Erlang Testing (Non Exported / Private) function of module using common test

Question

I have a module in Erlang which has functions that are not exported by Erlang. How can I test / call these functions using common test framework?

Answer 1

It's tricky with Common Test, but it's possible to use embedded EUnit test cases to test private functions in a module. You can then test the public interface using Common Test. Rebar will automagically discover embedded test cases when you run rebar test.

Here's an example:

-module(example).

-export([public/1]).

-ifdef(TEST).
-include_lib("eunit/include/eunit.hrl").
-endif.

%% This function will be tested externally using Common Test
public(Foo) ->
    private(Foo + 42).

%% This function is not reachable to CT, so it will be tested using EUnit.
private(Bar) ->
    Bar * 2.

%%% Tests
-ifdef(TEST).

private_test() ->
    ?assertEqual(10, private(5)),
    ?assertEqual(0, private(0)).

-endif.

On a side note, you might find Meck to your liking if you need to mock away a module (or parts thereof) when testing with EUnit.

For a gentle introduction to EUnit, see the Learn You Some Erlang chapter.

Answer 2

It is impossible. You can use -ifdef(TEST). preprocessor condition to export those functions only when compiling for testing.

Depending on your tooling, you might need to explicitly provide that TEST macro while compiling the modules. You can do that by using {d,'TEST'} compiler option or -DTEST compilation flag.