Clarify the current use of mock

[jasagredo]

There is a comment in the readme that mentions that mock is a hack for some reasons, and points to this issue advancedtelematic#236 in which one might get the impression that mock is actually not used in the library and one can just mock whatever answers if they don't use symbolic references.

Because this isn't actually
used by the library yet, it's not important for mock to return the correct
responses, e.g. having mock for Read return ReadValue 0 is completely
fine.

I should have added that the only important part is that Reference responses are generated by genSyms, because that is what increases the state counter that you mentioned in the beginning which is used for generating commands...

But for now, while mock is only used for genSyming references, it doesn't really matter.

I got the understanding that as I don't use references yet, I could just define the following:

mock :: Model Symbolic
     -> Action Symbolic
     -> GenSym (Response Symbolic)
mock _ _ = pure ResponseOk

But this catastrophically fails on the transition function if you don't expect that response for every command. I have the suspicion that didn't happen in the linked issue because the system didn't have to shrink as the test succeeded. Once you try to shrink, you would get incomprehensible pairings of cmd and resp (unless one realizes that mock is actually used!!).

The understanding I had was wrong, as there are indeed some uses of mock that require you to actually implement mock. I hope these changes in the README might give some hints to the reader to avoid future misunderstandings.

[stevana]

Thanks for the clarification!

I've forgot the details around mock to be honest, I wanted to remove it at some point as I saw it as a failed experiment, but Edsko's lockstep technique depends on it.