required_fields.ex

defmodule RequiredFields do
  defmacro __using__(_opts) do
    quote do
      def new(attributes) do
        Map.merge(%__MODULE__{}, attributes)
        |> validate
      end

      def validate(struct) do
        missing = missing_attributes(struct)
        if Enum.any? missing do
          raise "#{__MODULE__} requires #{Enum.join(missing, ", ")} field(s)"
        else
          struct
        end
      end

      defp missing_attributes(struct) do
        Enum.reduce(@required_attributes, [], fn (attr, missing) ->
          if Map.get(struct, attr) do
            missing
          else
            [attr | missing]
          end
        end)
      end
    end
  end
end

defmodule Walrus do
  defstruct name: nil, favorite_color: nil
  @required_attributes [:name]
  use RequiredFields
end

IO.puts inspect Walrus.new(%{name: "Frank"})
IO.puts inspect Walrus.new(%{name: "Frank", favorite_color: "purple"})
IO.puts inspect Walrus.new(%{favorite_color: "purple"})