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A mocking library for Erlang

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A mocking library for Erlang.


With meck you can easily mock modules in Erlang. Since meck is intended to be used in testing, you can also perform some basic validations on the mocked modules, such as making sure no function is called in a way it should not.

meck automatically renames existing modules in case they are loaded when you want to mock them, and restores them upon unloading of the mocked module. It is also possible to call the original functions from a mocked module using meck:passthrough/1 from inside an expectation.


meck requires rebar to build. Either install rebar by building it manually and putting it in your path or by using Agner (agner install rebar).

To build meck, got to the meck directory and simply type:

$ rebar compile

Make sure meck works on your platform (requires the eunit application, which is included by default in Erlang):

$ rebar eunit

Two things might seem alarming when running the tests: 1. Warnings emitted by cover and 2. an exception printed by SASL. Both are expected due to the way Erlang currently prints errors. The important line you should look for is All XX tests passed, if that appears all is correct.


To install meck permanently, use of Agner is recommended:

$ agner install meck

If you want to install your own built version of meck add the ebin directory to your Erlang code path or move the meck folder into your release folder and make sure that folder is in your ERL_LIBS environment variable.


Here's an example of using meck in the Erlang shell:

Eshell V5.7.5  (abort with ^G)
1> meck:new(dog).
2> meck:expect(dog, bark, fun() -> "Woof!" end).
3> dog:bark().
4> meck:validate(dog).

Exceptions can be anticipated by meck (resulting in validation passing). This is intended to be used to test code that can and should handle certain exceptions indeed does take care of them:

5> meck:expect(dog, meow, fun() -> meck:exception(error, not_a_cat) end).
6> catch dog:meow().
7> meck:validate(dog).

Normal Erlang exceptions result in a failed validation. The following example is just to demonstrate the behavior, in real test code the exception would normally come from the code under test (which should, if not expected, invalidate the mocked module):

8> meck:expect(dog, jump, fun(Height) when Height > 3 ->
                             (Height) ->
9> dog:jump(2).
10> catch dog:jump(5).
11> meck:validate(dog).

Here's an example of using meck inside an EUnit test case:

my_test() ->
    meck:expect(library_module, fib, fun(8) -> 21 end),
    ?assertEqual(21, code_under_test:run(fib, 8)),

Pass-through is used when the original functionality of a module should be kept. When the option passthrough is used when calling new/2 all functions in the original module will be kept in the mock. These can later be overridden by calling expect/3 or expect/4.

Eshell V5.7.5  (abort with ^G)
1> code:unstick_mod(string).
2> meck:new(string, [passthrough]).
3> string:strip("  test  ").

It's also possible to pass calls to the original function allowing us to override only a certain behavior of a function (this usage is compatible with the passthrough option). passthrough/1 will always call the original function with the same name as the expect is is defined in):

Eshell V5.7.5  (abort with ^G)
1> code:unstick_mod(string).
2> meck:new(string).
3> meck:expect(string, strip, fun(String) -> meck:passthrough([String]) end).
4> string:strip("  test  ").
5> meck:unload(string).
6> string:strip("  test  ").


Patches are greatly appreciated!

Should you find yourself using meck and have issues, comments or feedback please create an issue.

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