What's the meaning of :key parameter in add-watch function

Question

My problem is that with the doc and examples providede i can't understand the meaning of `:key` parameter or its possible values This is the official doc page of the function that I'm referring: http://clojuredocs.org/clojure_core/clojure.core/add-watch add-watch clojure.core ``` (add-watch reference key fn) ``` Adds a watch function to an agent/atom/var/ref reference. The watch fn must be a fn of 4 args: a key, the reference, its old-state, its new-state. Whenever the reference's state might have been changed, any registered watches will have their functions called. The watch fn will be called synchronously, on the agent's thread if an agent, before any pending sends if agent or ref. Note that an atom's or ref's state may have changed again prior to the fn call, so use old/new-state rather than derefing the reference. Note also that watch fns may be called from multiple threads simultaneously. Var watchers are triggered only by root binding changes, not thread-local set!s. Keys must…

Accepted Answer

It's basically just an identifier that you can use in calling code to identify the watch, in case you have more than one watches per reference. It's something that should have significance to your application code, but will be passed through by Clojure.

For instance:

```
user> (def a (atom 0))
#'user/a
user> (add-watch a
                 :count-to-3
                 (fn [k r old-state new-state]
                     (println "changed from" old-state "to" new-state)
                     (when (>= new-state 3)
                       (remove-watch a :count-to-3))))
#<Atom@3287a10: 0>
user> (dotimes [_ 5] (swap! a inc))
changed from 0 to 1
changed from 1 to 2
changed from 2 to 3
nil
user> @a
5
```

What's the meaning of :key parameter in add-watch function

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