What is ZIO and how does it compare to Cats Effect?

ZIO (Zero-Dependency Input/Output) is a functional effects library for Scala. The core type is ZIO[R, E, A] — an effect requiring environment R, failing with E, succeeding with A. Advantages: built-in dependency injection via ZLayer, typed error channel (not just Throwable), structured concurrency with Fiber, comprehensive standard library (Streams, Queue, Ref, STM). Cats Effect: uses IO[A] with untyped errors, more modular (separate libraries for streams, queues), wider ecosystem adoption. Comparison: ZIO is more batteries-included; Cats Effect is more composable with the broader ecosystem (fs2, http4s). Both support fiber-based structured concurrency with millions of lightweight fibers. ZIO's typed error channel allows exhaustive error handling at compile time — a significant advantage for domain modeling. Most new Scala functional projects choose one or the other as their effect system foundation.