gomock - why do recorder methods use interface{}?

[Jonathan Yu]

Hi everyone,

I've been using gomock to generate mock objects (in source mode), and it's been pretty great - way better than maintaining similar code on my own. One thing that I find curious, though, is that expectations are recorded using a generic interface{} type, rather than the original type.

For example, in the sample mock_user.go file, we have:

func (_m *MockIndex) Anon(_param0 string) {

_m.ctrl.Call(_m, "Anon", _param0)

}

func (_mr *_MockIndexRecorder) Anon(arg0 interface{}) *gomock.Call {

return _mr.mock.ctrl.RecordCall(_mr.mock, "Anon", arg0)

}

Does anyone know why the Recorder interface is generated with interface{}? Doesn't this mean that we lose type safety when defining expectations? After all, you can do: mock.EXPECT().Anon(123) even though the method must be called with a string parameter, right?

Jonathan

--

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[Julian Phillips]

On 05/04/2017 18:06, Jonathan Yu wrote:
> Hi everyone,
>
> I've been using gomock to generate mock objects (in source mode), and
> it's
> been pretty great - way better than maintaining similar code on my own.
> One
> thing that I find curious, though, is that expectations are recorded
> using
> a generic interface{} type, rather than the original type.
>
> For example, in the sample mock_user.go

> <https://github.com/golang/mock/blob/master/sample/mock_user/mock_user.go>

> file, we have:
>
> func (_m *MockIndex) Anon(_param0 string) {
> _m.ctrl.Call(_m, "Anon", _param0)
> }
>
> func (_mr *_MockIndexRecorder) Anon(arg0 interface{}) *gomock.Call {
> return _mr.mock.ctrl.RecordCall(_mr.mock, "Anon", arg0)
> }
>
> Does anyone know why the Recorder interface is generated with
> interface{}?
> Doesn't this mean that we lose type safety when defining expectations?
> After all, you can do: mock.EXPECT().Anon(123) even though the method
> must
> be called with a string parameter, right?

Because you _don't_ have to use a value of the appropriate type when
setting up the expectations, e.g.:

"mock.EXPECT().Anon(gmock.Any())" or
"mock.EXPECT().Anon(gomock.Not("invalid"))"

in fact "mock.EXPECT().Anon("123")" is actually a shortcut for
"mock.EXPECT.Anon(gomock.Eq("123"))"

(see https://godoc.org/github.com/golang/mock/gomock#Matcher)

HTH,

--
Julian

[Jonathan Yu]

Thanks so much for your answer, Julian, that makes total sense! I guess the examples I've seen so far are missing these more "advanced" use cases :)

Appreciate the quick and informative reply.