How to mock objects in Erlang using Meck?

Question

Okay, I'm using Meck and I'm lost. My first language (that I've been writing for about 7 months) is Ruby, so I can't seem to wrap my brain around Meck mocking yet. I do get Ruby mocking though. Hoping someone can help me. Also, I've only been writing Erlang for a week.

Updated Code (but mocking still isn't working)...

I have a Erlang ~~console_io~~ prompter module that looks like this:

    -module(prompter).
    -export([prompt/1, guess/0]).

    prompt(Message) ->
      console_io:gets(Message).

    gets() ->
      {_, [Input]} = io:fread("Enter:  ", "~s"),
      Input.

    guess() ->
      Guess_Input = gets(),
      Guess_List = convert_guess_to_list(Guess_Input).

    convert_guess_to_list(Guess_Input) ->
      re:split(Guess_Input, "", [{return, list}, trim]).

My test now looks like this:

    -module(prompter_test).
    -include_lib("eunit/include/eunit.hrl").

    guess_1_test() ->
      meck:new(prompter),
      meck:expect(prompter, gets, fun() -> "aaaa" end),
      ?assertEqual(prompter:guess(), ["a","a","a","a"]),
      ?assert(meck:validate(prompter)),
      meck:unload(prompter).

The error I'm getting is this:

    Eshell V5.9.3.1  (abort with ^G)
    1> prompter_test: guess_1_test (module 'prompter_test')...*failed*
    in function prompter:guess/0
      called as guess()
    in call from prompter_test:guess_1_test/0 (test/prompter_test.erl, line 10)
    in call from prompter_test:guess_1_test/0
    **error:undef

I want to mock (stub?) the gets function in my test so that gets will return "aaaa" and then when I assert on get_guess() it should equal ["a", "a", "a", "a"].

How do I do this?

score 12 · Accepted Answer · answered May 13 '13 at 10:28

There are two problems:

The prompter module has two exported functions, but you only mock one of them (gets) with meck:expect. By default, Meck creates a new module that only contains the functions that you explicitly mock. You can change that by using the passthrough option:
```
meck:new(prompter, [passthrough]),
```
When you mock the gets function, all module-prefixed calls (i.e. prompter:gets()) are intercepted, but Meck has no way (yet?) of intercepting internal calls (e.g. the gets() call in the guess function), so you would still get the unmocked version of the function. There is no completely satisfactory way to avoid this. You could change the call in guess to prompter:gets(), or you could move gets into a separate module and mock that.

Thank you so much! I tried to do add the module name to the function in the call ("change the call in guess to prompter:gets()"). That didn't work unfortunately. So, I ended up pulling out gets() to its own separate module that I could then mock per the instructions from kjw0188. — Kelly, May 13 '13 at 19:05

score 6 · Answer 2 · answered May 12 '13 at 19:32

The first line says to create a new mocked module, my_library_module:

    meck:new(my_library_module),

Next, we mock the function fib in my_library_module to return 21 when 8 is passed in:

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

We have some eunit assertions to test our mocked function. The code_under_test:run call is what you want to replace with the the function using your mocked module, and the 21 is the result you are expecting from the function call:

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

Then we unload the mocked module:

    meck:unload(my_library_module).

If you wanted to write the same test for your module, you could write:

my_test() ->
    meck:new(console_io),
    meck:expect(console_io, gets, fun() -> "aaaa" end),
    ?assertEqual(["a", "a", "a", "a"], console_io:get_guess()), % Uses console_io
    ?assert(meck:validate(console_io)),
    meck:unload(console_io).

Thank you so much! I totally get it now. However, the test still is not passing. I updated the question above with my new code/test and the error I'm getting. Hoping you can help me out. — Kelly, May 12 '13 at 22:13
What does meck:validate/1 actually do in the above examples? I can't work it out — , Jun 01 '17 at 09:15
There's another answer that might answer your question: https://stackoverflow.com/questions/43279705/whats-the-point-of-meckvalidate — kjw0188, Jun 14 '17 at 19:42

How to mock objects in Erlang using Meck?

2 Answers2