Keep your projects up-to-date with Scala Steward

Wednesday 10 July 2019

Frank S. Thomas

I’m excited to announce version 0.3 of Scala Steward and to introduce it to the Scala community! Scala Steward is a robot that helps you keep dependencies, sbt, and sbt plugins up-to-date by automatically creating pull requests on GitHub, GitLab, or Bitbucket that bump outdated version numbers.

How to use it

You can use it with any public Scala project on GitHub that uses sbt simply by adding it to this file and then wait for the first pull requests to come in a few hours later. Over 500 projects are already using this public instance of Scala Steward which I’m running as a free service for the whole Scala OSS ecosystem. Since Sep 9, 2018, when Scala Steward created its first pull request, it has created more than 11 900 pull requests of which over 8 300 are already merged!

Here is a screenshot of one of Scala Steward’s pull requests: screenshot of a Scala Steward pull request

Receiving automatic pull requests that bump versions of dependencies reduces maintenance if your code compiles without warnings and errors with the new version. In many cases, it is enough to review the changelog of the new version and to check that your automated tests pass before merging such pull requests. If you are adventurous, you can even let Scala Steward’s pull requests be merged automatically.

Running your own instance

You can also run your own instance of Scala Steward for your private repositories and join the companies that are already doing this. Unfortunately this isn’t well documented yet. Improving the documentation would make a great first contribution after you have figured out how to set it up.

Automatic code migrations with Scalafix

What if a new version of your dependencies contains breaking changes so that your code does not compile without errors or warnings anymore? In these cases automatic version bumps only notify you of a new version and make you aware that human intervention is needed to fix errors and warnings and to adapt the code to the new version.

Fortunately, we can improve on this situation by combining Scala Steward with the excellent Scalafix tool. Scalafix is a refactoring and linting tool that allows to rewrite code based on user-supplied rules. You can read more about Scalafix here and here. Scalafix rules can also be used for version migrations to help users updating to new versions and some libraries (like Cats, FS2, or http4s) are already providing such migrations. With Scala Steward, we now have the possibility to apply Scalafix migrations automatically to create pull requests like this:

diff --git a/build.sbt b/build.sbt
--- a/build.sbt
+++ b/build.sbt
 libraryDependencies := List(
-  "org.typelevel" %% "cats-core" % "0.9.0"
+  "org.typelevel" %% "cats-core" % "1.6.1"

diff --git a/src/main/scala/cats.scala b/src/main/scala/cats.scala
--- a/src/main/scala/cats.scala
+++ b/src/main/scala/cats.scala
 object cats {
-  (Option(1) |@| Option(2) |@| Option(3)).map(_ + _ + _)
+  (Option(1), Option(2), Option(3)).mapN(_ + _ + _)

Besides bumping the Cats version number, the Scalafix migration took care of the breaking changes that were introduced between Cats 0.9.0 and Cats 1.0.0 and updated the code so that it compiles cleanly with the new version. These automatic migrations have the potential to significantly reduce maintenance costs of version updates that contain breaking changes or deprecations.

Get involved

Scalafix migrations are great but we haven’t yet solved the problem of writing them automatically for version updates. :-) That means there are plenty of opportunities for new contributors to help library maintainers writing Scalafix migrations. And by integrating them into Scala Steward, their value is amplified by the number of projects that use Scala Steward. Even small contributions can already benefit a large chunk of the Scala ecosystem.

After you have written a Scalafix migration for your favorite library, Scala Steward needs to be made aware that it exists and for which version update it should be applied. Instructions for adding them to Scala Steward and more details about the Scalafix integration in Scala Steward can be found here. Once the migration has been added and the new version of the library is released, you can enjoy watching Scala Steward creating pull requests with changes from your Scalafix migration!

There are also enough opportunities to improve Scala Steward itself. The Scalafix integration, for example, has only been added recently and is still rough around the edges.


Scala Steward:

  • Can create automatic pull requests that bump dependency versions. This saves a lot of time.
  • In addition to the version bumps, it can migrate the code with Scalafix.
  • You can help the whole Scala ecosystem by writing Scalafix migrations.

Try out Scala Steward now!


Scala Steward wouldn’t exist without Roman Timushev’s great sbt-updates which is an integral part of Scala Steward. A special shout-out also goes to @impurepics who created the cute and iconic logo!

And many thanks to the 20+ contributors who made Scala Steward to what it is today: Alex, Anil Kumar Myla, Arulselvan Madhavan, Bayram Kiran, Cédric Chantepie, Christopher Davenport, Dale Wijnand, Daniel Pfeiffer, David Francoeur, Fabian, Filipe Regadas, Jakub Kozłowski, JCollier, Jeff Martin, kenji yoshida, Mark Canlas, Michael Wizner, Philippus Baalman, Piotr Gabara, Renato Cavalcanti, sullis, TATSUNO Yasuhiro, Thomas Heslin, and Thomas Kaliakos.

I’d also like to thank Andrea Magnorsky, David Francoeur, Lars Hupel, Seth Tisue, and Yifan Xing for proofreading this post.