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Mox is a library for defining concurrent mocks in Elixir.

The library follows the principles outlined in "Mocks and explicit contracts", summarized below:

  1. No ad-hoc mocks. You can only create mocks based on behaviours

  2. No dynamic generation of modules during tests. Mocks are preferably defined in your test_helper.exs or in a setup_all block and not per test

  3. Concurrency support. Tests using the same mock can still use async: true

  4. Rely on pattern matching and function clauses for asserting on the input instead of complex expectation rules

See the documentation for more information.


Just add mox to your list of dependencies in mix.exs:

def deps do
    {:mox, "~> 1.0", only: :test}

Mox should be automatically started unless the :applications key is set inside def application in your mix.exs. In such cases, you need to remove the :applications key in favor of :extra_applications or call Application.ensure_all_started(:mox) in your test/test_helper.exs.

Enforcing consistency with behaviour typespecs

Hammox is an enhanced version of Mox which automatically makes sure that calls to mocks match the typespecs defined in the behaviour. If you find this useful, see the project homepage.


Copyright 2017 Plataformatec
Copyright 2020 Dashbit

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


Mocks and explicit contracts in Elixir



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