[kagakuninja]

Because they have in the past declared "Java is not a functional language" and were hostile to the use of monads in the standard library. They are adding nice features, but the language will never be as clean and unified as Scala.

[jmaker]

Monads don’t go by the name, they must satisfy the axioms. How many people do it right? Even in Haskell lots of folks forget to check the axioms. Also, monads are hard to compose. I think there’s no reason to be like Cats or Arrow. It’s all already there, the mental shift is minimal in my opinion.

[kagakuninja]

"Monads don't compose" just means you should choose a single monad for most of your functions; in Scala that is usually Future, Either, Try, IO or ZIO. There are conversion methods between the most common monads.

Scala monads are fantastic IMO; Either-based programming is simple and eliminates tons of boilerplate. So is Cats, although the learning curve is steep.