QuickCheck for Clojure
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test.check is a Clojure property-based testing tool inspired by QuickCheck. The core idea of test.check is that instead of enumerating expected input and output for unit tests, you write properties about your function that should hold true for all inputs. This lets you write concise, powerful tests.

Releases and Dependency Information

Release notes for each version are available in CHANGELOG.markdown. Remember that prior to version 0.5.7, test.check was called simple-check.

As of version 0.9.0, test.check requires Clojure >= 1.7.0.

Please note a breaking change for ClojureScript in the 0.8.* releases.

Latest Version


[org.clojure/test.check "0.10.0-alpha2"]



Stable Version


[org.clojure/test.check "0.9.0"]



If you'd like to try a SNAPSHOT version, add the sonatype repository to your project.


Useful libraries


Let's say we're testing a sort function. We want to check that that our sort function is idempotent, that is, applying sort twice should be equivalent to applying it once: (= (sort a) (sort (sort a))). Let's write a quick test to make sure this is the case:

(require '[clojure.test.check :as tc])
(require '[clojure.test.check.generators :as gen])
(require '[clojure.test.check.properties :as prop])

(def sort-idempotent-prop
  (prop/for-all [v (gen/vector gen/int)]
    (= (sort v) (sort (sort v)))))

(tc/quick-check 100 sort-idempotent-prop)
;; => {:result true, :num-tests 100, :seed 1382488326530}

In prose, this test reads: for all vectors of integers, v, sorting v is equal to sorting v twice.

What happens if our test fails? test.check will try and find 'smaller' inputs that still fail. This process is called shrinking. Let's see it in action:

(def prop-sorted-first-less-than-last
  (prop/for-all [v (gen/not-empty (gen/vector gen/int))]
    (let [s (sort v)]
      (< (first s) (last s)))))

(tc/quick-check 100 prop-sorted-first-less-than-last)
;; => {:result false, :failing-size 0, :num-tests 1, :fail [[3]],
       :shrunk {:total-nodes-visited 5, :depth 2, :result false,
                :smallest [[0]]}}

This test claims that the first element of a sorted vector should be less-than the last. Of course, this isn't true: the test fails with input [3], which gets shrunk down to [0], as seen in the output above. As your test functions require more sophisticated input, shrinking becomes critical to being able to understand exactly why a random test failed. To see how powerful shrinking is, let's come up with a contrived example: a function that fails if it's passed a sequence that contains the number 42:

(def prop-no-42
  (prop/for-all [v (gen/vector gen/int)]
    (not (some #{42} v))))

(tc/quick-check 100 prop-no-42)
;; => {:result false,
       :failing-size 45,
       :num-tests 46,
       :fail [[10 1 28 40 11 -33 42 -42 39 -13 13 -44 -36 11 27 -42 4 21 -39]],
       :shrunk {:total-nodes-visited 38,
                :depth 18,
                :result false,
                :smallest [[42]]}}

We see that the test failed on a rather large vector, as seen in the :fail key. But then test.check was able to shrink the input down to [42], as seen in the keys [:shrunk :smallest].

To learn more, check out the documentation links.

clojure.test Integration

The macro clojure.test.check.clojure-test/defspec allows you to succinctly write properties that run under the clojure.test runner, for example:

(defspec first-element-is-min-after-sorting ;; the name of the test
         100 ;; the number of iterations for test.check to test
         (prop/for-all [v (gen/not-empty (gen/vector gen/int))]
           (= (apply min v)
              (first (sort v)))))


ClojureScript support was added in version 0.7.0.

The first test.check example needs only minor modifications for ClojureScript:

(ns cljs.user
  (:require [clojure.test.check :as tc]
            [clojure.test.check.generators :as gen]
            [clojure.test.check.properties :as prop :include-macros true]))

(def sort-idempotent-prop
  (prop/for-all [v (gen/vector gen/int)]
    (= (sort v) (sort (sort v)))))

(tc/quick-check 100 sort-idempotent-prop)
;; => {:result true, :num-tests 100, :seed 1382488326530}

The remaining examples need no further changes. Integrating with cljs.test is via the clojure.test.check.clojure-test/defspec macro, in the same fashion as integration with clojure.test on the jvm.

Developer Information

We can not accept pull requests. Please see CONTRIBUTING.md for details.



test.check runs in both jvm-clojure and clojurescript, so testing comprehensively requires several steps:

  • Run lein test to run the JVM tests (requires Leiningen)
  • Run lein cljsbuild once to run the ClojureScript tests (also requires node.js)
  • To run the same tests in a web browser, open (after running the above command) test-runners/run_tests_dev.html and test-runners/run_tests_adv.html and watch the javascript console for output
  • Run script/test-self-host to run the self-hosted ClojureScript tests (also requires node.js)

See also

Other implementations



test.check used to be called simple-check.

See migrating from simple-check.




YourKit is kindly supporting test.check and other open source projects with its full-featured Java Profiler. YourKit, LLC is the creator of innovative and intelligent tools for profiling Java and .NET applications. Take a look at YourKit's leading software products:


Copyright © 2014 Rich Hickey, Reid Draper and contributors

Distributed under the Eclipse Public License, the same as Clojure.