Strange error reporting when importing module

[rohanpujaris]

I have made a repo with minimal code to generate a strange error. Error seems strange to me. It is generated when we write code as following.

defmodule My.MyModule do
  defmodule My.CustomModule do
  end

  defmacro __using__(_) do
    quote do
      import My.MyModule
    end
  end
end

Below is the repo link with minimal code: https://github.com/rohanpujaris/strangerror_elixir

I know that the thing that I tried is little weird and chances of others trying it is very rare. But it took me nearly 3hr to figure out what was the issue. If I haven't looked into error reported by the elixir, I would have solved it earlier. I feel the error is highly misleading.

When we compile code from above repo it will return below error.

== Compilation error on file lib/strangeerror.ex ==
** (CompileError) lib/strangeerror.ex:2: module My.MyModule.My.MyModule is not loaded and could not be found
    expanding macro: My.MyModule.__using__/1
    lib/strangeerror.ex:2: StrangeError (module)
    (elixir) expanding macro: Kernel.use/1
    lib/strangeerror.ex:2: StrangeError (module)

I was trying to use behaviours and I declared module inside another and after writing few lines of code I compiled the project and got above error. I have also raised question in stackoverflow http://stackoverflow.com/questions/34471538/unable-to-understand-import-via-using-macro-elixir-phoenix/ . But after finding out the reason, I feel like its better to report it here as an issue.

Also there is other thing which i felt was strange. When I transformed above code as below the error was gone.

defmodule My.MyModule do

  defmacro __using__(_) do
    quote do
      import My.MyModule
    end
  end

 defmodule My.CustomModule do
 end
end

[josevalim]

Thank you for the report! When you define:

defmodule My.MyModule do
 defmodule My.CustomModule do
 end

  defmacro __using__(_) do
    quote do
      import My.MyModule
    end
  end
end

In order for My.CustomModule to work, Elixir will define an alias of My pointing to My.MyModule.My, so My.CustomModule becomes My.MyModule.My.CustomModule. That's how aliases and nested modules work, so not a bug. :)

[rohanpujaris]

Thanks for the quick reply.
I understood little bit about how nested module work.
I am still confused. Why does the error says My.MyModule.My.MyModule is not loaded and could not be found. Please can you explain from where does elixir brings My.MyModule.My.MyModule into picture. Also why does above code work if I define nested module My.CustomModule below the __using__ macro. It shouldn't be position dependent. I believe that it might not be a bug, but I am quite worried about the strange error report. I think issue might probably my lack of knowledge with elixir. But still i would like to clear out above things.

[josevalim]

It says My.MyModule.My.Module because your code now has an alias where My expands to My.MyModule.My. The order matters because aliases in Elixir are lexical (like imports and require).

[danhper]

Maybe we could improve Kernel.defmodule documentation about nesting as it is clear by reading it that

defmodule Foo do
  defmodule Bar do
  end
end

will create an alias to Foo.Bar in the scope, but not that

defmodule Foo do
  defmodule Bar.Baz do
  end
end

will create an alias to Foo.Bar which can indeed be confusing for the above example.

[rohanpujaris]

Thanks @josevalim for the explanation. Doubt cleared. I think I ignored the fact that aliase are lexically scoped after reading your first reply. Also i agree with @tuvistavie that improving documentation might help beginner like me.

[rohanpujaris]

Ohh there is an example here http://elixir-lang.org/getting-started/alias-require-and-import.html for explaining alias. Sorry for the trouble. I also updated my answer in SO http://stackoverflow.com/questions/34471538/unable-to-understand-import-via-using-macro-elixir-phoenix/. Thanks @josevalim