Managing Per-Thread State with Vars

When you call def or defn, you create 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 value of foo is printed from a new thread. ...

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