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:

 ​-> 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 ...

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