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At Faculty, we write most of the back-end services for our data science platform in Scala. To save on time spent discussing code style, we apply automatic formatting to our source code using scalafmt.

scalafmt is relatively easy to set up in IntelliJ, a popular IDE for Java and Scala, but many on our development team use Neovim as their primary code editor. This blog 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

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 run scalafmt through nailgun 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

To find out more about what Faculty can do for you and your organisation, get in touch.