Scalaz: Purely Functional Programming in Scala

It has been said that Scala is an object-functional hybrid language, and Scalaz takes the “functional” side of Scala as far as it will go. A library of pure data structures, type classes, highly generalized functions, and concurrency abstractions, Scalaz provides the functional programmer with familiar tools, and inspires the imperative programmer to eschew all side-effects. It is simultaneously a testament to the power of Scala and an exposition of its limitations.