If we want to get the values from a set that are not part of one or more other sets we must use the difference function in the clojure.set namespace. The function returns a set with all values from the first set that are different from values in other sets.

In the following example we use the difference with several sets:

(ns mrhaki.set.difference
  (:require [clojure.set :as set]
            [clojure.test :refer [is]]))

;; The difference function will take a first set
;; and leave out elements that are in the following set(s).
(is (= #{"Java"}
       (set/difference #{"Java" "Clojure" "Groovy"}
                       #{"Kotlin" "Groovy" "Clojure"})))

(is (= #{"Java"}
       (set/difference #{"Java" "Clojure" "Groovy"}
                       #{"Kotlin" "Groovy"}

;; When other sets do not contain values
;; from the first set, the result is the original set.
(is (= #{1 2 3}
       (set/difference #{1 2 3} #{4 5})))

Written with Clojure 1.10.1.