OCaml mock module generation

Question

We have quite some ocaml code with lots of hard coupled dependencies between modules. Lately, we've been moving to the functor approach to decouple those modules, so if we have module A that depends on module B and C, we go like this in the declaration :

module A: (B:B_Signature) (C:C_Signature) = struct ... end

Like this we can inject a 'mocked up' B or C module, answering those signatures, in module A for unit testing, and can still create the production module with the real B and C modules in it. The problem is that right now we have to type these mocked up modules out by hand, which is a bit of a blocker, as it's usually loads of boilerplate.

So, I'm looking for a way to generate 'mock' modules from an ocaml module, instead of typing them out by hand. What I mean with that is if I have a module with a few functions, having the following signature

   module type A =
    sig
    val f: string -> int -> string
    val g: string -> string -> int
    end

I would like to generate a mock implementation, for example like this

module A_mock =
struct
let f _s _i =
    ""
let g _s1 _s2 =
    0
end

So, this is a module that has both functions, with the same signature, but with implementations of f and g that ignore their arguments and return by default and empty string for function f and 0 for function g.

Those default values, empty string and zero, are just an example. I know that I will want to have this more configurable in the end, I want to create the mock and specify the return values for certain functions, maybe even be able to inspect the arguments with which the mocked functions were called and so on, but for now, for this simple case, I'm looking for a solution other than typing and implementing this all by hand.

I didn't find any frameworks that do this for OCaml.

I was trying to find out how do this maybe through camlp4 and code generation but the documentation seems to be kind of limited and I didn't really see whether it would even be possible.

So, my question, does anybody know of a framework or whether there is a code generation kind of way to create this kind of boilerplate modules from a signature in OCaml?

Kasper

Kapput(kaputt.x9c.fr) has a Mock module . But I doubt it's what you want. Isn't what you want more like a Spy? In which case, I think ppx or camlp4 should solve that need although the learning curve is steep currently. — nlucaroni
Yeah, indeed, it's more of a spy in this simple case, you're right. But in the end I want more "Mockito style", to quote the java counterpart, possibilities (so also telling the function what argument it has to return in which case, and so on and so on, which is more Mock I think). But indeed, the documentation of camlp4 is really not good at all (or I didn't find the good docs yet), so if someone could set me on the correct path regarding this simple spy module or so, that would be great. — Kasper
I recently switched jobs to where they use Mockito, so I'm becoming familiar with the whole Mock and Spy concepts so it's good to be reconfirmed I know what I'm talking about ;). I actually have been thinking a lot about this particular issue over the past few weeks and I have been sitting on it since it may be a waste of time under camlp4 and with ppx on the horizon. — nlucaroni
My favorite resource for learning Camlp4 is ambassadortothecomputers.blogspot.com/p/reading-camlp4.html — lukstafi
Alright, I decided to go the Camlp4 route to create some kind of mocking framework for ocaml based on mockito. If I get something working, I'll push it on github and post a link here, in case somebody would be interested. Right now, struggling a bit with camlp4, uphill battle. — Kasper

Kasper Kasper · Accepted Answer · 2014-02-21T10:03:42

Okay, I created the start of a camp4 library that would create a mock module. There's a wiki page on the github with a bit of explanation, and an example signature file and two scripts checked in on the master branch as well for now. Hope to be able to expand this and make it more robust in time, when we start using it more we'll likely get ideas about it as well.

https://github.com/KasperJanssens/mockaml

OCaml mock module generation

1 Answers