clojure.core/assert

View this page on ClojureDocs

Type: macro Added: Clojure 1.0 Examples: 3 Runnable: 0

([x] [x message])

Evaluates expression x and throws an AssertionError with optional message if x does not evaluate to logical true.

Assertion checks are omitted from compiled code if 'assert' is false.

Examples

by dakrone on 2010-07-31 . May have evaluation errors. ▶

(assert true) nil (assert false) java.lang.AssertionError: Assert failed: false (NO_SOURCE_FILE:0) (assert nil) java.lang.AssertionError: Assert failed: nil (NO_SOURCE_FILE:0) (assert 0) nil (assert [1 2 3]) nil (assert "foo") nil

by jakebasile on 2014-10-10 . May have evaluation errors. ▶

;; Messages can help you track down what went wrong. (assert (= 5 (+ 2 2)) "There are four lights!") AssertionError Assert failed: There are four lights!

by sofiiahitlan on 2024-01-04 . May have evaluation errors. ▶

;; AssertionError isn't an Exception so it should be caught properly: (try (let [some-with-cond? (fn [el] {:pre [(some? el)]} (some? el))] (some-with-cond? nil)) (catch AssertionError e "AssertionError caught.")) AssertionError caught. ;; catch Exception wouldn't work: (try (let [some-with-cond? (fn [el] {:pre [(some? el)]} (some? el))] (some-with-cond? nil)) (catch Exception e "AssertionError caught.")) Execution error (AssertionError) at loaders.core/eval26824$some-with-cond? (form-init14472288491470508923.clj:5587). Assert failed: (some? el)

Note by Steve Kuo

The documentation ("... and throws an exception if ...") is incorrect. It throws AssertionError which an Error and not an Exception. They both extend Throwable but Error is not an Exception.

Note by didibus

By default, assertions are turned on, since the value of *assert* defaults to true in Clojure.

Note by taehee-kim-sherpas

in shadow-cljs, All assertions are removed in release mode by default. In other words, assert is false in release mode. (see :elide-asserts option) It is applied to schema libraries like malli, malli.core/assert is removed on release mode.

See also

• clojure.spec.alpha/assert

---

Content from the matching ClojureDocs page, with authors credited on each contribution.