What is the difference between def and defonce in Clojure?

Question

What is the difference between def and defonce in Clojure?

When to use def over defonce or vice versa?

Answer 1

defonce skipped when the variable is already defined.

 user> (def a 1) ;;=> #'user/a user> a ;;=> 1 user> (def a 2) ;;=> #'user/a user> a ;;=> 2 user> (defonce b 1) ;;=> #'user/b user> b ;;=> 1 user> (defonce b 2) ;;=> nil user> b ;;=> 1 

Answer 2

Defonce only associates a name with a root value if the name does not have a root value.

For example, for example, Jay Fields blogs , it can be used together when you want to reload namespaces, but you may not need to reload everything.

 (defonce ignored-namespaces (atom #{})) (defn reload-all [] (doseq [n (remove (comp @ignored-namespaces ns-name) (all-ns))] (require (ns-name n) :reload )))