scalafmt in Neovim

At Faculty, where I work as an engineer developing our data science platform, we write most of our backend services in Scala. To save on time spent discussing code style, we’re trying out autoformatting of our code with scalafmt, with a configuration that reasonably closely reflects our current style.

scalafmt is relatively easy to set up in IntelliJ, a popular IDE for Java and Scala, however I and many on our development team use Neovim as our primary code editor. This article details the steps we took to get Neovim to apply scalafmt to Scala source code automatically on save.

These instructions are written for use on Mac OS – on other systems you’ll need to translate to use the appropriate package manager and init system.

1. Install nailgun

As responsiveness is important when running scalafmt from an editor, it’s recommended to run scalafmt through nailgun. Nailgun keeps scalafmt running on a local server to avoid paying a penalty to start up the JVM on each run.

To install nailgun with Homebrew:

brew install nailgun

2. Install scalafmt with coursier

If you don’t already have coursier, install it with Homebrew:

brew install --HEAD coursier/formulas/coursier

With coursier, install scalafmt and create an executable running the scalafmt service in nailgun:

coursier bootstrap \
  --standalone com.geirsson:scalafmt-cli_2.12:1.5.1 \
  -r bintray:scalameta/maven \
  -o /usr/local/bin/scalafmt_ng \
  -f --main com.martiansoftware.nailgun.NGServer

Create an alias to run the scalafmt CLI through nailgun with:

ng ng-alias scalafmt org.scalafmt.cli.Cli

Note: The above command installs the latest version of scalafmt at the time of writing. See the scalafmt docs for a command to install the most up to date version.

You can test that scalafmt is working by running the server with:

scalafmt_ng

and in a separate shell run

ng scalafmt --version

3. Create scalafmt service

We don’t want to have to manually run the scalafmt server every time we start the computer, so to have Mac OS run it as a service, create the following file in ~/Library/LaunchAgents/nailgun.scalafmt.plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>KeepAlive</key>
  <true/>
  <key>Label</key>
  <string>nailgun.scalafmt</string>
  <key>Program</key>
  <string>/usr/local/bin/scalafmt_ng</string>
  <key>RunAtLoad</key>
  <true/>
</dict>
</plist>

and enable the service with:

launchctl load -w ~/Library/LaunchAgents/nailgun.scalafmt.plist

4. Configure Neovim

Finally, we want to configure Neovim to automatically apply scalafmt formatting to our Scala source files.

Install neoformat with your favourite plugin manager. For example, with vim-plug, add to ~/.config/nvim/init.vim:

Plug 'sbdchd/neoformat'

then reopen Neovim and run :PlugInstall.

In ~/.config/nvim/init.vim, configure neoformat to use the scalafmt server with:

let g:neoformat_scala_scalafmt = {
        \ 'exe': 'ng',
        \ 'args': ['scalafmt', '--stdin'],
        \ 'stdin': 1,
        \ }

Finally, to configure Neovim to automatically apply formatting to Scala files on save, add the following line to ~/.config/nvim/init.vim:

autocmd BufWritePre *.{scala,sbt} Neoformat