ZIO VS Monix

scala has 2 healthy and pretty complete lib ecosystems : check out typelevel and ZIO. Both are FP oriented, which might not be your cup of tea at first glance but I would encourage you to try em out ! Softest introduction would be to start with the typelevel cats library and build up from there. The excellent Scala with Cats will ease you softly into an FP mindset. It's a bit dated and for scala 2 only but translating to Scala 3 is a very good exercise if you feel so inclined !

Last but not least, Scala is currently the language with one of the best effect systems in my opinion (https://zio.dev/). Kotlin for example has copied the approach with https://arrow-kt.io/ which I think is great actually. But when comparing Scala and Kotlin here, Scala wins by a large margin, it is a completely different world. It's like building a highly concurrent system in Erlang vs C.

Of course, if you don't want to learn things like union types, traits/typeclasses and effects (similar to async/await but more powerful) you will be annoyed by Scala. But once you learned them, you can never go back.

ZIO

Links: - https://dotty.epfl.ch/ - https://scala-native.org/en/stable/ - https://www.scala-js.org/ - https://typelevel.org/ - https://zio.dev/ - https://github.com/scala-native/scala-native/pull/3120 - https://github.com/lampepfl/dotty/pull/16517 - https://dotty.epfl.ch/docs/reference/experimental/index.html - https://scala-cli.virtuslab.org/ - https://scalameta.org/metals/ - https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html - https://www.scala-lang.org/blog/2023/04/18/faster-scalajs-development-with-frontend-tooling.html - https://www.scala-lang.org/blog/2022/08/17/long-term-compatibility-plans.html

I use ZIO (http://zio.dev) for Scala which makes parallel programming trivial.

Wraps different styles of asynchronicity e.g. callbacks, futures, fibers into one coherent model. And has excellent resource management so you can be sure that when you are forking a task that it will always clean up after itself.

Have yet to see anything that comes close whilst still being practical i.e. you can leverage the very large ecosystem of Java libraries.

The one advantage Rust has over Scala is that it detects data races at compile time, and that's a big time saver if you use low level thread synchronization. However, if you write pure FP code with ZIO or Cats Effect that's basically a non-issue anyway.

> It's a weakness of functional programming, not the language.

I would strongly object this.

Have you seen ZIO?

https://zio.dev/

But overall, I'm happier with Scala. It has not one, but two lively FP communities. I recommend that you pick one of them: ZIO or TypeLevel (Cats Effect, fs2, http4s, ...). To some extent, you can mix TypeLevel libraries in ZIO projects.

I think "better Haskell on JVM" (in contrast to "worse Haskell") is a good identity for Scala to have. (Please note that this is an intentional hyperbole.)

Of course, there are areas where Haskell is stronger than Scala (hint: modularity, crucial for good Software Engineering, is not one of them). And Scala has its own way of doing things, so just imitating Haskell won't work well.

Examples of this "better Haskell" are https://typelevel.org/cats-effect/ and https://zio.dev/ .

All together, Scala may be a better choice for you if you want to do Pure Functional Programming. And is definitely less risky (runs on JVM, Java libraries interop, IntelliJ, easy debugging, etc...).

None of the other languages you mentioned are viable in this sense (if also you want a powerful type system, which rules out Clojure).

I agree that Rust's identity is pretty clear: a modern language for use cases where only C or C++ could have been used before.

fs2 (reactive streaming, https://github.com/typelevel/fs2) written in Scala. It shows how nicely things can compose in a typesafe way if the language supports it.

And then, the opposite is Monix (https://monix.io/). It's also about reactive streaming and the API is great, but the inner code is ugly because it sacrifices readability/composability for performance.

The author is the creator of Monix and implemented the first version of cats-effect. He knows what he is doing.

I haven't had the pleasure to use it personally, but will probably check it out once Monix switches to CE3. As far as I'm aware, this requires a major rewrite of the library: https://github.com/monix/monix/issues/1502

Monix exposes Observable with similar API. It has a stronger FP influence but there are more similarities than differences.

The Observer is a bit different, it returns Future[Ack] for built-in back-pressure. We can Continue / Stop synchronously or asynchronously.

cats-effect or monix or ZIO for Scala.