macro.ex

defmodule MacroFun do

  defmacro a(str, do: block) do
    res = [a: str]
    {:__block__, _, calls} = block
    res = Enum.reduce calls, res, fn
      {f, c, a}, res when f in [:b, :c] -> {f, c, [res|a]}
    end
    res
  end

  def b(res, str) do
    res ++ [b: str]
  end

  def c(res, str) do
    res ++ [c: str]
  end

end

defmodule Run do
  import MacroFun

  def run do
    result = a("hello") do
      b("bleh")
      c("blah")
    end
    IO.inspect result
  end
end

Run.run

> elixir macro.ex 
[a: "hello", b: "bleh", c: "blah"]