The is macro is the core of the clojure.test library. It returns the value of its body expression, printing an error message if the expression returns a falsey value.

(defn square [x]
  (+ x x))

(require '[clojure.test :as t])

(t/is (= 0 (square 0)))
;;=> true

(t/is (= 1 (square 1)))
;;
;; FAIL in () (foo.clj:1)
;; expected: (= 1 (square 1))
;;   actual: (not (= 1 2))
;;=> false