Formatting Scala code in Visual Studio Code

Scala and JavaScript, a match made in heaven

Gabriele Petronella Blocked Unblock Follow Following Aug 16, 2017

Recently at buildo we grew tired of formatting our Scala code by hand and we started looking into Scalafmt. While discussing its adoption we identified a few non-negotiable requirements:

the CI should check that everything is properly formatted

the editor should allow the developer to format a file while working on it

The first requirement is easy to fulfill: we can write a CI task that runs scalafmt --test and it's done.

The second one is trickier: the editor is a personal choice and when talking about editors for Scala there’s usually little agreement.

No one at buildo has a taste for IDEs, so IntelliJ is not on the table (too bad, since it already had a Scalafmt plugin!). Some people use Sublime, some people Emacs, some other Vim, but there’s a good group of people using Visual Studio Code, a solid rich editor by Microsoft, which features also a good support for Ensime.

As many other modern editors, Visual Studio Code supports extensions, and they are written in TypeScript, a typed superset of JavaScript.

First attempt: invoking the scalafmt CLI

After understanding the basics of VSCode extensions, my first naive attempt was to simply “shell out” and invoke a local scalafmt process to format the file in the active text editor.

Something like scalafmt --std-out /path/to/file .

While this approach is very simple and it works, it also has a few downsides:

it requires installing scalafmt and having it available on the user's path, making the extension not self-contained.

and having it available on the user's path, making the extension not self-contained. it’s slow. Don’t get me wrong: Scalafmt is not slow per se, but spinning up a JVM for formatting a single file incurs in a very high overhead. Formatting a file while editing is an operation that should be cheap, especially if you enable it on save (like VSCode allows).

What can we do to make this faster and self-contained?

Second attempt: scalafmt… in JS?

Scalafmt already cross-compiles to Scala.js, and — if you think about it — it makes sense: its core functionality is to manipulate a string (the original source code) and return another string (the formatted source code). There’s nothing inherently JVM-specific about it, so it can work just fine in Node.js.

What was missing was an API to invoke it from JavaScript, so I rolled-up my sleeves and wrote one. The result is a Node.js package published on npm, which can be bundled in any Node application, including (surprise!) a VSCode extension.

A few nights later, here it is: https://github.com/gabro/vscode-scalafmt

You can install the extension only, it doesn’t require installing scalafmt globally. Also, it’s fast, since we’re not paying the JVM startup cost anymore! Formatting a file is an almost-immediate operation that you can do at any time without disrupting your workflow.

A word about JavaScript and Scala

Scala.js is a game changer, at least for the Scala community. It wasn’t until recently that I started thinking this, but when you look at what it makes possible, it becomes evident.

Recently I’ve talked about adding Scalameta support to AST Explorer, today I’m telling you about a VSCode extension. Tomorrow it will be something else:

a code browser?

a browser extension that formats your snippets on GitHub?

you name it

The truth is: you may not like JavaScript as a language (by the way, you should try TypeScript!), but let me state the obvious: JavaScript is the foundation of many tools we use today, ranging from the web to text editors.

Being able to integrate Scala-specific tools with these JS tools is like having superpowers.

So I’ll say it again: Scala.js is a game changer for the world of Scala tooling.

Thanks

I’d like to thank Ólafur Geirsson for authoring Scalafmt and answering my many questions on Gitter and GitHub.

A big thank you also to Sébastien Doeraene and the Scala.js team.

Finally, a shout-out to Microsoft for Visual Studio Code: it’s an incredibly solid piece of software that is a pleasure to hack on and work with.

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