This is the eighth major release in the Cats Effect 2.x lineage. It is fully binary compatible with all 2.x.y releases.
Unlike Cats Effect 2.2.0, which was a massive release with a ton of changes and improvements, Cats Effect 2.3.0 is a relatively modest release with only a few changes. As we continue to move towards Cats Effect 3.0, we are shifting efforts on Cats Effect 2 towards long term stability and maintenance, with more of the innovation pushed into the newer release lineage.
With that said, this is the first major Cats Effect 2 release which supports Scala 3! More specifically, this release has been published for both Scala 3.0.0-M1 and M2. We will continue to track the latest two Scala 3 milestones and release candidates until such time as Scala 3.0.0 final is released.
Notable New Features
Scala 3!
The most exciting new feature in this release is first-class support for Scala 3! It has long been possible to use Cats Effect 2 from projects compiled against Scala 3 (and Dotty before it) using the withDottyCompat utility. However, this would very quickly run against tricky corner cases, particularly with things like Resource. Thanks to the hard work of @vasilmkd, @bplommer, and @mpilquist, Cats Effect 2.3.0 now fully supports Scala 3 for all use-cases, including ScalaJS.
Furthermore, it is expected that source compatibility on downstream code has been preserved between Scala 2.13 and Scala 3.0, meaning that migrating to Scala 3 should be as easy as swapping the scalaVersion in your build definition. Cats Effect 3 has been building on Dotty for about six months now, and we're very excited to bring this functionality to the Cats Effect 2 lineage.
Print Non-Daemon Threads on Exit
This probably seems like a relatively trivial feature, but it actually solves a really annoying problem that was otherwise difficult to track down. The issue resolved here is when exiting an IOApp which, at some point, spawned Threads in user-code, and where those Threads are not setDaemon(true). If these spawned threads are still running when IOApp exits, the result is that the exit hangs while waiting for the threads to terminate. Particularly if you don't know what you're looking at, this can be quite an annoying problem to track down.
In an attempt to help out with this (unfortunately relatively common) scenario, IOApp will now detect active non-daemon threads when it reaches its exit point and print them to stderr, giving you a chance to track down the offenders and correct the situation, rather than just silently hanging.
User-Facing Pull Requests
- #1448 – Remove extra type param from
ApplicativeThrow(@bplommer) - #1379 – On JVM IOApp clean exit, optionally print non-daemon threads (@Daenyth)
- #1343 –
ApplicativeThrow/MonadThrow/BracketThrowaliases (@bplommer) - #1272 – Render symbolic operators in enhanced exceptions properly (@Avasil)
- #1242 – Override
Refcombinators for much better performance (@bplommer) - #1173 – Support for Cats Effect 2 on Dotty (@vasilmkd)
- #1436, #1408, #1377, #1286, #1276, #1245, #1216, #1184 – Numerous documentation improvements and enhancements! (@kubukoz, @visortelle, @molowny, @lrodero, @slouc, @neko-kai, @Daenyth, @RafalSumislawski)
- Special thanks to @lrodero here for the complete overhaul of our working tutorial example!
Special thanks to each and every one of you! Also extra-special thanks to @bplommer who did a ton of work on Resource, both in Cats Effect 2 and on the Cats Effect 3 development branch, to improve the safety and maintainability of the internal representation.