Required private modules don't survive quote blocks

[the-mikedavis]

Minimum example here.

If we define a private module:

# `lib/using_private/impl.ex`
import Defmodulep

defmodulep UsingPrivate.Impl, visible_to: [UsingPrivate] do
  # implementation functions for the `__using__/1` macro in
  # `lib/using_private.ex`

  def hello do
    IO.puts("hello")
  end
end

And define a quoted block that uses that private module:

# `lib/using_private.ex`
defmodule UsingPrivate do
  defmacro __using__(_opts) do
    quote do
      import Defmodulep
      requirep UsingPrivate.Impl, as: Impl

      def hello, do: Impl.hello()
    end
  end
end

And finally use that quoted block in a fresh module

# `lib/using_private/bad.ex`
defmodule UsingPrivate.Bad do
  use UsingPrivate
end

We get a compiler warning:

$ mix compile --force
==> defmodulep
Compiling 1 file (.ex)
Generated defmodulep app
==> using_private
Compiling 3 files (.ex)
warning: function Impl.hello/0 is undefined (module Impl is not available)
  lib/using_private/bad.ex:2

Generated using_private app

And an error in iex

iex(1)> UsingPrivate.Fixture.hello
** (UndefinedFunctionError) function Impl.hello/0 is undefined (module Impl is not available)
    Impl.hello()

The module is not defined!

If we write the same code without the quoted block, though, everything works as expected:

# `lib/using_private/good.ex`
defmodule UsingPrivate.Good do
  import Defmodulep
  requirep UsingPrivate.Impl, as: Impl

  def hello, do: Impl.hello()
end

[the-mikedavis]

The expansions:

-file("/vagrant/code/using_private/lib/using_private"
      "/good.ex",
      1).

-module('Elixir.UsingPrivate.Good').

-compile([no_auto_import]).

-export(['__info__'/1, hello/0]).

-spec '__info__'(attributes | compile | functions |
                 macros | md5 | module | deprecated) -> any().

'__info__'(module) -> 'Elixir.UsingPrivate.Good';
'__info__'(functions) -> [{hello, 0}];
'__info__'(macros) -> [];
'__info__'(Key = attributes) ->
    erlang:get_module_info('Elixir.UsingPrivate.Good', Key);
'__info__'(Key = compile) ->
    erlang:get_module_info('Elixir.UsingPrivate.Good', Key);
'__info__'(Key = md5) ->
    erlang:get_module_info('Elixir.UsingPrivate.Good', Key);
'__info__'(deprecated) -> [].

hello() -> 'Elixirp.UsingPrivate.Impl':hello().

-file("/vagrant/code/using_private/lib/using_private"
      "/bad.ex",
      1).

-module('Elixir.UsingPrivate.Bad').

-compile([no_auto_import]).

-export(['__info__'/1, hello/0]).

-spec '__info__'(attributes | compile | functions |
                 macros | md5 | module | deprecated) -> any().

'__info__'(module) -> 'Elixir.UsingPrivate.Bad';
'__info__'(functions) -> [{hello, 0}];
'__info__'(macros) -> [];
'__info__'(Key = attributes) ->
    erlang:get_module_info('Elixir.UsingPrivate.Bad', Key);
'__info__'(Key = compile) ->
    erlang:get_module_info('Elixir.UsingPrivate.Bad', Key);
'__info__'(Key = md5) ->
    erlang:get_module_info('Elixir.UsingPrivate.Bad', Key);
'__info__'(deprecated) -> [].

hello() -> 'Elixir.Impl':hello().

Looks like the aliasing of Impl is changed by the quoted block in bad.ex

[the-mikedavis]

Workaround:

defmodule UsingPrivate do
  defmacro __using__(_opts) do
    quote do
      require :"Elixirp.UsingPrivate.Impl", as: Impl
      def hello, do: Impl.hello()
    end
  end
end

But as you specifically say,

Note that bypassing such behaviour is extremely discouraged otherwise

[josevalim]

I am not quite sure if this is fixable. ;( have you tried using requirep outside of the quote, right above it, instead of inside? -- *José Valim* www.plataformatec.com.br Skype: jv.ptec Founder and Director of R&D

[the-mikedavis]

Aha @josevalim! I swore I tried that 😅

# `lib/using_private.ex`
defmodule UsingPrivate do
  defmacro __using__(_opts) do
    quote do
      def hello, do: Impl.hello()
    end
  end
end

# `lib/using_private/bad.ex`
defmodule UsingPrivate.Bad do
  import Defmodulep
  requirep UsingPrivate.Impl, as: Impl
  use UsingPrivate
end

$ mix compile --force
Compiling 4 files (.ex)
Generated using_private app

iex> UsingPrivate.Bad.hello
hello
:ok 

Works perfectly.

I notice that requireing Logger, for example, works just fine. Would this be something fixed by implementing defmodulep in core Elixir?

[josevalim]

We would need to make requirep part of Elixir, yeah. -- *José Valimwww.plataformatec.com.br <http://www.plataformatec.com.br/>Founder and Director of R&D*