# Module aliasing in macros

**URL:** <https://forum.elixirforum.com/t/module-aliasing-in-macros/10644>\
**Category:** Questions\
**Tags:** alias, macros\
**Created:** [December 6, 2017, 4:05pm UTC](https://forum.elixirforum.com/t/module-aliasing-in-macros/10644 "2017-12-06T16:05:36Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![nmichel](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/nmichel/32/8744_2.png) [@nmichel](https://forum.elixirforum.com/u/nmichel)\
**Post date:** [December 6, 2017, 4:05pm UTC](https://forum.elixirforum.com/t/module-aliasing-in-macros/10644/1 "2017-12-06T16:05:36Z")

</div>

Hi

There is something I don’t get about macros, modules and aliasing.  
Consider the following code snippets.

Module `Foo` exports macro `show/1` that just log the received AST fragment on standard output.

```elixir
defmodule Foo do
    defmacro show(frag) do
        IO.puts "#{inspect frag}"
        frag
    end
end

```

Then consider the following two useless modules `Outer` and `Outer.Inner`.

```elixir
defmodule Outer do
    defmodule Inner do
        def foo do
            :foo
        end
    end
end

```

Now lets play with aliasing.

```elixir
defmodule Test do
    import Foo

    alias Outer, as: A
    alias Outer.Inner, as: B
    alias A.Inner, as: C
    
    show A
    show B
    show C
    show C.foo()
    
    ast = quote do: A
    IO.puts "ast #{inspect ast}"
end

```

Here is the console log when I compile the code.

```elixir
iex(1)> c("./test/demo.ex")
{: __aliases__ , [counter: 0, line: 23], [:A]} # No aliasing info
{: __aliases__ , [counter: 0, line: 24], [:B]}
{: __aliases__ , [counter: 0, line: 25], [:C]}
{{:., [line: 26], [{: __aliases__ , [counter: 0, line: 26], [:C]}, :foo]}, [line: 26], []}
ast {: __aliases__ , [alias: Outer], [:A]} # quote => Aliasing info !
[Test, Outer, Outer.Inner, Foo]
iex(2)> 

```

What puzzles me, is that in `show/1` macro calls, module fragments do not hold any aliasing information, while when quoted, it does.

Why don’t I have aliasing information during macro expansion ?

Many thanks !

Nicolas -

---

<div class="post-metadata">

**Author:** ![nmichel](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/nmichel/32/8744_2.png) [@nmichel](https://forum.elixirforum.com/u/nmichel)\
**Post date:** [January 3, 2018, 10:53am UTC](https://forum.elixirforum.com/t/module-aliasing-in-macros/10644/2 "2018-01-03T10:53:14Z")

</div>

Hi,

I pursued my investigations further, and modified the `Foo.show/1` function as follow.

```elixir
defmodule Foo do
    defmacro show(frag = {: __aliases__ , _, refs}) do
        resolved = Macro.expand(frag, __CALLER__ )
        IO.puts """
        Alias
        frag \t #{inspect frag}
        resolved #{inspect resolved}
        """
        frag
    end

    defmacro show(frag = {{:., _, [path, target]}, _, _}) do
        resolved = Macro.expand(path, __CALLER__ )
        IO.puts """
        Call
        path \t #{inspect path}
        resolved #{inspect resolved}#{inspect target}
        """
        frag
    end
end

```

Basically I catch fragments bearing aliasing information to make some experiments.

Here is the output

```elixir
Alias
frag {: __aliases__ , [counter: 0, line: 38], [:A]}
resolved Outer

Alias
frag {: __aliases__ , [counter: 0, line: 39], [:B]}
resolved Outer.Inner

Alias
frag {: __aliases__ , [counter: 0, line: 40], [:C]}
resolved Outer.Inner

Call
path {: __aliases__ , [counter: 0, line: 41], [:C]}
resolved Outer.Inner:foo

ast {: __aliases__ , [alias: Outer], [:A]}

```

We can see that, though fragments do not bear aliasing information, `Macro.expand` resolves aliases at compile time.

Cheers

Nicolas -

P.S. The question may rise of knowing WHY I needed to have those resolved symbols. Well, I wanted to be able to take some decisions based on the knowledge that a function is exported by a certain module … it proved to be a wrong way, but the underlying purely technical is still interesting 🙂