Elixir V 1.18 behaviours and different builds warnings

Questions / Help
1

Hello all,

Where I work, we like to inject module dependencies during compile time.
So for example, I can have MyModule for dev and prod builds, but for test I might have MyModuleStub.

So let’s say I have a code like this:

# ...
@the_module Application.compile_env!(:my_app, :the_module)
#...
case @the_module.execute() do
  {:ok, response} -> do_something(response)
  :error -> do_something_else()
end

It works fine in dev and prod, however in test the module is a simplified version:

defmodule MyModuleStub do
# ...
  def execute, do: {:ok, fixture()}
end

Then, when compiling in test, we get the following warning:

     warning: the following clause will never match:

         :error

     because it attempts to match on the result of:

        MyModuleStub.execute()

     which has type:

         none()

     typing violation found at:
     │
     │       :error ->
     │       ~~~~~~~~~~~~~~~~

It seems the compiler warning is right…however, the situation is less than an ideal. Ideally, we should have a way to tell the compiler that we’re talking about a behaviour, or it should understand alone. Is there any way of suppressing this warning? Have you used modules this way? How did you work around for Elixir v1.18 warnings?

2

I’m not sure how to solve this with the current capabilities, but I think it’s funny that the main complaint about the type system is that it is too smart :grinning:

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3
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4

Thank you. It is what we ended up doing.

5

Another option, if Process.get(...) is too magical, is to pretend that you can return something else:

if :rand.uniform() >= 0 do
  # always true
  {:ok, ...}
else
  :error
end

Once we add behaviours to the type system, this warning will disappear, but for now we need to rely on one of the following approaches.

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