field :subtotal, :float do
  resolve fn _, _->
    batch({SomeModule, :batch_totals}, [:subtotal], fn results ->
      {:ok, results.subtotal}
    end)
  end
end

field :discount, :float do
  resolve fn _, _->
    batch({SomeModule, :batch_totals}, [:discount], fn results ->
      {:ok, results.discount}
    end)
  end
end

field :grand_total, :float do
  resolve fn _, _ ->
    batch({SomeModule, :batch_totals}, [:subtotal, :discount], fn results ->
      grand_total = results.subtotal - results.discount
      {:ok, grand_total}
    end)
  end
end

# in SomeModule
def batch_totals(_, desired_fields) do
  desired_fields
  |> List.flatten
  |> Enum.uniq
  |> Map.new(fn
    :subtotal -> {:subtotal, expensive_func1()},
    :discount -> {:discount, expensive_func2()},
  end)
end