RubyMoney/money

require 'money/bank/base'

class Money
  module Bank
    # Raised when trying to exchange currencies
    class DifferentCurrencyError < Error; end

    # Class to ensure client code is operating in a single currency
    # by raising if an exchange attempts to happen.
    #
    # This is useful when an application uses multiple currencies but
    # it usually deals with only one currency at a time so any arithmetic
    # where exchanges happen are erroneous. Using this as the default bank
    # means that that these mistakes don't silently do the wrong thing.
    class SingleCurrency < Base

      # Raises a DifferentCurrencyError to remove possibility of accidentally
      # exchanging currencies
      def exchange_with(from, to_currency, &block)
        raise DifferentCurrencyError, "No exchanging of currencies allowed: #{from} #{from.currency} to #{to_currency}"
      end
    end
  end
end