Clojure: High Performance JVM Programming

Dynamic var binding and state

The fourth kind among the Clojure's reference types is the dynamic var. Since Clojure 1.3, all the vars are static by default. A var must be explicitly declared so in order to be dynamic. Once declared, a dynamic var can be bound to new values on per-thread basis. Binding on different threads do not block each other. An example is shown here:

(def ^:dynamic *foo* "bar")
(println *foo*)  ; prints bar
(binding [*foo* "baz"] (println *foo*))  ; prints baz
(binding [*foo* "bar"] (set! *foo* "quux") (println *foo*))  ; prints quux

As the dynamic binding is thread-local, it may be tricky to use in multi-threaded scenarios. Dynamic vars have been long abused by libraries and applications as a means to pass in a common argument ...