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clojure / test.check


QuickCheck for Clojure



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.

test.check used to be called simple-check.

Releases and Dependency Information

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.


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



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

Version numbers

test.check version numbers start where simple-check left off: 0.5.7.


Migrating from simple-check

See migrating from simple-check.

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 '[ :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]
            [ :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.

Release Notes

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

See also...

Other implementations



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Copyright © 2014 Rich Hickey, Reid Draper and contributors

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