Rich Hickey is the designer of the Clojure programming language, which he released publicly in 2007 after working on it independently for about two and a half years. In Clojure’s own design rationale, written in the first person, he explains that he set out to build “a Lisp for Functional Programming symbiotic with an established Platform designed for Concurrency” because he could not find an existing language that combined all four of those properties.
Hickey’s central design conviction is that mutable state is the chief source of accidental complexity in software. The Clojure rationale states plainly that “if a data structure can be mutated, dangerous to presume it won’t be,” and so Clojure makes its core data structures immutable and persistent by default. Rather than eliminating change entirely, Hickey designed explicit reference types and a software transactional memory system so that state transitions are coordinated and visible.
These ideas are most associated with a series of conference talks Hickey gave over the following years, in which he argued for separating the simple from the easy and for treating values, not mutable objects, as the stable units a program reasons about. He later co-authored “A History of Clojure” for the ACM HOPL conference, documenting how the language and its community grew.
Hickey also created Datomic, a database built around the same principles of immutable, accumulating values, and he stewarded Clojure’s development for many years before stepping back from day-to-day leadership.