In 2016’s Curry-On keynote, David Nolen elected Clojure’s efficient immutable collections to his shortlist of four transformative technologies originating from research, which made the language a success story in the long run. "Oldies, but goodies", Nolen said. The foundations of nowadays efficient immutable collections, which are based on hash-tries, were pioneered in the late 50s and early 60s. However, rejuvenated research interest in this topic just met the enthusiasm of functional programmers roughly a decade ago, after Clojure and Scala broadly started embracing well-performing functional data structures. This talk will quickly go back to the origins of prefix trees, to then introduce the basic ingredients it takes to build efficient immutable collections that are based on hash-tries. The main part of this talk will pickup where Clojure and Scala left off: focusing on current research involving hash-tries, and projecting where the journey might go. Engineers, researchers, and programming language enthusiasts, let’s have a talk!