[erlang-questions] mockgyver -- yet another mocking library

Klas Johansson <>
Sun Nov 6 12:19:30 CET 2011

Hi Adam,

On Fri, Nov 4, 2011 at 4:54 PM, Adam Lindberg
<> wrote:

> Nice to see more mocking tools popping up!


>  - Is it tied to EUnit or generic?

It's generic (the ?MOCK macro is generic, although the
?WITH_MOCKED_SETUP macro is intended for use from eunit).

>  - What happens when a function is called with other parameters than those expected?

Nothing.  If I write a "guard" like this:

    ?WAS_CALLED(lists:nth(2, [a, b, c])),

that call will succeed if the function was called with those
parameters once.  If it was called with those parameters for example
twice (or never) it'll fail (like an assert macro in eunit).  If it
was called once with those parameters, and once with another set of
parameters it still succeeds.

If you want to check that the function was only called once with those
parameters and no other parameters you can do something like this:

   [[2, [a, b, c]]] = ?WAS_CALLED(lists:nth(_, _)),

since ?WAS_CALLED (as well as ?WAIT_CALLED and ?GET_CALLS) will always
return a list of argument lists.

Not as concise.  Perhaps a better syntax for those cases will make it
into a future version. :-)

>  - How's the beam renaming working out for you so far? Have you seen any cases in
>   where it hasn't been able to find all occurrences of a module name?

So far it's done its job, but it's not really bullet proof.  Haven't
had the need to call the original module frequently enough to bump
into problems.  The algorithm only replaces the atom within the atom
table, but not within the constant pool.  mockgyver works for modules
both with and without debug_info, but I've been thinking of changing

* use debug_info when available
* otherwise, resort to replacing within the beam code

It's not been an issue so far, but if people start using this and bump
into problems I'll prioritize it.

>  - really nice syntax (thanks to parse transforms).


> Is there a functional API?

Yes and no.  There is, but it's not documented and may change.
However, nothing is set in stone. If there are good reasons for adding
such an interface it'll be done.


> On 2011-11-02, at 21:47 , Klas Johansson wrote:
>> Hi all,
>> I'd like you to meet mockgyver -- an Erlang tool which will make it
>> easier to write EUnit tests that need to replace or alter (stub/mock)
>> the behaviour of other modules.
>> mockgyver aims to make that process as easy as possible with a
>> readable and concise syntax.
>> mockgyver is built around two main constructs: ?WHEN which makes it
>> possible to alter the behaviour of a function and another set of
>> macros (like ?WAS_CALLED) which check that a function was called with
>> a chosen set of arguments.  Let's redefine pi to 4:
>>       ?WHEN(math:pi() -> 4),
>>       4 = math:pi(),
>> Use pattern matching to check that a function was called with certain arguments:
>>       ?WAS_CALLED(lists:reverse([a, b, c])),
>> ... or if you don't care about the arguments:
>>       ?WAS_CALLED(lists:reverse(_)),
>> The library has been in use for a year for a bunch of Erlang
>> applications, except for a couple of recent additions.
>> A short tutorial as well as docs and many more examples[1] in markdown
>> format on github:
>>    https://github.com/klajo/mockgyver
>> Cheers,
>> Klas
>> [1] https://github.com/klajo/mockgyver/blob/master/doc/mockgyver.md
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