[lilactown]

IMO this is one of the main benefits of Clojure from a lisp-er perspective: it actually takes multithreading seriously in its design. It basically chose the "yes add the machinery" option, but built the machinery into the design of the language.

For instance, vars (i.e. global bindings) are mutable storage containers that can be bound on a per-thread basis. `(def foo "bar")` creates a new var and interns it in the current namespace, binding a root value "bar". Using it in code looks up the current value of it, either in thread-local storage or in the root binding. Any thread looking up the root binding of the var will see a newly bound value immediately.

However! Data in Clojure is immutable, which means that even if a running program started by looking up the current value of a var (or any of the other concurrency-safe containers in Clojure), once it has that value it is guaranteed not to change underneath you.

e.g. if you have a process that looks up the current value of a var and does some work with it for 5s, an in-flight process will not be altered if the var is redefined in the middle of the work. Other processes could even be kicked off that would alter the var, and it will have no affect on each other unless you explicitly synchronize, since the data inside the var is immutable.