test.ex

defmodule TestApp.Serializer do
  defmacro __using__(_) do
    quote do
      defimpl Msgpax.Packer, for: __MODULE__ do
        def transform(struct) do
          type = String.replace_prefix(to_string(__MODULE__), "Elixir.Msgpax.Packer.TestApp.Data.", "")
          Msgpax.Packer.Map.transform(Map.merge(Map.from_struct(struct), %{"__type" => type}))
        end
      end
    end
  end
end