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https://github.com/eproxus/meck

A mocking library for Erlang
https://github.com/eproxus/meck

erlang meck mocking-library testing

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A mocking library for Erlang

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Meck


A mocking library for Erlang





  

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  * [Features](#features)

  * [Examples](#examples)

  * [Use](#use)

  * [Manual Build](#manual-build)

  * [Caveats](#caveats)

  * [Contribute](#contribute)



## Features



See what's new in [0.8 Release Notes][release_notes_0.8].



  * Dynamic return values using sequences and loops of static values

  * Compact definition of mock arguments, clauses and return values

  * Pass through: call functions in the original module

  * Complete call history showing calls, return values and exceptions

  * Mock validation, will invalidate mocks that were not called correctly

  * Throwing of expected exceptions that keeps the module valid

  * Throws an error when mocking a module that doesn't exist or has been

    renamed (disable with option `non_strict`)

  * Support for [Hamcrest][hamcrest] matchers

  * Automatic backup and restore of cover data

  * Mock is linked to the creating process and will unload automatically

    when a crash occurs (disable with option `no_link`)

  * Mocking of sticky modules (using the option `unstick`)



## Examples



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



```erlang

Eshell V5.8.4  (abort with ^G)

1> meck:new(dog, [non_strict]). % non_strict is used to create modules that don't exist

ok

2> meck:expect(dog, bark, fun() -> "Woof!" end).

ok

3> dog:bark().

"Woof!"

4> meck:validate(dog).

true

5> meck:unload(dog).

ok

6> dog:bark().

** exception error: undefined function dog:bark/0

```



Exceptions can be anticipated by Meck (resulting in validation still passing).

This is intended to be used to test code that can and should handle certain

exceptions indeed does take care of them:



```erlang

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

ok

6> catch dog:meow().

{'EXIT',{not_a_cat,[{meck,exception,2},

                    {meck,exec,4},

                    {dog,meow,[]},

                    {erl_eval,do_apply,5},

                    {erl_eval,expr,5},

                    {shell,exprs,6},

                    {shell,eval_exprs,6},

                    {shell,eval_loop,3}]}}

7> meck:validate(dog).

true

```



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):



```erlang

8> meck:expect(dog, jump, fun(Height) when Height > 3 ->

                                  erlang:error(too_high);

                             (Height) ->

                                  ok

                          end).

ok

9> dog:jump(2).

ok

10> catch dog:jump(5).

{'EXIT',{too_high,[{meck,exec,4},

                   {dog,jump,[5]},

                   {erl_eval,do_apply,5},

                   {erl_eval,expr,5},

                   {shell,exprs,6},

                   {shell,eval_exprs,6},

                   {shell,eval_loop,3}]}}

11> meck:validate(dog).

false

```



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



```erlang

my_test() ->

    meck:new(my_library_module),

    meck:expect(my_library_module, fib, fun(8) -> 21 end),

    ?assertEqual(21, code_under_test:run(fib, 8)), % Uses my_library_module

    ?assert(meck:validate(my_library_module)),

    meck:unload(my_library_module).

```



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`.



```erlang

Eshell V5.8.4  (abort with ^G)

1> meck:new(string, [unstick, passthrough]).

ok

2> string:strip("  test  ").

"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 defined in):



```erlang

Eshell V5.8.4  (abort with ^G)

1> meck:new(string, [unstick, passthrough]).

ok

2> meck:expect(string, strip, fun

    ("foo") -> "bar";

    (String) -> meck:passthrough([String])

end).

ok

3> string:strip("  test  ").

"test"

4> string:strip("foo").

"bar"

5> meck:unload(string).

ok

5> string:strip("foo").

"foo"

```



## Use



Meck is best used via [Rebar 3][rebar_3]. Add Meck to the test dependencies

in your `rebar.config`:



```erlang

{profiles, [{test, [{deps, [meck]}]}]}.

```



### Manual Build



Meck uses [Rebar 3][rebar_3]. To build Meck go to the Meck directory

and simply type:



```sh

rebar3 compile

```



In order to run all tests for Meck type the following command from the same

directory:



```sh

rebar3 eunit

```



Documentation can be generated through the use of the following command:



```sh

rebar3 edoc

```



### Test Output



Normally the test output is hidden, but if EUnit is run

directly, two things might seem alarming when running the tests:



  1. Warnings emitted by cover

  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.



## Caveats



### Global Namespace



Meck will have trouble mocking certain modules since it works by recompiling

and reloading modules in the global Erlang module namespace. Replacing a

module affects the whole Erlang VM and any running processes using that

module. This means certain modules cannot be mocked or will cause trouble.



In general, if a module is used by running processes or include Native

Implemented Functions (NIFs) they will be hard or impossible to mock. You may

be lucky and it could work, until it breaks one day.



The following is a non-exhaustive

list of modules that can either be problematic to mock or not possible at

all:



* `erlang`

* `supervisor`

* All `gen_` family of modules (`gen_server`, `gen_statem` etc.)

* `os`

* `crypto`

* `compile`

* `global`

* `timer` (possible to mock, but used by some test frameworks, like Elixir's

  ExUnit)



### Local Functions



A meck expectation set up for a function _f_ does not apply to the module-

local invocation of _f_ within the mocked module. Consider the following module:



```erlang

-module(test).

-export([a/0, b/0, c/0]).



a() ->

  c().



b() ->

  ?MODULE:c().



c() ->

  original.

```



Note how the module-local call to `c/0` in `a/0` stays unchanged even though the

expectation changes the externally visible behaviour of `c/0`:



```erlang

3> meck:new(test, [passthrough]).

ok

4> meck:expect(test,c,0,changed).

ok

5> test:a().

original

6> test:b().

changed

6> test:c().

changed

```



### Common Test



When using `meck` under Erlang/OTP's Common Test, one should pay special

attention to this bit in the chapter on

[Writing Tests](https://erlang.org/doc/apps/common_test/write_test_chapter.html):



> `init_per_suite` and `end_per_suite` execute on dedicated Erlang processes,

> just like the test cases do.



Common Test runs `init_per_suite` in an isolated process which terminates when

done, before the test case runs. A mock that is created there will also

terminate and unload itself before the test case runs. This is because it is

linked to the process creating it. This can be especially tricky to detect if

`passthrough` is used when creating the mock, since it is hard to know if it is

the mock responding to function calls or the original module.



To avoid this, you can pass the `no_link` flag to `meck:new/2` which will unlink

the mock from the process that created it. When using `no_link` you should make

sure that `meck:unload/1` is called properly (for all test outcomes, or

crashes) so that a left-over mock does not interfere with subsequent test

cases.



## Contribute



Patches are greatly appreciated! For a much nicer history, please [write good

commit messages][commit_messages]. Use a branch name prefixed by `feature/`

(e.g. `feature/my_example_branch`) for easier integration when developing new

features or fixes for meck.



Should you find yourself using Meck and have issues, comments or feedback please

[create an issue here on GitHub][issues].



Meck has been greatly improved by [many contributors](https://github.com/eproxus/meck/graphs/contributors)!



### Donations



If you or your company use Meck and find it useful, a [sponsorship][sponsors] or [donations][liberapay] are greatly appreciated!



  

   

   Sponsor on GitHub

   

  

  

    

      Donate using Liberapay

    

  



[release_notes_0.8]: https://github.com/eproxus/meck/wiki/0.8-Release-Notes

[hamcrest]: https://github.com/hyperthunk/hamcrest-erlang

[rebar_3]: https://github.com/erlang/rebar3

[issues]: http://github.com/eproxus/meck/issues

[commit_messages]: http://chris.beams.io/posts/git-commit/

[sponsors]: https://github.com/sponsors/eproxus

[liberapay]: https://liberapay.com/eproxus/