impure-function.clj

(require '[clojure.test :refer [is]])

(defn random-beatle []
  (case (rand-int 4)
    0 "John"
    1 "Paul"
    2 "George"
    3 "Ringo"))

(defn hello-beatle []
  (str "Hello " (random-beatle)))

(with-redefs [random-beatle (fn [] "Alex")]
  (is (= "Hello Alex" (hello-beatle))))

;; but be careful - this doesn't always work as you may expect
;; it depends on where the future is dereferenced

(is (= "Hello Alex" @(with-redefs [random-beatle (fn [] "Alex")]
                       (future (hello-beatle)))))

FAIL in () (form-init6579728494720041389.clj:204)
expected: (= "Hello Alex" (clojure.core/deref 
                            (with-redefs [random-beatle (fn [] "Alex")] 
                              (future (hello-beatle)))))
  actual: (not (= "Hello Alex" "Hello Paul"))

;; but it works when the future is dereferenced inside with-redefs
(is (= "Hello Alex" (with-redefs [random-beatle (fn [] "Alex")]
                      @(future (hello-beatle)))))
true