Proposal: implementation of struct: option for defmock

[ityonemo]

In some cases I have found myself wanting to be able to clone a struct from an implementation module to build a mock. This PR creates a :struct option for defmock, which drags the source module's struct into the mock implementation. Before cloning, the struct-bearing module is checked to see if it matches the behaviours in the mock module. Hopefully this fits in the spirit of mocks being nouns, not verbs, and doesn't interfere too much with the nominal operation of mox!

[josevalim]

Hi @ityonemo. Apologies but I am not convinced this should be the case.

If there is a struct in your mock, then the struct is part of the contract, an it should be properly defined as part of your expectations. Ideally, since this is a mock, I don't think you should be using structs at all, and instead define the proper functions that work on these values.

[ityonemo]

no problem! I just had a protocol functions that detected %module{} and wanted to stub them... I'm probably just using protocols wrong.

eg:

defprotocol Proto do
  def func(t)
end

defimpl Proto, for: MyModule do
  def func(thing = %module{}) do
   # lets me selectively stub in implementation testing
    module.do_something(thing)
    module.do_something_else(thing)
  end
end

defmock Mock, for: MyModule.Api, struct: MyModule
defimpl Proto, for: Mock do
  defdelegate func(thing), to: MyModule
end