Most Ruby code makes heavy use of mutable state, which often contributes to long term maintenance problems. Mutability can lead to code that's difficult to understand, libraries and applications that aren't thread-safe, and tests that are slow and brittle. Immutability, on the other hand, can make code easy to reason about, safe to use in multi-threaded environments, and simple to test. Doesn't that sound nice?
This talk answers the question "why immutability?", covers the building blocks of immutable code, such as value objects and persistent data structures, as well as higher order concepts that make heavy use of immutability, such as event sourcing and pure functions, and finally discusses the tradeoffs involved in going immutable.