interop-cats VS Http4s

If you're using http4s, don't use ZIO. Yes, ZIO has an interop-cats module, so you can do this. But using http4s means you're working in the conceptual framework the Typelevel ecosystem is based on (and what interop-cats does can be characterized as "describe ZIO's implementation in those terms, so the Typelevel ecosystem can make heads or tails of it.") This is essentially all cost and no benefit: you can't avoid understanding the Typelevel ecosystem if you use http4s (at least, no more than you can by using cats-effect), and you don't get any of the value proposition of ZIO (interop-cats gives you Typelevel typeclass instances for the RIO type alias, which means your error channel is rooted in Throwable, and you're faced with the most complex part of the ZIO ecosystem: ZLayer, which the Typelevel ecosystem doesn't use and doesn't need). Finally the ZIO ecosystem is still quite immature, and this brings us to documentation. There is not (yet!) anything comparable to:

These are necessarily oversimplifications. In particular, the ZIO ecosystem offers the relevant instances of cats-effect typeclasses to support use of the ZIO type in the cats-effect ecosystem.

However, ZIO continues to offer cats-effect type classes and I certainly have no doubt cats-effect 3 continues to benefit from John's contributions. Furthermore, I likewise don't doubt the value of the ZIO ecosystem generally, and John's success in building the ZIO community speaks for itself. I personally have chosen to remain closer to the other, let's say "classical," pure FP ecosystems, partially for historical (or, if you prefer, "sunk cost") reasons, but partially because I'm satisfied the value of the Haskell/Typelevel/PureScript/fp-ts/etc. interplay warrants it.

It seems that zio/interop-cats faces a similar issue.

http4s is a Typelevel project, and therefore falls into the "program in Scala as if it were Haskell" category. Many people find this off-putting, but honestly, I think with the resources listed above, this is the option at the best intersection of "mature" and "well-documented" available in Scala. The reason it's off-putting to many people is that Haskell-style pure FP isn't mainstream, so it isn't so much a matter of learning a new technology as it is a matter of learning a new paradigm, which necessarily means surfacing and unlearning things you already know, and perhaps confronting the uncomfortable feeling that things you thought were "fundamental," "have to be that way," aren't, and don't. I personally found this process liberating. But not everyone does.

If you want a mature REST API library, I recommend http4s. Be aware, though, that it’s based on purely-functional programming with Cats, cats-effect, and fs2, so if you’re not familiar with them or aren’t prepared to commit to the paradigm, the learning curve may be daunting, seem pointless, or both.

See also cats-effect and fs2. cats-effect gives you your IO Monad (and IOApp to run it with on supported platforms). fs2 is the ecosystem’s streaming library, which is much more pervasive in functional Scala than in Haskell. For example, http4s and Doobie are both based on fs2.

Most important/known: * https://http4s.org/ - an HTTP client/server * https://github.com/typelevel/fs2 - streaming * https://github.com/tpolecat/doobie - JDBC

Scala devs are too busy wondering about free monads and F[Request[F] => Response[F]]. I am very pleased by http4s, Doobie, ScalaJS, and the whole ecosystem, really: https://http4s.org

In reality, your performance issues will not be http4s, but something else. That being said, there are improvements that http4s can and is making, and I'm quite excited about the future 1.0 release, which has some important and fundamental performance improvements already, like a a 125% performance improvement on the plaintext benchmark from https://github.com/http4s/http4s/pull/6091 - and finally, yes, akka-http does have very good performance, but you can also get good performance out of http4s.

http4s for REST services.

This is a long list of misunderstandings I don’t have the patience to unpack. Instead, let me refer you to the links in my top comment in the thread, then suggest you learn at least http4s, a purely-functional web service library that’s been used in production for a decade or so now.

The most popular nowadays are - I guess - akka-http and http4s. You can also use Play if you don't want to start from scratch but prefer a framework-based approach.

There are functional frameworks: https://http4s.org/