How does one start a thread in Clojure?

Question

I\'ve read a lot about how great Clojure is when it comes to concurrency, but none of the tutorials I\'ve read actually explain how to create a thread. Do you just do (.start (T

Answer 1

Programming Clojure doesn't address that question until page 167: "Use Agents for Asynchronous Updates".

Before you go starting threads, please note that Clojure will multitask on its own, given half a chance. I've written programs blithely ignorant of concurrency and found that when conditions are right, they occupy more than one CPU. I know that's not a very rigorous definition: I haven't explored this in depth yet.

But for those occasions when you really do need an explicit separate activity, one of Clojure's answers is apparently the agent.

(agent initial-state)

will create one. It's not like a Java Thread in terms of being a code block waiting to be executed. Instead, it's an activity waiting to be given work to do. You do this via

(send agent update-fn & args)

The example does

(def counter (agent 0))

counter is your name and handle for the agent; the agent's state is the number 0.

Having set that up, you can send work to the agent:

(send counter inc)

will tell it to apply the given function to its state.

You can later pull the state out of the agent by dereferencing it:

@counter will give you the current value of the number that started out at 0.

Function await will let you do something like a join on the agent's activity, should it be a long one:

(await & agents) will wait until they're all done; there's also another version that takes a timeout.