I have also done some experiments with the BEAM file. See the little beam_file project.

With BeamFile you can create byte, erl, and elixir-code (with the limitation discussed here) from the beam file.

Example:

defmodule Example.Math do
  @moduledoc "Math is Fun"

  def add(number_a, number_b), do: number_a + number_b

  def odd_or_even(a) do
    if rem(a, 2) == 0 do
      :even
    else
      :odd
    end
  end
end

iex> {:ok, code} = BeamFile.elixir_code(Example.Math)
iex> IO.puts(code)
defmodule Elixir.Example.Math do
  @moduledoc """
  Math is Fun
  """

  def add(number_a, number_b) do
    :erlang.+(number_a, number_b)
  end

  def odd_or_even(a) do
    case(:erlang.==(:erlang.rem(a, 2), 0)) do
      false ->
        :odd

      true ->
        :even
    end
  end
end

My project works with a lot of code but not all code:

The problem is that the transformation from Elixir to BEAM bytecode loses information. A simple example:

a = 1
a = 2

will result in bytecode somewhat like

a = 1
a1= 2

Because rebinding variables isn’t possible in bytecode, so Elixir fakes it. There are plenty of such examples. Decompiling BEAM bytecode to Erlang is fairly straightforward and precise, but getting back to the original Elixir AST is, I think, almost impossible. The simpler solution would be to just stash the AST in a BEAM segment (the format is pretty extensible so while I’m not sure whether this is already happening somewhere, I’m sure it is not too hard to do this).