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What is the difference between def and defonce in Clojure?

What is the difference between def and defonce in Clojure?

When to use def over defonce or vice versa?

Solution

  • defonce is skipped when 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