Scalaz is a mature, purely functional programming library for Scala, providing a comprehensive set of type classes (Functor, Monad, Traverse, Foldable, etc.), monad transformers (OptionT, StateT, WriterT, etc.), effect types (IO, ST, RegionT), efficient immutable data structures (IList, Tree, FingerTree, Heap, Dequeue, etc.), and syntactic enrichment for all of the above. It is organized into modular sub-projects (core, effect, iteratee, scalacheck-binding) and supports cross-compilation to JVM, JavaScript, and Native platforms across Scala 2.13 and 3. The library is designed for production use and serves as both a practical toolkit and a reference implementation of category theory concepts in Scala. While primarily a library, it contains advanced research-level abstractions (free monads, cofree comonads, finger trees) that make it a valuable resource for functional programming education.