Seven Concurrency Models in Seven Weeks

Day 1: Atoms and Persistent Data Structures

A pure functional language provides no support for mutable data whatsoever. Clojure, by contrast, is impure—it provides a number of different types of concurrency-aware mutable variables, each of which is suitable for different use cases. These, in concert with Clojure’s persistent data structures (we’ll cover what persistent means in this context later) allow us to avoid many of the problems that traditionally afflict concurrent programs with shared mutable state.

The difference between an impure functional language and an imperative language is one of emphasis. In an imperative language, variables are mutable by default and idiomatic code modifies them frequently. In an impure functional ...