What are some uses of Clojure metadata?

Question

How have you used metadata in your Clojure program?

I saw one example from Programming Clojure:

(defn shout [#^{:tag String} message] (.toUp         


        

Answer 1

For diversity some answer, which does not concentrate on interaction with the language itself:

You can also eg. track the source of some data. Unchecked input is marked as :tainted. A validator might check things and then set the status to :clean. Code doing security relevant things might then barf on :tainted and only accept :cleaned input.