCaml - Learn OCaml.

Before you begin that tutorial, you will need to know a few things:

  • Make sure you are using the my-first-switch switch. Go back to the previous section if you don’t remember how to select the my-first-switch switch.

  • You don’t need to use rlwrap. Instead use with-dkml utop in your my-first-switch switch; it is much easier to work with! Do an extra opam install utop when it asks you to install the graphics package and the ocamlfind packages.


When you want to use OCaml tools from your project, use with-dkml to reliably get those tools to work on Windows. We already do this on your behalf for opam and dune, but some tools like ocamlc, ocamlopt and utop need help to find the Microsoft compiler or UNIX binaries or the right Windows paths. So don’t guess; just get in the habit of using with-dkml!

So with-dkml ocamlopt -o helloworld rather than ocamlopt -o helloworld And with-dkml utop rather than utop. Et cetera.

Now go follow A First Hour with OCaml - Learn OCaml!

Integrated Development Environment (IDE)

Installing Visual Studio Code

Installing an IDE like Visual Studio Code will let you navigate the code in your SDK Projects, see the source code with syntax highlighting (color), get auto-complete to help you write your own code, and inspect the types within your code.

If you haven’t already, download and install Visual Studio Code from its website. For Windows 64-bit you will want to choose the “User Installer” “64-bit” button underneath the Windows button, unless you have Administrator access to your PC (then “System Installer” is usually the right choice):


Windows Development Environment Virtual Machine users (you will know if you are one of them) already have Visual Studio Code bundled in the virtual machine.

Installing the OCaml Plugin

Once you have Visual Studio Code, you will want the OCaml plugin.

In the File > Preferences > Extensions view (or press Ctrl Shift X), type ocamllabs.ocaml-platform in the search box to find and install:

#### OCaml Platform
* Official OCaml language extension for VSCode

Now you need to quit ALL Visual Studio Code windows (if any), and then restart Visual Studio Code.

After that, in the File > Preferences > Settings view (or press Ctrl ,), select User > Extensions > OCaml Platform.

Then uncheck OCaml: Use OCaml Env.


Do not forget to uncheck OCaml: Use OCaml Env

This setting is a legacy option that may disappear in future versions of the OCaml Plugin. For now, if you don’t uncheck the option, you will not see your Opam switches in Visual Studio Code.