defmodule DSL do

  defmacro extend_struct struct_mod, keyword do
    quote do
      defstruct Keyword.merge(Map.to_list(Map.from_struct(unquote(struct_mod).__struct__)), unquote(keyword))
    end
  end

end

defmodule DSLTest do
  use ExUnit.Case

  defmodule BaseStruct do
    defstruct a: 1, b: "two"
  end

  defmodule TestStruct do
    import DSL

    extend_struct BaseStruct, some: "extra", key: "with overrides", b: "three"
  end

  test "it can merge structures" do
    ts = %TestStruct{}
    map = Map.from_struct ts
    assert map == %{a: 1, b: "three", key: "with overrides", some: "extra"}
  end

end