20170508_expanding_macros.md

Elixir - Expanding macros

Today I learned a thing about macros and how they expand.

I thought that the Elixir AST at compile time would expand any macros until only non-macro code was left.

But I was wrong. Elixir will only expand nested macros, when the inner most macro call is the body of the outer macro.

As an example look at this code:

defmodule Macros do
  defmacro delegate_to_forty_two do
    quote do
      Macros.forty_two
    end
  end

  defmacro forty_two do
    quote do
      42
    end
  end

  defmacro does_not_fully_expand do
    quote do
      Macros.forty_two + Macros.forty_two
    end
  end
end

We can use Macro.expand/2 and Macro.to_string/1 to see how the macros expand:

iex(4)> quote(do: Macros.delegate_to_forty_two) |> Macro.expand(__ENV__) |> Macro.to_string
"42"

This fully expands to 42. When trying with Macros.does_not_fully_expand in which :+ is the body of the quoted expression, we will get another result:

iex(5)> quote(do: Macros.does_not_fully_expand) |> Macro.expand(__ENV__) |> Macro.to_string
"Macros.forty_two() + Macros.forty_two()"

I would have expected this to expand to 42 + 42, but I was wrong and I learned something new today.