(defn unfnil
  [f]
  (fn [& args]
    (let [val (apply f args)]
      (if (empty? val)
        nil
        val))))

unfnil is a kind of a reverse of fnil from Clojure.

For example, fnil can be used with conj like this: (fnil conj #{}) which will replace the first argument to conj with #{} if it is nil. Useful if we want a set instead of a list, because conj with nil produces a list.

(update {:alarm true} :days (fnil conj #{}) :monday)
=> {:alarm true, :days #{:monday}}

unfnil is the reverse, for instance (unfnil disj) with #{:a} :a produces a nil instead of an empty set. Unlike fnil, unfnil works with collections only.

(update *1 :days (unfnil disj) :monday)
=> {:alarm true, :days nil}