mocks_calculator.ex

defmodule MyApp.Calculator do
  def convert(amount, currency_from, currency_to) do
    with {:ok, rate_currency_from} <- MyApp.Rates.get_current_rate(currency_from),
         {:ok, rate_currency_to}   <- MyApp.Rates.get_current_rate(currency_to),
         value                      = amount * rate_currency_from / rate_currency_to do
      {:ok, value}
    end
  end
end

defmodule MyApp.Rates do
  def get_current_rate(currency) do
    # do external API requests
    {:ok, result}
  end
end