[hocuspocus]

The core difference is that Cats Effect is a fairly extensible effect type, with a full type class hierarchy. It's easy for compatible libraries to use and provide only what they need.

JDG argues against the "tagless final" pattern and brings some valid points, but in practice they aren't a problem if you don't overdo it. So in my case, I don't see the appeal in switching to a less mature ecosystem, at least based on this premise alone.

Before using Cats Effect/http4s/fs2/... I worked a few years using the Akka ecosystem (Play and then Akka http, streams, clustering, event-sourcing, ...) and while the underlying technology is solid, I was not a big fan of the experience overall. To me, the Typelevel ecosystem has mostly figured out what Scala should look like today.

If people find an answer in Zio, that's great, it doesn't have to be zero-sum.