Programming Clojure, 2nd Edition

Managing Per-Thread State with Vars

When you call def or defn, you create a dynamic var, often called just a var. In all the examples so far in the book, you pass an initial value to def, which becomes the root binding for the var. For example, the following code creates a root binding for foo of 10:

(def ^:dynamic foo 10)
-> #'user/foo

The binding of foo is shared by all threads. You can check the value of foo on your own thread:

foo
-> 10

You can also verify the value of foo from another thread. Create a new thread, passing it a function that prints foo. Don’t forget to start the thread:

user=> (.start (Thread. (fn [] (println foo))))
-> nil
| 10

In the previous example, the call to start returns nil, and then the ...