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ocaml-on-windows
OCaml on Windows
Read about the state of OCaml on Windows and our roadmap to improve Windows support.
getting-started
2021-05-27 21:07:30 +0000

OCaml on Windows

We recommend installing opam 2.2 for new users; it comes with full Windows support.

There are a couple scenarios when other OCaml installers may be useful:

  • when you want to develop applications and either are unfamiliar with Unix or care more about stability and ease-of-use than the latest compiler, you can use Diskuv OCaml
  • when you want only to run, not develop, applications, you can use Docker or WSL2

The recommendations are based on the availability table below:

  • Tier 1 is fully supported with the latest compilers
  • Tier 2 is supported but maintained when possible
  • Tier 3 is user supported
╭──────────────────────────────────────────────────────────────────────────╮
│ Tier   │ OCaml Version and Environment   │ Support                       │
│ ------ │ ------------------------------- │ ----------------------------- │
│ Tier 1 │ OCaml 5 with Opam 2.2           │ Full support.                 │
│ Tier 2 │ 4.12.1 with Diskuv OCaml        │ Supported on select versions. │
│ Tier 3 │ 5 with WSL2                     │ User supported.               │
│ Tier 3 │ 5 with Docker                   | User supported.               │
╰──────────────────────────────────────────────────────────────────────────╯

Installation Environments

Diskuv OCaml

Diskuv OCaml ("DKML") is a distribution of OCaml that supports software development in pure OCaml. The distribution is unique in its:

  • full compatibility with OCaml standards like opam, Dune, and OCamlfind.
  • focus on "native" development (desktop software, mobile apps, and embedded software) through support for the standard native compilers, like Visual Studio and Xcode.
  • ease-of-use through simplified installers and simple productivity commands. High school, college, and university students should be able to use it
  • security through reproducibility, versioning, and from-source builds.

To install DKML, briefly review the following:

  • You need to stay at your computer and press "Yes" for any Windows security popups. After the DKML installer finishes installing two programs (Visual Studio Installer and Git for Windows), you can leave your computer for the remaining one (1) hour.

  • First time installations may get a notification printed in red. If you see it, reboot your computer and then restart your installation so that Visual Studio Installer can complete. The notification looks like:

    - FATAL [118acf2a]. The machine needs rebooting.
- ...
- >>  The machine needs rebooting. <<<
-         ...
-         FATAL [5f927a8b].
-         A transient failure occurred.
-         ...
-         >>  A transient failure occurred. <<<

  • You may be asked to accept a certificate from Open Source Developer, Gerardo Grignoli for the gsudo executable that was issued by Certum Code Signing CA SHA2.

Then download and run:

Check that OCaml is installed properly with the following commands in your shell (PowerShell or Command Prompt). The line beneath the $ command shows the desired output for both the OCaml version and the toplevel version:

$ where.exe ocaml
C:\Users\frank\AppData\Local\Programs\DiskuvOCaml\0\usr\bin\ocaml.exe

$ ocaml -version
The OCaml toplevel, version 4.12.1

To learn more about Diskuv OCaml, see the official Diskuv OCaml documentation.

opam-repository-mingw

opam-repository-mingw is an opam repository containing patches for packages to build and install on Windows as well as mingw-w64 and MSVC compiler variants (in both 32 and 64-bit). For a long time this has been maintained by @fdopen along with installers to create a custom Cygwin environment with opam and OCaml installed.

As of August 2021, the repository will no longer be updated. It is still useful as an overlay to the default opam repository. This means if a package exists in opam-repository-mingw the opam client will use that information, otherwise it will fall through to opam-repository. To add opam-repository as an underlay to your opam setup (assuming you followed the manual installation from the OCaml for Windows site), you can use:

opam repo add upstream https://opam.ocaml.org --rank 2 --all-switches --set-default

This assumes you only have the opam-repository-mingw repository for this switch set with a priority of 1.

WSL2

If you only need to run OCaml programs on a Windows machine, then the simplest solution is to use the Windows Subsystem for Linux 2 (WSL2). WSL2 is a feature that allows Linux programs to run directly on Windows. WSL2 is substantially easier and faster to use than WSL1. Microsoft has comprehensive installation steps for setting up WSL2.

After you have installed WSL2 and chosen one Linux distribution (we suggest Ubuntu LTS), you can follow the Get Up and Running With OCaml: Installation for Linux and macOS steps.

Docker Images

The ocaml/opam Docker Hub repository now contains regularly updated Windows images. This includes images using msvc and mingw. If you are comfortable with Docker, this might be an easier way to get a working Windows environment on your machine.

Editor Support for OCaml on Windows

Visual Studio Code on Windows

If you use the recommended DKML installer, you will need to:

  1. Go to File > Preferences > Settings view (or press Ctrl ,)
  2. Select User > Extensions > OCaml Platform
  3. Uncheck OCaml: Use OCaml Env

If you use WSL2, you will remotely connect to your WSL2 instance from Visual Studio Code. Microsoft has a useful blog post that covers getting WSL2 and Visual Studio Code connected.

Vim and Emacs on Windows

For Vim and Emacs install the Merlin system using opam:

$ opam install merlin

The installation procedure will print instructions on how to link Merlin with your editor.

If you use Vim, the default cygwin Vim will not work with Merlin. You will need install Vim separately. In addition to the usual instructions printed when installing Merlin, you may need to set the PATH in Vim:

let $PATH .= ";".substitute(system('opam config var bin'),'\n$','','''')