I'm a Go developer so I have no hot takes but all of the Scala projects where I work are in ZIO
this post was submitted on 10 Jun 2023
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Cats seems to still be the market leader, but ZIO is probably easier to get into
Various places I've interviewed at had a blanket ban on ZIO, which I found odd
The effect system is pretty foundational. Libraries built on Cats-Effect can be used with ZIO (with caveats), due to the exposed type classes, but ZIO libraries can't be comfortably used alongside Cats-Effect-driven libraries. You tend to pick either one ecosystem, or the other. Picking both creates a mess, and you need strong leadership that can keep clean boundaries between the modules of the project. We see that in our $work project by combining Typelevel / Cats-Effect libraries with Akka, but there the separation is cleaner because Akka has a clear purpose in our project, without much overlap with the used Typelevel libraries.
I may be biased, but Typelevel libraries interoperate better with the rest of the ecosystem, and they are more mature, but I'm also certain this depends on personal experience and the projects we've been working on. In other words, it's fine to pick ZIO, if you like it better, the problem is with combinations, IMO.
I only have limited time with ZIO, and used Cats without IO, but ZIO seems to be a little bit easier to grasp. I like that it kinda does everything that Option/Either/IO does in one, makes me feel like I won't be dealing with as much nesting, and I wont need the monad transformers too. At my job we used Future instead of IO and monad transformers were used a lot. They weren't too bad, but every now and then i'd step back and ask "is this really the best?" I'm not in a Scala job right now, I'm really hoping that in the future there will be just one library for this kind of thing, I hate having divided ecosystems.
Sharing from my personal experience — with the Typelevel libraries, you don't really need monad transformers if you don't like them.
For dependency injection, personally, while being a big fan and contributor of Typelevel, I prefer to use Scala as an OOP language, too, as OOP is already great for dependency injection. And I'm always surprised by the lengths people go to avoid passing parameters to constructors or to functions. Although I will say that the use of Resource for managing the lifecycle of resources is a Scala super-power.
For errors, there are different approaches, no one size fits all, but personally, I never use EitherT or OptionT, and that's a false choice being presented by these 2 communities. You don't have to pick between EitherT or ZIO because you can choose neither.
Domain / input errors have to be designed with care, and you usually don't need short-circuiting for those. And as an alternative, what you can do instead is to design errors out of existence. I'm sharing some design advice for that here: https://alexn.org/blog/2022/04/18/scala-oop-design-sample/
Probably just the nature of where I work and the Scala circles I'm in, but until I asked the question here it seemed like Cats was in the lead for market share.