Installing Unison locally

Learn how to install Unison locally to solve Exercism's exercises on your own machine

Installation Options

The current Unison release is available for Mac OS X, 64-bit Linux, and Windows users! We hope that if you are trying out Unison you'll come talk to us in the Exercism forum or in the Unison language slack. Come ask questions, and report issues you might encounter! We want you to have a welcoming and positive experience when getting started! 😊

Unison can be downloaded with homebrew, Nix, or directly via packaged binaries. All of these download options are described in the Installation instructions page of our website. Once you have the UCM installed, head back here!

Option 1: Using Homebrew

First install homebrew if you haven’t already.

Then from the command line enter these commands:

brew tap unisonweb/unison
brew install unison-language

This will install the Unison codebase manager executable, ucm. If you’re upgrading from a previous version, just do

brew upgrade unison-language.

Note: if you get prompted for a GitHub username and password at this point, make sure you spelled unisonweb/unison correctly.

Option 2: Install manually for Mac and Linux users

Linux and Mac manual install links are provided here under the respective Linux and Mac manual download section.

You'll be running a command in your shell that looks something like this, but the link above will contain the most up-to-date binary:

mkdir unisonlanguage
curl -L --output unisonlanguage/ucm.tar.gz
tar -xzf unisonlanguage/ucm.tar.gz -C unisonlanguage

Option 3: Install manually for Windows users

  1. Set your default terminal application to “Windows Terminal” for best results. Search for “Terminal” in Settings, or follow this how-to.
  2. If you’re not sure you already have it, install Git:
  • Via winget: winget install --id Git.Git -e --source winget
  • If you are on Windows 10 and don’t have winget, you can install it from the Microsoft Store, and then run the command above.
  1. Download the UCM with the link provided here and extract it to a location of your choosing.
  2. Run ucm.exe

Option 4: Nix installation

Head to and follow the instructions in the readme there.

Create your Unison codebase

Run ucm to initialize a Unison codebase in $HOME/.unison. This is where Unison will store function definitions, types, namespaces, and so on. By default, the Unison Codebase Manger (abbreviated the "UCM") will begin downloading the standard library, called base, which you'll use to write Unison code. It may take a moment! 🕰

Unison workflow

When you want to write Unison code, you'll issue the ucm command to start the codebase manager. The UCM will then observe changes to any .u suffixed file in the directory where it is called from. For example, you might be implementing functions in a file called scratch.u in a directory called ~/exercism/unison/my-exercise/. You should cd into ~/exercism/unison/my-exercise/ and run ucm. This will ensure that the UCM is watching your scratch.u file for changes.

If you're ever confused about what directory the ucm is watching for file changes under, it's printed in the welcome screen upon starting the cli.

👀 I'm watching for changes to .u files under /Users/myUser/exercism/unison/my-exercise

See more workflow tips here!

Editor support

You can use any text editor to write your .u files. Unison has LSP support, so your favorite IDE can help out with things like types on hover and auto-complete. A few popular editors with Unison support are listed below:


Using vim-plug:

  1. Install vim-plug if you haven't already.
  2. Add the following to your .vimrc: Plug 'unisonweb/unison', { 'branch': 'trunk', 'rtp': 'editor-support/vim' }
  3. Issue the vim command :PlugInstall.

For more information run :help unison from within vim or view the online help doc.


From the console, run:

apm install unisonweb/atom-unison

VS Code

  1. Bring up the Extensions view by clicking on the Extensions icon in the Activity Bar on the side of VS Code, or with ⇧⌘X.
  2. Search for "unison"
  3. Install the Unison VS Code extension


Install unison-mode.