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mockfn is a library supporting mockist test-driven-development in Clojure. It is meant to be used alongside a regular testing framework such as clojure.test.

Framework-agonostic usage

In order to use mockfn, it's enough to require it in a test namespace.

(:require [mockfn.macros :as mfn]
          [mockfn.matchers :as matchers]

This will bring mockfn features in scope for this namespace.

Stubbing Function Calls

The providing macro replaces functions with mocks. These mocks return preconfigured values when called with the expected arguments.

(testing "providing"
  (providing [(one-fn) :result]
    (is (= :result (one-fn)))))

As presented below, a mock (one-fn) can be configured with different returns for different arguments.

(testing "providing - one function, different arguments"
  (providing [(one-fn :argument-1) :result-1
              (one-fn :argument-2) :result-2]
    (is (= :result-1 (one-fn :argument-1)))
    (is (= :result-2 (one-fn :argument-2)))))

It's also possible to configure multiple mocks, for multiple functions, at once.

(testing "providing with more than one function"
  (providing [(one-fn :argument) :result-1
              (other-fn :argument) :result-2]
    (is (= :result-1 (one-fn :argument)))
    (is (= :result-2 (other-fn :argument))))))


If you would like to call the value instead of returning it, use mockfn.macros/calling:

(testing "providing - calling mocked value as a function"
  (providing [(one-fn) (calling #(throw (ex-info "one-fn failed" {:args 'none})))]
    (is (thrown? ExceptionInfo (one-fn)))))


When mocking it is sometimes useful to set some mocks to point to their original implementation. This can be done by using mockfn.macros/unmocked:

(testing "providing - using unmocked to default to original function"
  (providing [(one-fn :argument-1) unmocked
              (one-fn :argument-2) :result-2]
    (is (thrown? ExceptionInfo (one-fn)))))

Private functions

Private functions can be mocked by following Clojure's convention of referring them by their var, prefixing them with #'.

(testing "providing - private function"
  (providing [(#'private-fn) :result]
    (is (= :result (#'private-fn)))))

Verifying Interactions

The verifying macro works similarly, but also defines an expectation for the number of times a call should be performed during the test. A test will fail if this expectation is not met.

(testing "verifying"
  (verifying [(one-fn :argument) :result (exactly 1)]
    (is (= :result (one-fn :argument)))))

Notice that the expected number of calls is defined with a matcher.

Argument Matchers

Mocks can be configured to return a specific value for a range of different arguments through matchers.

(testing "argument matchers"
  (providing [(one-fn (at-least 10) (at-most 20)) 15]
    (is (= 15 (one-fn 12 18))))))

Syntax sugar for clojure.test

Support for clojure.test is provided in the mockfn.clj-test namespace.

(:require [clojure.test :refer :all]
          [mockfn.clj-test :as mfn]
          [mockfn.matchers :as matchers]

The mockfn.clj-test/deftest and mockfn.clj-test/testing macros replace clojure.test/deftest and clojure.test/testing and support a flatter (as in not nested) mocking style using mockfn.clj-test/providing and mockfn.clj-test/verifying:

(mfn/deftest deftest-with-builtin-mocking
  (is (= :one-fn (one-fn)))
    (one-fn) :one-fn)

  (mfn/testing "testing with built-in-mocking"
    (is (= :one-fn (one-fn)))
    (is (= :other-fn (other-fn)))
      (other-fn) :other-fn (exactly 1))))

Notice that in order to leverage the built-in support for mocking in these macros, it's necessary to use the providing and verifying versions provided at the mockfn.clj-test namespace.

Built-in Matchers

The following matchers are included in mockfn:


(exactly expected)

Matches if actual value is equal to the expected value.


(at-least expected)

Matches if actual value is greater or equal than the expected value.


(at-most expected)

Matches if actual value is less or equal than the expected value.



Always matches.


(a expected)

Matches if actual value is an instance of the expected type.


(pred odd?)

Matches when the predicate applied to the actual value results in a truthy value.

Functions will automatically be interpreted as pred matchers, and hence don't need to explicitly be wrapped in the pred matcher.

External Matchers

Functions placed in the argument position of mocks will be treated as pred matchers by default. Thus, one can make use of external predicate functions to perform matching.


matcher-combinators is a library for comparing nested datastructures and pretty printing mismatch diffs. It can be used to specify mockfn argument matching behavior, for example:

(require '[matcher-combinators.standalone :refer [match?]]
         '[matcher-combinators.matchers :as m])

(testing "matcher-combinator example"
  (providing [(one-fn (match? (m/in-any-order [1 3 2]))) :result-unordered]
    (is (= :result-unordered (one-fn [3 2 1])))))

Quirks and Limitations

While providing and verifying calls can be nested, all required stubs and expectations for a single mock must be defined in the same call. Mocking a function in a inner providing or verifying call will override any definitions made in the outer scope for the tests being run in the inner scope.

(testing "nested mocks"
  (providing [(one-fn :argument-1) :result-1]
    (providing [(one-fn :argument-2) :result-2
                (other-fn :argument-3) :result-3]
      (is (thrown? ExceptionInfo (one-fn :argument-1)))
      (is (= :result-2 (one-fn :argument-2)))
      (is (= :result-3 (other-fn :argument-3))))
    (is (= :result-1 (one-fn :argument-1))))))